Requirements Definition for Future DIAL Instruments - Emits

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Requirements Definition for
Future DIAL Instruments
Final Report
14 July 2005
Compiled by
Gerhard Ehret and Christoph Kiemle
Institut für Physik der Atmosphäre (IPA),
DLR, Oberpfaffenhofen
ESA RFQ Reference: IMT-CSO/FF/fe/03.887; RFQ/3-10880/03/NL/FF
Requirements Definition for Future DIAL Instruments, Final Report
1
ESA STUDY CONTRACT REPORT
ESA CONTRACT No
10880/03/NL/FF
SUBJECT: Requirements Definition for
Future DIAL Instruments
* ESA CR( )No
* STAR CODE
No of volumes
This is Volume
No
CONTRACTOR: IPA,
Deutsches Zentrum für
Luft- und Raumfahrt
CONTRACTOR'S
REFERENCE
ABSTRACT:
This study reports on the background for the definition of a future space-borne DIAL system. Target
species are the greenhouse gases (GHG) CO2, CH4 and N2O, ozone (O3), and the meteorological
parameters pressure (p) and temperature (T). Observational requirements were established by analysing the
scientific needs and user requirements. Ozone and the GHG were also investigated in the context of political
conventions. For the latter information on long-term (5-10 year) emissions are required. For CO2 and CH4
the knowledge on the total surface flux should be better than 20% on a spatial scale of 1000*1000 km2
averaged over 5 years. For N2O the requirement is more relaxed (~ 100%). The scientific needs for the GHG
were translated to observational requirements by running tracer experiments and inverse modelling. The
performances of current and future passive sensors were evaluated by literature research and compared to the
requirements defined in this report. Performance models of different classes of active optical instruments
were established. Direct and heterodyne detection principles as well as pulsed and random modulated (RM)
cw laser sources have been applied. The error characteristics of the different sensors were investigated by
analytical approaches and computer simulations. The best concept was selected. For this, different hardware
technologies were traded and design concepts for entering in a future breadboarding phase were proposed.
The key results of this study can be summarised as follows: The temperature DIAL applied in the oxygen
A-band is not suited for space-borne measurements. Passive sensors meet at least the threshold requirement
(e.g. AIRS/ HSB/ IASI) which is 1 K accuracy and 1 km vertical resolution. Surface pressure cannot be
measured by passive sensors to sufficient accuracy. Oxygen - DIAL is expected to meet the EUMETSAT
requirements for global NWP (1 hPa, 100 km horizontal resolution). The ozone UV-DIAL meets the
threshold requirements in the stratosphere. The UTLS can be covered only in parts. Soundings in the
troposphere would require a big instrument which is not available from current technology. Passive sensors
fail in meeting the stringent vertical resolution threshold/target requirement for O3 which is 2/1 km in the
stratosphere and UTLS and 3/1 km in the in the troposphere. The method based on Integrated Path
Differential Absorption (IPDA) is expected to meet the stringent target requirement for CO2 which is 0.75
ppmv at 1.6 µm and 1.5 ppmv at 2 µm for a 50 km integration length over land and ocean under all climate
conditions. The target requirements can also be achieved for an active CH4 IPDA instrument. For N2O only
the threshold requirements can be met. Range-resolved atmospheric backscatter DIAL would require a large
instrument for both CO2 and CH4. An overall systematic error of 0.32 ppmv for the pulsed and 0.4 ppmv
for the random modulated cw instrument was found for CO2. With passive systems the target requirements
can only be met for CH4 sounding in the solar backscatter region. Both direct detection at 1.6 µm and
random modulated cw IPDA at 2 µm were selected for investigations on instrument level. The instrument
demands for a 1.6µm IPDA sensor are expected to be higher than for the other approach.
Recommendations on future activities include further studies on the availability of auxiliary data such as
surface pressure, line parameters, surface reflectance over sea and the variability over land, and on the
suppression of spectral impurity for the direct detection instrument. Furthermore the performance of the
random modulated technique for the CW instrument needs to be demonstrated. The observational
requirements for soundings over the ocean and for the systematic error need to be consolidated.
Names of authors: G. Ehret (DLR), C. Kiemle (DLR) M. Wirth (DLR), A. Fix (DLR), H. Volkert
(DLR) S. Houweling (SRON), M. Heimann (MPI Jena), J. Lelieveld (MPI Mainz), V. Wulfmeyer
(UHOH), A. Behrendt (UHOH), M. Buchwitz (IUP Bremen), H. Bovensmann (IUP Bremen), U.
Kummer (ASG), H.R. Schulte (ASG)
NAME OF ESA STUDY MANAGER
ESA BUDGET HEADING
DIV: DIRECTORATE:
2
Final Report Contents
REQUIREMENTS DEFINITION FOR FUTURE DIAL INSTRUMENTS ................................................... 1
INTRODUCTION................................................................................................................................................. 5
1
SCIENTIFIC REQUIREMENTS................................................................................................................ 7
1.1
TEMPERATURE AND PRESSURE ................................................................................................................ 7
1.1.1
Relevance of Temperature and Pressure for global NWP ............................................................ 7
1.1.2
Deficiencies of Current Observing System ................................................................................... 7
1.1.3
Observational Requirements for Temperature .............................................................................. 9
1.1.4
Observational Requirements for Pressure..................................................................................... 9
1.2
OZONE (O3)........................................................................................................................................... 11
1.2.1
Scientific Relevance ..................................................................................................................... 11
1.2.2
International Agreements ............................................................................................................ 13
1.2.3
Deficiencies of Current and near-term O3 Data Availability ..................................................... 14
1.2.4
Observational Requirements for O3 ............................................................................................ 14
1.3
THE GREENHOUSE GASES CO2, CH4 AND N2O .................................................................................... 16
1.3.1
Scientific Relevance .................................................................................................................... 16
1.3.2
Scientific Requirements and International Agreements ............................................................. 18
1.3.3
Observational Methods................................................................................................................. 19
1.3.4
Observational Requirements for a DIAL Instrument ................................................................. 21
1.3.5
Limitations.................................................................................................................................... 27
1.3.6
Harmonization of DLR and IPSL CO2 requirements ................................................................. 28
2
INSTRUMENT PERFORMANCE ........................................................................................................... 31
2.1
PASSIVE SENSORS .................................................................................................................................. 31
2.1.1
Temperature Profiling.................................................................................................................. 31
2.1.2
Pressure Soundings...................................................................................................................... 32
2.1.3
Ozone Profiling ........................................................................................................................... 33
2.1.4
Greenhouse gases ......................................................................................................................... 35
2.1.5
Summary on Passive Systems ...................................................................................................... 38
2.2
ACTIVE OPTICAL SENSORS ..................................................................................................................... 39
2.2.1
The Physics of Measurement ....................................................................................................... 39
2.2.2
Instrument Performance Analysis ............................................................................................... 43
2.2.3
Overview on Sources of Random and Systematic Errors............................................................ 46
2.2.4
Temperature-DIAL....................................................................................................................... 52
2.2.5
Pressure-DIAL ............................................................................................................................. 54
2.2.6
Ozone-DIAL ................................................................................................................................. 56
2.2.7
Greenhouse Gases ........................................................................................................................ 58
2.2.8
Performance Summary on Active Systems .................................................................................. 63
2.3
PERFORMANCE SUMMARY OF ACTIVE AND PASSIVE SENSORS .............................................................. 68
2.3.1
Temperature ................................................................................................................................. 68
2.3.2
Pressure ........................................................................................................................................ 68
2.3.3
Ozone ............................................................................................................................................ 68
2.3.4
Carbon Dioxide CO2 .................................................................................................................... 68
2.3.5
Methane (CH4)............................................................................................................................. 69
2.3.6
Nitrous Oxide (N2O) .................................................................................................................... 69
2.3.7
Potential Synergy Between Passive and Active Sensors.............................................................. 69
2.4
PARAMETER SELECTION ........................................................................................................................ 71
2.4.1
Selection Criteria.......................................................................................................................... 71
2.4.2
Potential Advantage of an Active Sensor for CO2 ...................................................................... 71
2.4.3
Need for Auxiliary Data ............................................................................................................... 72
3
3
POTENTIAL INSTRUMENT CONCEPTS FOR CO2 .......................................................................... 73
3.1
CW HETERODYNE IPDA WITH RANGING CAPABILITY .......................................................................... 73
3.1.1
Detailed Instrument Requirements .............................................................................................. 74
3.1.2
Critical Instrument Sub-Systems ................................................................................................. 75
3.1.3
Expected Measurement Performance of a Coded CW IPDA ..................................................... 77
3.2
PULSED INCOHERENT IPDA INSTRUMENT............................................................................................ 78
3.2.1
Instrument Requirements............................................................................................................. 78
3.2.2
Critical Instrument Sub-Systems ................................................................................................. 80
3.2.3
Expected Measurement Performance of Pulsed IPDA at 1.6 µm............................................... 82
3.3
PERFORMANCE SYNTHESIS .................................................................................................................... 84
4
TECHNOLOGY STATUS, RISKS AND LIMITATIONS ..................................................................... 84
4.1
TRANSMITTER SUB-SYSTEM .................................................................................................................. 84
4.2
RECEIVER SUB-SYSTEMS ....................................................................................................................... 85
4.2.1
Telescope ...................................................................................................................................... 85
4.2.2
Blocking Filter.............................................................................................................................. 85
5
SUMMARY OF BASELINE RECOMMENDATION ............................................................................ 86
6
STUDY CONCLUSION AND RECOMMENDATIONS ........................................................................ 87
6.1
SUMMARY .............................................................................................................................................. 87
6.2
RECOMMENDATIONS ON FOLLOW-ON ACTIVITIES ................................................................................. 87
6.2.1
Validation of Statistical Albedo Variations ................................................................................. 87
6.2.2
Error Analysis of Supporting Data............................................................................................. 88
6.2.3
Breadboarding Activities .............................................................................................................. 88
7
REFERENCES............................................................................................................................................ 90
7.1
SCIENTIFIC REQUIREMENTS ................................................................................................................... 90
7.1.1
Temperature and Pressure........................................................................................................... 90
7.1.2
Ozone ............................................................................................................................................ 90
7.1.3
Greenhouse Gases ........................................................................................................................ 90
7.2
PERFORMANCE OF PASSIVE SYSTEMS .................................................................................................... 92
7.3
PERFORMANCE OF ACTIVE SYSTEMS ..................................................................................................... 96
8
DOCUMENTS AND TECHNICAL NOTES RELATED TO THE STUDY......................................... 98
4
Introduction
Lidar (Light Detection and Ranging) is regarded as an innovative component of the meteorological
and environmental observing system. It bears the potential to directly sample the four-dimensional
variability of the atmosphere with unprecedented accuracy and spatial resolution. This measurement
technique can establish independent sets of data products of various atmospheric parameters and
species that will help to answer scientific questions related to global change analysis of the Earth's
climate, atmospheric chemistry, atmospheric dynamics and the hydrological cycle.
Today atmospheric parameter are mainly measured by the operational radiosonde network and in-situ
sensors mounted on civil and research aircraft, research balloons or by passive remote sensing
instruments flown on satellites. All these techniques have their limitations. In-situ sensors do not
provide global coverage and passive remote sensing suffers from too coarse vertical resolution as well
as from retrieval complexity and associated error structures. Furthermore, both observational methods
need enormous calibration efforts to reduce biases in their measurements.
Active systems such as Lidars allow the direct measurement of wind speed, humidity, surface pressure
and profiles as well as temperature profiles without requiring additional model assumption. Lidar
instruments have the potential to measure major greenhouse gases such as CO2, CH4, N2O and O3.
Common to all Lidar instruments and a driver of feasibility is the availability of the appropriate
hardware operating in the spectral region where trace gases possess absorption bands of desired
strength. In general, large aperture telescopes and high power laser pulses are needed to achieve the
desired measurement sensitivity. The latter has to be generated very efficiently due to the limited
resources on a satellite. In the receiver chain very sensitive detectors are also required.
For the measurement of atmospheric parameters the Differential Absorption Lidar (DIAL) technique
proved to be well suited. When operating DIAL from a satellite platform, the laser transmitter emits
pulsed radiation at, at least, two closely neighboured wavelengths down to the Earth's atmosphere
where scattering and absorption processes attenuate the radiation. A small part is scattered back to the
receiver telescope on the satellite. The time interval between transmitted pulses and received signals
accurately determines the distance between satellite and scattering volume. By using wavelengths
lying close to the centre of an absorption line and just outside of the absorption peak the concentration
of corresponding species can be calculated directly from comparison of the received intensities.
On the other hand, if temperature sensitive absorption lines are selected, the temperature profile can be
derived from DIAL measurements. A DIAL measurement performed in the wing of the line, where
pressure broadening dominates the absorption feature, enables the measurement of ground pressure
and pressure profiles.
Profiles can be measured above cloud tops, between scattered clouds and also below clouds when
these are optically thin. An extended range can be realised by multi-wavelength systems running on
several spectral lines with different absorption cross sections. A high range-resolution is obtained by
transmission of short pulses in connection with range-gating of the received signals.
DIAL instruments are sophisticated and contain several components with novel and demanding
technologies. For operation on ground and airborne platforms, various instruments have been realised.
Space-borne lidar systems are beginning to be tested, while they have been the subject of extensive
investigations in Europe since the mid 1970ies. As mission and instrument concepts we mention
ATLID, an atmospheric backscatter lidar payload of the EARTHCARE mission, ALADIN, a Doppler
wind lidar and WALES, a water vapour DIAL. ALADIN has been recommended by the European
5
Earth Observation community for implementation in the frame of the Earth Explorer Atmospheric
Dynamics Mission. WALES has been studied at the level of a phase A study.
The objective of this study is to provide the background for the definition of a future space-borne lidar
system capable to monitor greenhouse gases, ozone (O3) and also pressure (p) and temperature (T).
Target species are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) which have been
recognized as the most important of the Earth’s greenhouse gases, the concentration of which is being
modified directly by human activities. Ozone protects life on Earth by absorbing ultraviolet radiation
at stratospheric levels. Near the surface it is a pollutant responsible for photochemical smog and a
greenhouse gas which contributes to climate change.
According to the Statement of Work (SOW) this study comprises three topics. First adequate
observational requirements for the particular parameter and a hypothetical mission in the timeframe >
2012 are analysed and defined. The greenhouse gases CO2, CH4 and N2O are investigated also in the
context of the Kyoto protocol. Second the performance of passive sensors is evaluated in a literature
survey and compared to the expected performance of active sensors. For the latter detailed computer
simulations are performed and the best concepts are selected. In the third part of this study different
hardware technologies are investigated and design concepts for entering in a future breadboarding
phase are proposed. This task also includes a preliminary instrument conceptual design study for the
major sub-systems, the identification of critical areas, and a preliminary risk analysis. Finally
recommendations on future activities are given.
The results of this study are documented in a bundle of Technical Notes where the findings are
detailed.
6
1 Scientific Requirements
1.1
Temperature and Pressure
1.1.1
Relevance of Temperature and Pressure for global NWP
Global numerical weather prediction (NWP) is in daily operational use and has reached a remarkable
quality level for 5 to 7 days ahead. The behaviour of the atmosphere is governed by a set of physical
laws, as the conservation of mass, momentum, angular momentum and energy, which can be
expressed as prognostic equations. The basic variables, which determine the large scale motions of the
atmosphere, are the three-dimensional fields of pressure, temperature, horizontal wind and direction
and humidity. A solution of the prognostic equations provides a description of the future state of the
atmosphere – a forecast – derived from a current state (initial values), which then can be interpreted in
terms of "weather", i.e. sunshine, cloudiness, rain and wind.
Today, numerical weather prediction models are very effective in the description of the state of the
atmosphere on a global basis. In the future, enhanced computer resources and advanced assimilation
schemes will be available and enable to resolve smaller scale meteorological processes even on a
global basis. This raises the need for improved observations of both temperature and pressure
particularly in terms of spatial resolution, accuracy and timeliness. For pressure the determination of
reliable surface values appears to be of highest importance to improve the semi-empirical data of the
southern oceans, while sufficiently accurate measurements of the high stratospheric values below 10
hPa are not considered as feasible. Global numerical weather prediction is seen as the central focus for
daily applications. Implicitly this also includes the larger scale application of climatological
monitoring and smaller scale regional NWP demands for the following reasons:
i)
climatological analyses using basic meteorological variables like temperature and pressure
necessitate consistent time series exceeding a decade; these are currently established at large
weather centres (e.g. ECMWF or NCEP) by re-analyzing 15, 40 or even 50 year long spans of
raw data, including satellite observations as far as they are available; this means that even in
the future long-term data sets for climate analysis will be determined from global weather
data, the observational requirements of which are steered by NWP applications;
ii)
regional NWP uses high resolution limited area models (current horizontal grid mesh below
10 km); they are nested in global NWP models, which provide the initial data and the forecast
boundary values for the entire forecast period; the impact of space borne sensors to regional
NWP will also in the future follow the link via global NWP, while future mesoscale data
assimilation will focus on surface based remote sensing (e.g. by precipitation radar networks;
furthermore, regional NWP mainly concerns well instrumented areas over land).
1.1.2
Deficiencies of Current Observing System
Radiosonde network
The traditional pressure and temperature measurements are made in-situ, at surface stations on land
(13504), ships (1607) and ocean buoys (2941), with radio-sondes ascending on meteorological
balloons (590; with 100 to 200 data points each) or with help of commercial airliners (40598). The
numbers in brackets indicate the amount of relevant observations entering the operational data
assimilation scheme of the European Centre for Medium-Range Weather Forecasts (ECMWF) on 7
7
Nov. 2003, 00 UTC; they are used to give some idea about the size of the datasets, although differents
weights are assigned to the different types of observations. Though low in number the radio-sondes
ascents are still the backbone of free atmosphere in-situ measurements, as a single one delivers a
complete vertical profile with instruments calibrated by a meteorological service. A serious deficiency
is the uneven geographical coverage with large data-void regions, especially at higher latitudes in the
southern hemisphere.
Satellite Remote Sensing
A variety of instruments on geostationary and polar orbiting satellites provide passive remote sensing
data, i.e. radiances in certain spectral bands. These are increasingly fed into operational data
assimilation schemes, either as indirect model variables, but directly observed radiances, or as direct
variables (e.g. temperature) after some retrieval process. The highest similarity to radio-sonde
temperature observations have the ATOVS sounding points (300596 altogether), which are sampled
by the NOAA polar orbiting satellites; different wavelengths allow to infer the temperature in different
height layers, but the vertical resolution is far lower than for radio-sonde observations.
One of the major accomplishments in satellite remote sensing foreseen for the upcoming decade is the
exploitation of the entire infrared spectrum at unprecedented spectral resolution using spectrometers or
interferometer. AIRS (Atmospheric Infrared Radiance Sounder) on AQUA is the first in a series of
spectrometers, which has already been lauched. IASI (Improved Atmospheric Sounding in the
Infrared) is another candidate scheduled for launch in 2005 on the METOP platform. It remains to be
evaluated to what extent these multi-channel instruments will impact operational NWP once their data
are regularly fed into the assimilation schemes.
Surface pressure can so far not be remotely sensed from space; a detailed feasibility study was
provided by O’Brien (2002). Such observations would be valulable, especially on the southern
hemisphere (Eyre et al., 2002) as is strongly suggested by the beneficial impact of PAOBs prepared by
the BMRC of Australia. These artificial surface pressure observations are manually constructed from
conventional surface pressure analyses combined with satellite images.
In general, space borne measurements are still lacking the required vertical resolution, which is likely
to improve considerably when active sensors (e.g. lidars) become operationally available.
Requirements for data assimilation
Nowadays advanced assimilation schemes are of similar complexity as the NWP model itself, i.e. the
algorithm which carries the starting values to the desired forecast result (Kalnay, 2002). All
assimilations start from a model generated first guess, typically a short range forecast from the last
analysis time. All available measurements are then used to transform the first guess to the initial data
for a particular NWP run by either optimum interpolation (OI), three-dimensional variation (3D-Var)
or four-dimensional variation (4D-Var) methods (Bouttier and Courtier, 1999). OI and 3D-Var assume
synchronized or synoptic observations while 4D-Var can handle asynoptic measurements taken during
a time window of 6 to 12 hours by applying the iterative application of the adjoint of the NWP model,
however at a significant increase of computational expense.
The large European weather centres (DWD, ECMWF, Météo-France, UK Met.Office) apply different
versions of the indicated assimilation schemes for their daily global modelling efforts. Therefore a
proper judgement of the impact of future satellite sensors has to take into account this variety today
and as envisaged for the coming decades. Furthermore, a clear distinction has to be made between the
requirements of global medium-term (10 to 14 d; Eyre et al, 2002) and regional short-term forecasting
(up to 3 d; Gustafsson et al., 2001).
8
To meet future NWP objectives following key observational requirements have been derived from
literature survey.
1.1.3
Observational Requirements for Temperature
Special emphasis is put on a well specified height assignment of the envisaged future space
instruments. When satellite derived vertical profiles come close to the still unmatched standard of
conventional radiosonde profiles concerning a height assignment reliable in the order of 100 to 10 m, a
major breakthrough in NWP applications can be expected. Currently a vast number of satellite data
enter the NWP data assimilation schemes, but only a small fraction of independent pieces of
information are retained, while all quality checked radiosonde profile provide up to 100 independent
data points in the vertical although they are typically counted as a single entry in statistics of
operational data (e.g. by ECMWF). For this reason, active remote sensing with advanced lidar
techniques appears to be promising. The observational requirements for global NWP are given in
Table 1.1-1 for four altitude ranges.
Table 1.1-1: Observational requirements for global NWP for space borne temperature sensors broken
down by the atmospheric layers Lower Trosposphere (LT), Higher Trosposphere (HT), Lower
Stratosphere (LS), and Higher Stratosphere (HS); taken from [TN 110]
Parameter
Temperature
Altitude range
LT
HT
LS
HS
pressure height
[hPa]
1000–500
500–100
100–10
10 – 1
approx. height range
[km]
0–5
5 – 15
15 – 35
35 – 50
1
1
2
3
0.1
0.1
0.2
0.3
200
250
Vertical sampling1
[km]
Height assignment
[km]
Horizontal domain
Horizontal sampling
Dynamic range
global
2
[km]
50
100
[ K]
Precision3 (1 standard deviation) [ K]
180 - 300
1
1
2
2
Bias
[ K]
< 0.5
< 0.5
< 0.5
< 0.5
Observation cycle4
[ h]
12
12
12
12
[ h]
3
3
3
3
Timeliness5
1
vertical sampling requirement for regional NWP = 0.5 km
horizontal sampling requirement for regional NWP = 15 km
3
precision requirement for regional NWP = 0.5 °K
4
observation cycle for regional NWP = 1 h
5
timeliness for regional NWP = 1 h
2
1.1.4
Observational Requirements for Pressure
Atmospheric pressure at a certain level provides integrated information of the mass distribution above.
Surface pressure at a point equals the gravitational force (weight) per unit area of the air column
above. For global NWP the pressure and temperature distribution follow to a high degree the
9
principles of hydrostatic balance, geostrophic balance and thermal-wind balance. The observational
requirements for pressure are divided in three altitude ranges (Table 1.1-2) and the surface (Table 1.13).
Table 1.1-2: Observational requirements for global NWP for space borne pressure sensors broken
down the atmospheric layers Lower Troposphere (LT), Higher Troposphere (HT), and Lower
Stratosphere (LS); taken from [TN 110].
Parameter
Pressure Profile
Altitude range
Surface
LT
HT
LS
pressure height
[hPa]
1000-500
500-100
100-10
approx. height range
[km]
0–5
5 – 15
15 – 35
1
1
2
0.01
0.01
0.01
0.01
global
global
global
global
50
50
100
200
1050-950
1050-500
500-100
100-10
<1
0.6
0.2
0.1
[ hPa]
<0.5
<0.5
<0.2
<0.1
Observation cycle
[ h]
12
12
12
12
Timeliness4
[ h]
3
3
3
3
Vertical sampling
[km]
Height assignment
[km]
Horizontal domain
Horizontal sampling1
Dynamic range
[km]
[ hPa]
2
Precision (1 standard deviation) [ hPa]
Bias
3
1
horizontal sampling requirement for regional NWP = 15 km
observation cycle for regional NWP = 1 h
3
timeliness for regional NWP = 1 h
2
A refined set of observational requirements (table 1.1-3) for the surface pressure has been derived
from harmonising with the findings of a recent EUMETSAT study, which was provided by ESA.
These requirements cover the categories threshold (thresh), target (tar) and breakthrough for the
categories now casting, regional, and global NWP independently. We assigned highest relevance to
global NWP (see argumentation above) and note that the surface pressure breakthrough requirements
for global NWP from EUMETSAT study are close to our findings in this issue as indicated in table
1.1-2 above. The EUMETSAT table 1.1-3 has finally been used to establish the instrument
performance with respect to the surface pressure observation.
Table 1.1-3: Observation requirements for surface pressure for nowcasting, regional and global NWP
according the EUMETSAT; study taken from [TN110]
Accuracy
Spatial res.
Repeat cycle
Timeliness
Breakthrough
Surface
(tresh/tar)
(tresh/tar)
(tresh/tar)
(tresh/tar)
Pressure
Now
2 / 1 hPa
10 / 1 km
30 / 10 min.
30 / 10 min.
2 hPa, 10 km,
Casting
30 min.
NWP
1 / 0.1 hPa
50 / 3 km
3 / 0.5 h
No spec
1 hPa, 30 km, 3
Regional
hrs.
NWP
3 / 0.5 hPa
250 / 15 km
12 / 1 h
4/1h
1 hPa, 100 km,
Global
6h
10
1.2
Ozone (O3)
1.2.1
Scientific Relevance
Ozone (O3) plays a key role in our atmosphere, as it attenuates radiation from the ultraviolet (UV) to
the infrared (IR) part of the spectrum, thus protecting life on Earth and affecting the greenhouse effect
and dynamical processes. Furthermore O3 controls oxidation reactions that cleanse the atmosphere
from natural and anthropogenic gas emissions, although under some conditions it also acts as a
noxious air pollutant. The abundance and distribution of O3 are determined by a large range of
processes that can interact non-linearly to source and sink gases. The role and control of O3 in the
stratosphere and troposphere are distinctly different as a result of its uneven vertical concentration
profile.
Stratospheric O3 and UV radiation
In the stratosphere between about 10-15 and 50 km altitude, molecular oxygen and ozone completely
absorb UV-C radiation (200-280 nm wavelength). Ozone moreover absorbs most UV-B (280-315 nm)
so that only a few percent reaches the earth’s surface. UV-B is of particular relevance because it
damages biological tissue. UV-A (315-400 nm) is not strongly attenuated by O3, whereas it also
influences human health (e.g. sunburn). About 90% of atmospheric O3 is in the stratosphere, hence its
depletion by nitrogen oxides and halocarbons has raised serious concern about the trend in surface
UV-B irradiance. For each percent decrease in total ozone, erythemally effective UV-B radiation
increases by about 1.3% (WMO, 1999). Because of the exceptionally cold conditions in the Antarctic
stratosphere, polar stratospheric clouds (PSCs) can be formed, on which halogen compounds are
activated from inert reservoir species. These halogens, notably chlorine from chlorofluorocarbons
(CFCs), cause dramatic O3 loss during the Antarctic spring because of the optimum between low
temperatures and the availability of sunlight, causing the “ozone hole”). Since the Arctic stratosphere
is more dynamic and warmer, springtime O3 loss is less dramatic. The relatively strong dynamics in
the northern hemisphere during late winter and early spring, however, can give rise to “mini-holes”
that sometimes reach Europe. At present, the total global stratospheric O3 loss amounts to 5-10% with
higher values at the poles and lower values in the tropics. Although the long-lived CFCs are phased
out under the Montreal Protocol and its amendments, the stratosphere becomes colder, moister and its
dynamics may change with increasing CO2 and climate change. The global concentrations of shortlived halocarbons moreover increase strongly, and a fraction may reach the stratosphere. The net effect
of these changes is yet unclear.
Tropospheric O3 and air pollution
Tropospheric ozone plays a key role in oxidation processes and air quality. It is transported downward
from the stratosphere, yet the main source in the troposphere is in situ photochemical formation
through the oxidation of carbon compounds, being catalyzed and controlled by nitrogen oxides. The
sum of these compounds (NOx) is often used because NO and NO2 rapidly equilibrate (NOx ≡
NO+NO2). The main NOx sources are fossil fuel combustion, biomass burning, soil microbial activity
and lightning, whereby the global anthropogenic emissions dominate the natural ones by a factor of 3
to 4. The anthropogenic NOx emissions primarily originate from fossil fuel use. If the emissions take
place in air that is still relatively pristine, however, the O3 formation per unit NOx added is much
more efficient than in NOx-enriched air. Therefore, even though most NOx is emitted in the Northern
Hemisphere, O3 increases in the Southern Hemisphere may also be substantial, as indicated by
satellite measurements and chemistry-transport model simulations.
11
Ozone is a main component of photochemical smog. In western Europe about half of the smog
precursors is associated with road transport and most of the rest with industry and fossil energy use.
The photochemical smog damages human health, agricultural crops and ecosystems. Paradoxically,
while stratospheric O3 protects life on Earth from harmful UV radiation, O3 near the surface has
adverse effects because of its high reactivity. The European 8-hourly air quality standard for ozone is
53 ppbv. Nitrogen dioxide (NO2) is also an oxidant that is controlled together with O3 through
national and European air pollution directives. Because oxidant build-up is dependent on solar
radiation, it is notorious during summertime episodes, typically under anticyclonic (stagnant) cloudfree conditions. Ozone levels in western Europe have increased strongly, in particular in the 1960s and
1970s. Meanwhile some countries report slight reductions, although air quality standards are still
violated during the summer episodes. In parts of southern Europe, on the other hand, the EU air
quality limit is exceeded throughout summer, with an upward tendency. Trend analyses of ship
measurements over the Atlantic Ocean during the period 1977-2002 have shown that O3 increases at
northern mid-latitudes have been relatively small (i.e. not statistically significant). However, at
subtropical and tropical latitudes as well in southern mid-latitudes, upward O3 trends are substantial
(Lelieveld et al., 2004).
.
Model simulations for present conditions and for the year 2025, applying the IPCC IS92a base
scenario for atmospheric chemistry projections, indicate continued increases of troposperic O3
(Lelieveld and Dentener, 2000). In spite of considerable efforts by environmental protection agencies,
surface ozone is expected to further increase in many areas In western Europe and the USA some
reductions of episodic peak O3 values have recently been attained. In many other countries, however,
rapid economic developments are associated with strong pollution increases, particularly in Asia.
Since many of these countries are located in subtropical latitudes, the high UV levels in summer can
boost photochemical air pollution.
Surface ozone in the northern hemisphere may increase substantially in the next decades, in particular
at subtropical latitudes; thus the observed O3 trend is predicted to continue. The model simulations
furthermore suggest O3 increases in western Europe and the USA, even though O3 precursor
emissions in these regions have been assumed to remain nearly constant in the emission scenario. The
expected growth of O3-precursor emissions in Asia, however, will likely cause a large-scale O3
increase, enhancing the hemispheric background, so that pollution control efforts in other regions,
including the USA and Europe, can be overpowered.
Climate effects of O3
Stratospheric ozone depletion in the past 25 years has caused a negative radiative forcing (i.e. cooling
effect), which also influences the troposphere. This is mostly caused by O3 loss in the lower
stratosphere at mid-latitudes, which decreases the local greenhouse effect (IR radiation absorption), in
turn reducing the heating of the troposphere-surface system. Ozone absorption of IR radiation mostly
takes place in the 9.6 µm wavelength region. The reduction of solar UV radiation absorption by O3 in
the lower stratosphere also plays a role, however, its effect on climate is counteracted by the
consequent increased solar radiation penetration into the troposphere.
In the troposphere large-scale O3 formation in smog-type chemistry, propelled by strong
anthropogenic NOx emissions, enhances the greenhouse effect, which causes a positive radiative
forcing of climate, which is comparable with that of methane. The influence of O3 changes on climate
is strongest when they take place near the tropopause. This is related to the strong temperature contrast
between the lower troposphere (9.6 µm emission) and the relatively cold tropopause (9.6 µm
absorption and re-emission to space). This means that the climate sensitivity to O3 changes is greatest
12
at the tropopause, i.e. the part of the atmosphere where the O3 concentration gradients are largest and
where O3 trends change sign; hence the observational systems needed to detect these changes must
provide high vertical resolution and sensitivity.
Upper troposphere and lower stratosphere (UTLS)
The location of the tropopause strongly influences the total O3 column, e.g. as observed from space.
High column O3 at mid-latitudes is usually associated with cyclones (low altitude tropopause),
whereas low column O3 is associated with anticyclones (high tropopause) (Lelieveld and Dentener,
2000). The combination of tropospheric warming by the enhanced greenhouse effect and stratospheric
cooling by ozone loss, is likely to affect the location of the tropopause. Indeed, meteorological
reanalysis data as well as observations by balloon soundings indicate that the tropopause altitude has
increased over the past decades. It has furthermore been suggested that tropopause height changes may
be a sensitive indicator of anthropogenic climate effects. Active remote sensing measurements of O3
and temperature profiles will be prerequisite to quantify these changes on a global scale.
An additional factor of UTLS coupling is that climate change is accompanied by stratospheric cooling,
caused by increasing CO2 and loss of O3. This can be associated with an intensification of the polar
vortex in the lower stratosphere, which may enhance O3 destruction, thus counteracting the effect of
decreasing chorine loading through the Montreal Protocol. Note that model calculations by several
groups assess these effects strongly differently (Steil et al., 2003). This might be particularly relevant
for the Northern Hemisphere, where the tropospheric wave activity, driving stratospheric dynamics, is
stronger than in the Southern Hemisphere. Although stratospheric ozone destruction is much stronger
over Antarctica than over the Arctic, the northern polar stratosphere is close to the threshold of
developing an ozone hole. Another interesting aspect is the unexpected Antarctic vortex breakup in
September 2002, associated with a major stratospheric warming event triggered by enhanced
tropospheric wave activity. It is not known to what extent this phenomenon is associated with climate
change.
Changes in upward propagating waves from the troposphere clearly affect the stratospheric diabatic
circulation, with consequences for stratosphere-troposphere exchange (STE) of ozone. Since STE is an
important source of ozone to the troposphere, future changes may add to the expected large-scale
increase of pollutant O3 formation. Since the O3 lifetime is relatively long in the upper troposphere,
intercontinental O3 transport in this part of the atmosphere has the important potential to contribute to
global change. In summary, especially the UTLS is very sensitive with respect to ozone and
temperature changes, whereas present measurement systems have great difficulty in providing the
required resolution.
1.2.2
International Agreements
Ozone and a number of its chemical precursors (or destroyers) are subject to several international
agreements. The Montreal Protocol to the 1985 Vienna Convention of the UN for the Protection of the
Ozone Layer is a prototype of an international agreement that has had a major impact on pollution
emissions and the global environment. It marks the consensus reached between all parties involved,
based on emerging scientific understanding. Several amendments (London 1990, Copenhagen 1992,
Beijing 1999) have further restricted the production of CFCs, and the full phase-out of emissions has
started in 2003.
On a European level the UN Convention on Long-Range Trans-boundary Air Pollution (LRTAP),
signed in 1979, and its protocols in the 1980s and 90s provide another example of such an agreement.
13
LRTAP limits the emission of ozone precursor gases such as NOx and NMHCs. Although the
Convention does not in itself call for any binding commitments to reduce specific pollutants, the 49
participating countries have committed to "limit and, as far as possible, gradually reduce and prevent
air pollution," and, to achieve this, use "the best available technology which is economically feasible".
Furthermore, the LRTAP parties have committed to monitor air pollution, including ozone. The IPCC
scientific consensus reports have provided the basis for the UN Framework Convention on Climate
Change (UNFCCC), signed during the 1992 Rio Earth Summit. In 1997 the parties met in Kyoto to
negotiate the agreement, which has now been ratified by many countries. Although ozone has not been
considered in the Kyoto Protocol, its climate impact is increasingly recognized so that it is likely that it
will be included in future climate agreements.
1.2.3
Deficiencies of Current and near-term O3 Data Availability
As a consequence of the high variability of atmospheric O3, a relatively dense network of
measurement stations is required, for which a combination of ground-based, balloon- and space-borne
instruments is employed. Direct measurements include chemical and optical techniques that are used
in surface networks and balloon sounding systems. Although these measurements are performed in
many locations, they are concentrated in populated regions, in particular in the developed world. Part
of the “gap” over developing countries and over the oceans is filled by using commercial – i.e.
passenger – aircraft. However, only few airliners participate in this endeavour, whereas the flight
routes usually involve limited “corridors” in the Northern Hemisphere. Remote sensing measurements
include ground-based active and passive systems. The Network for the Detection of Stratospheric
Change (NDSC) applies lidars, UV-VIS and Fourier Transform Infrared (FTIR) spectrometers to
monitor the ozone layer, predominantly at middle and high latitudes. The network contains a dozen
lidars that can measure stratospheric ozone. NDSC also contributes to the validation of satellite
measurements. In addition, O3 is measured from a variety of current space-borne instruments
(GOMOS, MIPAS, SCIAMACHY), using passive remote sensing techniques on polar orbiting
satellites. Although several of these instruments can measure O3 profiles according to the
specifications, as yet it has not been possible to retrieve more than 1-2 layers in the troposphere. It is
particularly difficult to detect O3 gradients near the tropopause, a critical region for atmospheric
chemistry and climate. Ozone trend analysis has shown that the presently available measurements
from space are not sufficiently sensitive to detect O3 trends in the tropical lower troposphere
(Lelieveld et al., 2004). Such analyses will be even more difficult for the extra-tropical troposphere.
1.2.4
Observational Requirements for O3
As analysed in the previous paragraphs ozone (O3) is a key compound in the chemistry and radiation
transfer in our atmosphere. It has a pronounced vertical profile, with a maximum at about 20-25 km in
the stratosphere, and a steep gradient in the upper troposphere – lower stratosphere (UTLS) region.
Important environmental issues can be summarised as follows:
•
•
•
•
Stratospheric O3 loss and consequent increased ultraviolet radiation near the surface;
Climate effects by changing O3 in the troposphere and stratosphere, and feedbacks;
with increasing CO2;
Regional O3 formation in the troposphere and photochemical air pollution;
Hemispheric O3 increases in the troposphere by growing precursor emissions in
developing countries, in particular those with emerging economies.
14
These issues are partly subject of international agreements, e.g. UN Conventions. Table 1.2-1
summarises the threshold and target observational requirements for following applications:
1. Tropospheric O3 and climate research;
2. Air quality (i.e. chemical weather) forecasting;
3. Stratospheric O3 monitoring and research;
4. UV radiation monitoring and forecasting;
5. UTLS chemistry and dynamics (including strat-trop exchange) research;
6. Numerical modelling and weather forecasting;
7. Compliance with international agreements
Table 1.2-1: Threshold and target observational requirements for O3 measurements; taken from
[TN130]. Target observational requirements are listed in parentheses.
Parameter
Ozone measurements
Altitude range [km]
Troposphere
UTLS
Stratosphere
0-10
8-16
10-50
Applications1
1,2,6,7
1,4,5,6
1,3,4,6,7
Vertical resolution [km]
3(1)
2(1)
2(1)
Horizontal domain
global
Horizontal resolution [km]
20 (10)
50 (10)
100 (10)
Dynamic range in mixing ratio [ppbv]
10-100
50-1000
103-104
Precision (1 standard deviation) [%]
< 20 (10)
< 20 (10)
< 20 (10)
Accuracy (systematic error) [%]
< 10 (5)
< 10 (5)
< 10 (5)
Timeliness [h]
3
6
6
1
The numbers indicate the applications listed above.
It should be noted that stratospheric O3 measurements remain to be of key interest, for example to
monitor the compliance with the Montreal Protocol and its amendments. The lower stratosphere is of
particular relevance because the future development of the ozone layer is linked to climate change
through PSC abundance and the stability of the polar vortex. To resolve the vortex edge and the
streamers that arise from vortex erosion in late winter and early spring, the measurements should be
performed at a minimum of 100 km resolution in the horizontal direction; the vertical resolution
should be at least 2 km to improve the performance of planned passive sounders (e.g. GOME-2,
HIRDLS, SAGE-III, TES). The UTLS region is of great interest for long-range air pollution transport,
the effects of air traffic, stratosphere-troposphere coupling and climate research applications. The
horizontal resolution of space-borne lidar measurements should be about 50 km or better and the
vertical resolution must be 1-2 km. Some vertically resolved information will be provided by the
SAGE follow-up instruments and the SCIAMACY limb sounder, however, the spatiotemporal
resolution of these missions within the troposphere is relatively low. The combined O3 column
information from planned operational missions (e.g. METOP) and the lidar retrieved profiles can be
used to improve assimilation data sets for many applications, including UV radiation monitoring and
forecasting. Numerical weather forecasting will profit from stratospheric wind data retrieved from O3
measurements. Furthermore, there is increasing evidence for a dynamical influence from the
stratosphere on the troposphere, for example, through the meridional heating gradient, affected by
15
chemical O3 destruction and solar variability, which indirectly influence the strength of the jet stream.
This application also requires spatially resolved information about the O3 distribution, which will
support medium- and extended-range weather forecasting.
1.3
The Greenhouse Gases CO2, CH4 and N2O
1.3.1
Scientific Relevance
Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have been recognized by the Working
Group I of the International Panel of Climate Change (IPCC) as the most important of the earth’s
greenhouse gases, whose concentration has been directly modified by human activities. Measurements
on Antarctic ice cores have documented an atmospheric increase of all three gases since the beginning
of the industrial revolution: for CO2 from 280 ppmv to about 370 ppmv, for CH4 from 700 ppb to
about 1750 ppb and for N2O from 270 ppb to about 315 ppb today. Together, the concentration
increases of these three trace gases have induced an additional global annual mean radiative forcing of
more than 2 Wm-2 over the time period of 1750 to present. The concentration increase of these
greenhouse gases can be clearly related to human activities related to energy production and land use
practices. An increasing world population and its needs for energy and food production will foster
increased greenhouse gas emissions over the next decades which will lead to a further rise in
atmospheric concentration and an associated increased radiative forcing of the climate of the earth.
Hence in order to better predict the behaviour of this system, a more accurate knowledge of the
sources and sinks of these gases in terms of location, magnitude and underlying processes is essential.
In addition to this fundamental scientific question, the establishment of the Kyoto protocol is also
challenging the science. The national emission reductions that have been agreed to in the Kyoto
protocol and the subsequent Conferences of the Parties (COPs) obviously necessitate the development
of methods to quantify and verify in a scientific credible way the committed emission reductions and
the impact of declared measures to curb source or enhance sink processes of the different greenhouse
gases (e.g. by afforestation). Unfortunately, up to now this is not possible, at least on larger scales than
a few hectares of forest.
Carbon dioxide (CO2)
The major anthropogenic sources of CO2 are the burning of fossil fuels and changes in land use (e.g.
deforestation for the establishment of agricultural areas). Precise atmospheric measurements, however,
show that only about half of the emitted CO2 remains in the atmosphere, the remainder being taken up
both by the oceans and by various processes on land. The exact quantification of the uptake on the
surface of the earth, and in particular its spatial and temporal variability, is still very poorly known,
despite intensive carbon cycle research over the last few decades (Prentice et al., 2002).
The problem of the exact quantification of the surface sinks of anthropogenic CO2 is complicated by
the fact that the aforementioned human perturbation is superimposed on the global, natural carbon
cycle, consisting of large carbon stores in the atmosphere (primarily CO2, ~750PgC), on land
(vegetation, soils, ~3’800PgC) and in the oceans (mostly dissolved inorganic carbon, ~38’000PgC).
There exist very large exchange fluxes between these reservoirs globally amounting to about
60 PgCa-1 between the atmosphere and the land (primarily photosynthesis and respiration) and about
80 PgCa-1 between the atmosphere and the oceans (gas exchange driven by the partial pressure
16
difference between air and sea). 1 These natural processes vary enormously in space (e.g. induced by
the temperature dependent solubility in the ocean) and time (e.g. induced by the annual cycle of
photosynthesis in mid- and high latitudes) and generate thus a complicated surface flux pattern of
source and sink regions that varies in time. Globally integrated, these natural fluctuations have been
relatively modest as witnessed by a rather stable atmospheric concentration level during the Holocene
before the anthropogenic perturbation. However, according to first coupled carbon cycle - climate
simulations, these natural carbon fluxes might experience significant modifications in the case of a
warming world, most likely reflecting a moderate or even strong positive feedback in the earth system
(Cox et al., 2000, Friedlingstein et al., 2003).
Compared to the natural exchange fluxes the anthropogenic perturbation fluxes are relatively small:
the global emissions from fossil fuel burning in the year 2000 are about 6.6 PgCa-1 and the CO2
emissions induced by changes in land use are estimated to lie between 1 and 2 PgCa-1. Globally the
anthropogenically induced uptake fluxes by the oceans and by land are even smaller: each estimated at
about 2 PgCa-1 during the 1990’s. Considering the large natural background fluxes, the detection of
this anthropogenic sink signal is very difficult. On the other hand, already an accurate quantification of
the natural spatio-temporal pattern of sources and sinks would be scientifically extremely valuable, as
it would allow a much better validation of coupled carbon cycle – climate models than heretofore
possible.
Methane (CH4)
CH4 is a powerful greenhouse gas, which has a Greenhouse Warming Potential (GWP) of 23 relative
to CO2 on a time scale of 100 years. The major anthropogenic sources of CH4 are the emissions from
energy production, landfills, waste treatment, cattle, rice fields and incomplete biomass burning. In
addition, there are significant natural CH4 emissions from wetlands. The global total emissions in
1998 are estimated at about 600 TgCH4 yr-1, of which the anthropogenic emissions comprise about 2030%. A large fraction of the emitted CH4 is destroyed in the atmosphere by the chemical reaction with
OH. In recent years the imbalance between the sources and sinks of CH4 has decreased from about 22
TgCH4 yr-1 in the early 1990’s to an insignificant value at present, resulting in approximately constant
concentration levels since 1999 (Dlugokencky et al., 2003). Because this transition is not well
understood future scenarios have become even more uncertain. The relatively short atmospheric
lifetime of CH4 of about 12 years make this greenhouse gas particularly interesting for a short-term
reduction of the radiative forcing by the anthropogenic greenhouse gases and its emissions are also
covered by the Kyoto protocol.
Scientifically, CH4 is also closely coupled to the global climate system. A global warming in particular
in Arctic regions might foster the melting of permafrost soils which contain significant amounts
carbon in organic form which under anaerobic conditions might be converted to CH4 and partially
released to the atmosphere. There exist also very large deposits of CH4 as hydrates on ocean shelves
that are vulnerable to ocean warming. Paleo records indicate that both processes have been important
feedbacks in the climate system. The development and implementation of a system for the detection of
CH4 emissions in these vulnerable areas have a high scientific priority.
Nitrous oxide (N2O)
Nitrous oxide constitutes a greenhouse gas that may be more relevant in the longer-term future
because of its long atmospheric lifetime of 120 years. It has a high GWP of 296 relative to CO2 on a
17
time scale of 100 years. The major anthropogenic sources (total ~4 TgN yr-1) are agriculturally used
soils (fertilization), cattle and feedlots, biomass burning and several industrial sources. These sources
are superimposed on natural sources from the ocean and from soils totalling about 10 TgN yr-1.
Overall, the anthropogenic sources comprise about 40% of the total N2O emissions; however the
existing literature numbers are very uncertain. N2O is destroyed primarily in the stratosphere by photo
dissociation. Many of the different source mechanisms and how they vary as functions of natural and
anthropogenic environmental parameters are very poorly known. A regional quantification of N2O
emissions by inverse modelling have not yet been possible because the atmospheric observation
network is much less developed for N2O concentration measurements as it is for CO2.
1.3.2
Scientific Requirements and International Agreements
The main political issues and scientific questions regarding the evolution of long-lived greenhouse
gases in the atmosphere call for the quantification of long-term (5-10 year) emissions (i.e. mean flux
and trend) and their spatial distribution. In addition, to improve the scientific understanding of the
underlying processes, the monitoring of shorter-term variability is also important, such as seasonal
cycles and in particular inter-annual variability, which documents the climate impact on the sources
and sinks of the greenhouse gases.
In order to define target and threshold accuracy levels of the greenhouse gas surface fluxes, a
distinction must be made between
(1) the scientific objective of an adequate quantification for the understanding of the relevant
processes controlling sources and sinks of the greenhouse gases, and
(2) a “Kyoto objective” for the independent verification of national emission inventories
determined by bottom-up methods.
Both objectives are different for the different greenhouse gases because of a different mix of source
and sink processes.
In case of CO2 the dominant anthropogenic source is the combustion of fossil fuels, which can be
determined rather accurately from energy production statistics. The accuracy of these estimates is
considered to be better than 5% globally, and, in developed countries, better than a few percent on a
national level. Since the present commitments under the Kyoto protocol are on the order of an 8%
reduction with respect to the emissions in the base year of 1990, a very high accuracy is needed for a
credible verification. Matching this accuracy with a top-down method clearly is unrealistic with the
present technology.
The Kyoto protocol also permits to account certain terrestrial sinks to the national CO2 emission
inventory, such as e.g. afforestation measures. In most places of the world, on a 5 year time scale, the
fluxes regulated by the Kyoto protocol are significantly smaller than 20% of the natural terrestrial
fluxes. Since the Kyoto regulated terrestrial fluxes cover only certain sectors, a top-down verification
method that “sees” only the total flux will not be able to detect these without additional information.
However, if in a future stage the Kyoto protocol were revised to include all terrestrial fluxes in the
national budgets (“full carbon accounting”), then addressing the second objective as listed above by
means of a top-down method would become feasible by subtracting estimates of the fossil emissions
from the total flux. Nevertheless, the required accuracy level is very high. For CO2 the current
uncertainty with which regional scale (106 km2, 5 year average) terrestrial fluxes can be estimated is
about 30 – 100 gCm-2a-1 (Rödenbeck et al., 2003). This is in the same order of magnitude as the typical
total terrestrial flux in many parts of the world. Even if integrated over entire continents the present
accuracy is not better than about 50% (Gurney et al., 2002, Rödenbeck et al., 2003). On the other
hand, achieving the main objectives requires an accuracy of better than 20% of the total flux, on a
18
spatial scale of 106km2, averaged over 5 years. At least such an accuracy is indispensable in order to
meet the more demanding, Kyoto relevant second main objective (Cihlar and Denning, 2002, Cihlar et
al., 2003).
For CH4 the situation is somewhat different, in that the anthropogenic emissions associated with fossil
fuels (gas leaks, incomplete combustion) are much less certain as compared to the case of CO2, and
other anthropogenic emissions from agricultural practices (rice cultivation and cattle grazing) are of
similar magnitude. The natural background fluxes from wetlands are relatively minor (globally only
about 30% of the total fluxes), but this fraction varies substantially in different parts of the globe. A
contribution to the Kyoto objective necessitates at least an accuracy of better than 20% of the total
flux.
For N2O the relatively poor understanding of the sources makes bottom-up estimates highly uncertain,
which, in return, would make already low accuracy top-down source estimates valuable, at least for
the scientific objective (1) above. Based on the required accuracy needed for bottom-up accounting
provided by the Subsidiary Body for Scientific and Technological Advice (SBSTA) of the IPCC
(SBSTA, van Amstel, 2004, preprint) already an accuracy of 100% of the 5 year average net
continental N2O flux would provide useful information.
In summary, based on the considerations outlined above, we thus specify a target requirement
accuracy of the continental flux estimates as 20% of the total averaged flux for CO2, and CH4 and as
100% of the total averaged flux for N2O, on a spatial scale of 106 km2, averaged over 5 years. This
target requirement reflects a typical, global average continental situation on the 106 km2 spatial scale.
A more specific target accuracy requirement for different parts of the world, also taking regionally
different existing source-sink knowledge into account could be specified, but was not deemed
appropriate at this time for this study.
1.3.3
Observational Methods
There exist essentially two approaches for the regional quantification of greenhouse gas sources and
sinks. In the “bottom-up” approach one may extrapolate point measurements or statistical information
using GIS information and/or remotely sensed properties of the surface (e.g. vegetation index). This
up-scaling inevitably necessitates a more or less complex model of the surface processes controlling
the surface exchange fluxes and emissions. This is the approach that currently is used to quantify
national greenhouse gas budgets that have to be reported for the assessment of the Kyoto
commitments. However, with the current state-of-the-art it is practically impossible to verify these
national budgets by an independent method.
Conversely, the “top-down” approach uses the relatively fast atmospheric mixing as a natural
integrator: the complex surface source-sink flux patterns are mirrored in atmospheric concentration
variations of CO2, CH4 and N2O, which are rapidly smoothed when moving away from local features
and thus in general reflect only the larger scale source-sink distribution. Hence atmospheric
greenhouse gas concentration measurements may be used to infer the surface sources and sinks,
thereby using an “inverted” model of atmospheric transport and mixing. This approach has been
successfully applied to infer continental and ocean basin scale CO2 sources and sinks from
atmospheric in situ concentration measurements at a global network of atmospheric background
stations (e.g. Gurney et al., 2002, Rödenbeck et al., 2003), and it has been used to constrain the
emission patterns of CH4 (Hein et al., 1997, Houweling et al. 1999). The N2O source distribution has
not yet been addressed with this method, primarily because the network of N2O observations is sparser
than for the other gases and the measurements themselves are of lesser quality.
19
At present the top-down approach in general is severely limited by the small number of observing
stations. The present network of measurement sites encompasses about 100 sites, most of which are
located at the surface on remote mountains, remote coasts and islands. At most sites, atmospheric
mixing ratio measurements are performed with a frequency of weekly to monthly. In addition, there
exist about 10 sites where regular vertical atmospheric flask sampling for the greenhouse gases is
performed by means of aircraft. Current efforts focus on the establishment of continuous measuring
systems, mostly on tall towers (200-400m) in Europe and North America. However, many interior
regions of the continents, particularly in the Tropics, are not sampled at all. It is for this reason, that a
space borne sensing technique, if accurate enough, would tremendously improve the top-down
estimates.
Forward model simulations
The atmospheric transport model TM3 (Heimann and Körner, 2003) has been used to calculate the
temporal and spatial variability of CO2, CH4 and N2O in the atmosphere based on realistic scenarios of
surface sources and sinks. Model simulations were performed on a 2x2 degree horizontal resolution
and 28 vertical layers from the surface to the top of the atmosphere. Meteorological fields were
derived from The National Centre of Environmental Research (NCEP) reanalysis (Kalnay, 1996). 3D
concentration fields were stored in 2 hour intervals for Jan.-Dec. 2003. This archive of model output
allowed to simulate the signal that would be observed by any prototype satellite instrument taking into
accounts its orbit, viewing mode, spatial and temporal coverage, precision, etc. In this study,
measurement precisions have been quantified that are needed to observe characteristic features of the
archived concentrations. Following procedure have been applied: 1) Total columns are derived using
the instrument specific weighting function (see Figure 1.3-1). 2) The columns are averaged to the
requirement-specified temporal and spatial scale. 3) Increasing levels of random noise are added until
predefined features of interest disappear in the noise (see Figure 1.3-3).
Inverse calculations
Atmospheric inverse modelling has been used to calculate the anticipated reduction in surface flux
uncertainty resulting from the use of DIAL measurements. The satellite is assumed to fly in a polar
orbit, similar to that of the upcoming OCO mission. Two different DIAL viewing modes are
distinguished: 1) hard target return 2) atmospheric backscattering. A globally uniform vertical
weighting function has been applied that (mode 1) peaks at the surface and gradually decreases with
altitude or (mode 2) peaks at 500m altitude, and has a triangular shape with a 1 km FWHM (see Figure
1.3-1). The weighting function for hard target DIAL has been derived from the on-line DIAL retrieval
of CO2 at 2.051 micron. This wavelength has been carefully selected to optimize the weighting of the
planetary boundary. It has been assumed that a similar surface sensitivity can be achieved for CH4 and
N2O. The general set-up and boundary conditions used for the DIAL-based inversions of sources and
sinks was equivalent to the OCO inversion reported by Houweling et al. (2004), except for the applied
weighting function. In summary, prior source strengths of CO2, CH4 and N2O are prescribed, in line
with currently accepted estimates. Monthly flux regions are specified at 8x10 degree horizontal
resolution (~106 km2). Measurements at the resolution of the required spatial and temporal scale are
aggregated to weekly ensembles at 8x10 degree.
To determine the relation between inversion performance and precision, a series of inversions have
been carried out, spanning a range of measurement precisions. The inversion performance (Pinv) is
defined as the average reduction in flux uncertainty over the continents calculated as
Pinv = σ apost,8x10,year abs( f apri,8x10,year ) ,
20
with σapost,8x10,year the uncertainty of ƒapri,8x10,year, the annual prior flux at 8x10 degree. A spline fit
through the inverse modelling results is used to interpolate between the selected precisions (see Figure
1.3-1). To extrapolate the inversion performance from 1 year to the time scale of the flux requirement
the inversion performance is assumed to improve with the square root of the number of years. To
account for loss of information by aggregating the measurements to the 8x10 degree resolution of the
inversion, forward model calculations have been carried out at different resolutions. The decrease in
variability going from the required resolution to 8x10 degree has been used as a measure of
information loss. The inverse modelling derived precision requirements have been relaxed by this
factor, which turned out to be as large as ~2.
Figure 1.3-1: Vertical weighting functions as used in the inverse calculations. Dark blue, green and
light blue; Hard Target DIAL at respectively 1579 nm, 1572 nm and 2051 nm; Red, atmospheric
backscattering DIAL. For reference a typical weighting function of passive TIR is given in black
(Hard Target weighting functions generally resemble those of passive NIR).
1.3.4
Observational Requirements for a DIAL Instrument
Random error
Tables 1.3-1 and 1.3-2 list the calculated observational requirements for hard target and atmospheric
backscatter DIAL. The requirements specify the instrument performance that is needed to fulfil the
target requirement on the accuracy of the fluxes. Figure 1.3-2 presents an example of how the target
requirements were derived. The technical threshold requirements represent a lower bound to
instrument performance beyond which no significant scientific insights are expected be gained
anymore. To assess this threshold, the random error that is added to forward model generated
concentration fields has been increased to a level at which the detection of continental scale
concentration gradients remains just feasible. Note that, at this noise level, global scale features, like
the north to south gradients and seasonal cycles, might still be readily detectable (see Figure 1.3-3).
Those, however, are not expected to lead to any new insights as they are already monitored at high
accuracy by the existing in situ surface measurement networks. It has been verified that the threshold
21
precisions, as derived from the procedure outlined above, correspond to the point where the standard
deviation of measurement noise equals that of the concentration variability over the continents.
Table 1.3-1: Observational requirements for the random error for column content measurements;
taken from [TN 120]
CO2 (ppmv)#
Target
Threshold
1
3.5
week
month
Precision
Temporal
res.
Horizon.
200km
res.
Vertical
column
res.
#: dry air mixing ratio
Target
10
week
CH4 (ppb)
Threshold
35
month
Target
0.1
week
N2O (ppb)
Threshold
0.5
month
500km
200km
500km
200km
500km
column
column
column
column
column
Table 1.3-2: Observational requirements the random error for range-resolved DIAL measurements
taken from [TN 120]
CO2 (ppmv)#
Target
Threshold
5
10
week
month
Precision
Temporal
res.
Horizon.
200km
res.
Vertical
bnd.layer
res.
CH4 (ppb)
Target
Threshold
100
100
week
month
N2O (ppb)
Target
Threshold
0.5
1.0
week
month
500km
200km
500km
200km
500km
bnd.layer
bnd.layer
bnd.layer
bnd.layer
bnd.layer
The required time and space scales are selected with the intention to resolve synoptic scale variability.
Synoptic scale variability is considered an important mode of atmospheric variability that, in our view,
should be resolved. Smaller scale variability, for example the diurnal cycle, may also be important but
this is beyond the scope of the coarse scale inverse analysis performed here. Generally, one should
realize that different combinations of measurement precision and scale might allow fulfilling the flux
requirements (as part of a trade off). The results of our inversions indicate that the target flux
requirements can be fulfilled without resolving the diurnal cycle. The target spatial resolution of 2x2
degree was mainly guided by constraints on the coverage of a polar orbiting satellite. Again, the
results indicate that flux requirements can be fulfilled under this condition and therefore they do not
dismiss a polar orbit. The threshold scales do not address the synoptic variability, but will be needed to
resolve seasonal changes.
22
Figure 1.3-2: Target requirements for CO2; taken from [TN 120]
Figure 1.3-3: Model simulated column averaged CO2 as would be observed by hard target DIAL for
July, without noise (left panel) and with threshold level noise right panel);taken from [TN 120]
A sensitivity study has been carried out to investigate the benefit of measuring in the wing of the
absorption line instead of the centre. Theoretically, measurements in the wing are more sensitive to
pressure broadening of absorption lines and will therefore improve the sensitivity to the planetary
boundary layer. Figure 1.3-4 shows the increase in instrument performance that is gained by
measuring in the line wing instead of the line centre. These results show that pressure broadening,
which a DIAL system could take advantage of relax the required target precision by about a factor 2
(see Table 1.3-3).
23
Figure 1.3-4: Sensitivity of the instrument performance to the exploitation pressure broadening: taken
from [TN 120]
Table 1.3-3: Column measurements with hard target DIAL of CO2 at different wavelengths;
taken from [TN 120]
Precision
Temporal res.
Horizon. res.
Vertical res.
HT at 1.6 μm (ppmv)#
Target
Threshold
0.5
2.5
week
month
200km
500km
bnd.layer
bnd.layer
HT at 2.1 μm (ppmv)
Target
Threshold
1
3.5
week
month
200km
500km
bnd.layer
bnd.layer
Another sensitivity study has been carried out to quantify the importance of accurate measurements
over the oceans for estimating continental fluxes. Figure 1.3-5 shows the effect of ten-fold less precise
measurements over the oceans. It indicates that at continental measurement precisions near the target
precision the instrument performance is only slightly affected by a change in precision over the
oceans. At threshold precision, however, accurate ocean measurements become more important. Note
that the factor 10 may be a too pessimistic assumption for the DIAL performance over sea. It does not
take into account a possible contribution of specular reflection to the measured signal, for lack of any
experimental data.
24
Figure 1.3-5: Instrument performances with and without a ten-fold relaxation of the precision over the
oceans (for Hard Target DIAL at 2051 nm); taken from [TN 120]
Systematic errors
The great advantage of satellite instruments over surface measurements is evidently the large the
number samples that could compensate for a loss of accuracy. This will only be true, however,
provided that the level of systematic error remains sufficiently low. The impact of systematic errors on
the estimated fluxes is largely determined by its characteristic pattern in space and time. This is
explained by the fact that the information on sources and sinks is not contained in the absolute level of
the concentrations, but rather in spatial and temporal gradients. For example, a constant systematic
offset will not alter the inverse calculated fluxes, while a bias that increases with time will. One could
think of several sources of systematic error acting simultaneously, each with its own characteristic
error pattern. If these errors become significant, the measurements might become very difficult to
interpret.
To quantify the potential effect of systematic errors, we have focused on – what are expected to be the dominant modes of systematic error. These modes are listed in Table 1.3-4. The second column of
this table shows the level of systematic error that will contribute the same amount of uncertainty to the
estimated fluxes as the random component of the error at target precision, calculated as follows
Pinv = Δf apost,8x10,year abs( f apri,8x10,year ),
with Δfapost,8x10,year the difference in the calculated a posteriori flux with and without adjustments of the
measurements to account for systematic errors. As can be seen in the table the tolerable level of
systematic error varies substantially among the different modes.
25
Table 1.3-4: Summary of systematic error requirements; taken from [TN 120]
CO2 (ppmv)#
Aerosols
0.2 (per unit AOT)
Linear trend
0.6 (per year)
Seasonal cycle
0.1 (seasonal amplitude)
Orbital cycle
0.7 (North-South difference)
Quasi random
1
Systematic errors by aerosols arise from a perturbation of the optical path after reflection on aerosol
particles. As a first and very rough attempt to quantify the potential impact of this error, all
measurements that enter the inversion have been perturbed by the local Aerosol Optical Thickness
(AOT). For this purpose, a model simulated seasonally varying climatological aerosol field has been
used that includes dust, soot, sulfate and sea salt (F.-M. Breon, personal communication).
It has been assumed that aerosol induced errors scale linearly with AOT. Afterwards the difference
between inversion-derived posterior fluxes with and without aerosol error is quantified. Theoretical
considerations point at an expected retrieval error of about 0.1 ppmv per Aerosol Optical Thickness
(AOT) of 0.1 (F.-M. Breon, personal communication), which exceeds our requirement (see Table 1.34). Moreover a recent analysis of Sciamachy observed CO2 indicates that aerosol induced errors may
be substantially larger (Houweling et al, 2005). It means that satellite systems that measure CO2 in the
NIR will not comply with our target flux requirement, unless the errors related to aerosols can be
corrected.
Figure 1.3-6: Model estimated influence of aerosols on inversion calculated CO2 fluxes (left panel).
Assumed annual mean aerosol optical thickness (right panel), the difference between the calculated
annual CO2 fluxes with and without aerosols; taken from [TN 120].
As can be seen in Table 1.3-4, errors that vary coherently over larger scales (linear trend, orbital cycle)
tend to have less stringent error requirements than those varying systematically on smaller scales (e.g.
aerosols). Generally the results of the first class show less ‘dipoles’ as the left panel of Figure 1.3-6,
and the ‘burden’ of the error is be shared coherently by many 8x10 degree flux elements. Integrated
over larger regions, however, these errors grow in importance, while aerosol-like errors tend to cancel
out partly.
The discussion is complicated by potential methods that might be used to correct for systematic error.
The residual systematic error that is left after correction has a rather unpredictable pattern. These
errors would to some extent resemble random errors (‘quasi random’ error). Therefore, it is assumed
that these errors, like real random errors, should not exceed the target precision. Similarly, it should be
possible to correct DIAL measurements for aerosols by selecting the ground return only. Therefore,
26
instead of a problem, the influence of aerosols may be considered a potential advantage of the DIAL
method over passive NIR methods.
Finally, it was attempted to estimate systematic errors arising from the use of ECMWF surface
pressure to quantify the air mass factor. In a recent analysis using an ensemble of model simulations
which was performed at ECMWF the 1σ variation in the surface pressure accounts to ~0.7 hPa or an
equivalent error of 0.25 ppmv CO2 ,globally. (Anderson, private communication, ECMWF). Although
this provides some indication of accuracy it is not a measure of systematic error. Further, it is not
guaranteed that the global pattern of uncertainty reflects that of systematic error. Therefore, at this
stage we weren’t able to make a proper assessment of the potential impact of systematic errors in
surface pressure on CO2 flux estimates. If the accuracy of ECMWF surface pressure is not dominated
by systematic errors (which one might expect), then the impact may remain limited. However, this
should be confirmed by further investigations, which we considered to be outside the scope of this
study.
1.3.5
Limitations
The inversion computations suffer from some known limitations. For example, a globally uniform
measurement uncertainty has been assumed, while, for example, for atmospheric backscattering DIAL
this will depend on the amount of backscattering material in the form of aerosols, which may vary
substantially. Secondly, the limited resolution of the inverse calculations, even though much better
than many other algorithms that are currently in use, introduces some amount of uncertainty. We have
tried to account for this limitation using the results of forward simulations at a higher resolution, but
acknowledge that this correction is rather uncertain.
It should be emphasized that the presented model analysis implicitly assumes that transport model
errors remain at insignificant levels. For the current generation of atmospheric transport models,
however, this is certainly not guaranteed. In addition, the influence of model errors on the estimated
fluxes will vary with the selected satellite measurement technique. More research will be needed in
this field, but it is known that the parameterization of vertical mixing is associated with relatively large
uncertainties. Therefore, transport models errors might become significant for measurements that rely
on an accurate simulation of the exchange between the boundary layer and the free troposphere and
the time scale of vertical mixing in the stratosphere. For this reason, night time measurements that are
sensitive to the planetary boundary layer will be more difficult to interpret than daytime
measurements. For the dusk-dawn orbit that was proposed for active instrumentation it means that the
dusk measurements will be easier than dawn measurements. It is not obvious, however, that another
choice of 12 hour separated measurements should clearly be favourable over dusk-dawn.
Finally, it is acknowledged that the present analysis of systematic errors is a first attempt in this
direction and admittedly rather crude. Clearly more research is needed in this field. An obvious
shortcoming is the statistical treatment of the measurement error, which has not been adjusted to the
actual biases that are introduced to the measurements. This may partly explain the oscillating
behaviour of the flux estimates (the ‘dipoles’) that are shown in Figure 1.3-6. A more coherent
adjustment of the fluxes to the perturbed measurements would likely relax the systematic
requirements. Besides these technical aspects there remains the limitation that the systematic errors
that may result from the use of ECMWF surface pressures have yet been addressed adequately.
27
1.3.6
Harmonization of DLR and IPSL CO2 requirements
The observational requirements for CO2 derived in this study have been compared to a parallel study
on a space-borne CO2 Lidar performed by IPSL. From detailed analysis of the different sets of
requirements it was found that they agree fairly well with respect to the random error and the sampling
characteristics when the same definitions are used for observations over land.
However, major differences have been found for the needs over the ocean. In our study it could be
demonstrated by simulations that the required measurement precision can be significantly relaxed over
the ocean by about a factor of 10 if the target species would be sources and sinks over land. In
contrast, the IPSL study claims that a similar precision over the ocean would be required as demanded
for measurements over land. This issue needs further analysis.
There are also some differences found for the approach each team has been used to derive the
requirements for the systematic error. While both teams agreed on a target requirement of 0.1 ppmv
the definition of the threshold requirement is still TBD.
As a result from harmonising both teams agreed to the need on two separate sets of observational
requirements which address CO2 measurements in the 1.6 and 2.05 µm spectral region. The weighting
function at 2.05 µm is more favourable for measurements in the low troposphere where the sources
and sinks reside. This yields to a significant relaxation of the required measurement precision at this
wavelength compared to measurements at 1.6 µm. It turns out that the difference is about a factor of 3
for the IPSL-study and a factor of 2 in case of the DLR-study which both would have a strong impact
on the size of the instrument.
To limit the representation errors and to reduce biases a 50 km along track resolution was found to be
more appropriate for the problem than the 200 km integration length as suggested earlier by the DLR
study. In summary, the following tables 1.3-5 and 1.3-6 list the observational requirements for an
active optical instrument operating at either 1.6 or 2.05 µm as agreed by both study teams. Note that
these tables assume a 50% reduced coverage due to the occurrence of clouds.
28
Table 1.3-5: Summary of observational requirements for a hard target Lidar at 2 µm; taken from
[TN 120]
CO2
Target
Threshold
Needed atmospheric variable
Dry air mixing ratio
Accuracy, random comp.
1.5 ppmv
5 ppmv
Accuracy, systematic comp.
~0.1 ppmv (*1)
*1
Sampling
50 km along track
50 km along track
Spatial resolution
<1 km for individual shots
Local time
No strong scientific requirement
Satellite orbit
Polar – geosynchronous
Geophysical data product
Column averaged mixing ratio
Lidar pointing
Nadir
Nadir
Measurement sensitive to
Total column CO2 content [g.cm-2] Total col. CO2 content [g.cm-2]
Additional geophysical data product needed (*2)
Surface pressure accuracy
< 1 hPa
< 1 hPa
Water vapour accuracy
< 1 g/cm-2
< 1 g/cm-2
*1 preliminary result, needs further analysis
*2 These geophysical data products are not necessary per-se, but appear most useful to convert the
CO2 column (in number of molecules per cm2), to a column concentration (in g/g or mol/mol).
29
Table 1.3-6: Summary of observational requirements for a hard target Lidar at 1.6 µm;
taken from [TN120]
CO2
Target
Threshold
Needed atmospheric variable
Accuracy, random comp.
0.75 ppmv
2.5 ppmv
Accuracy, systematic comp.
~0.1 ppmv (*1)
*1
Sampling
50 km along track
50 km along track
Spatial resolution
Local time
Satellite orbit
Geophysical data product
Lidar pointing
Nadir
Nadir
Measurement sensitive to
Total column CO2 content [g.cm-2] Total col. CO2 content [g.cm-2]
Additional geophysical data product needed (*2)
Surface pressure accuracy
< 1 hPa
< 1 hPa
Water vapour accuracy
< 1 g/cm-2
< 1 g/cm-2
*1 preliminary result, needs further analysis
*2 these geophysical data products are not necessary per-se, but appear most useful to convert the
CO2 column (in number of molecules per cm2), to a column concentration (in g/g or mol/mol).
30
2 Instrument Performance
Space-borne remote sensing has a relative long history. Particular passive instruments have been
extensively used to explore the state of state of the Earth atmosphere on a global basis. They are
preferable operated in limb or nadir viewing mode from LEO satellites or at reduced performance also
from geostationary platforms. Scanning capabilities can easily be implemented which helps to increase
global coverage. A further advantage is related to the ability of measuring multiple species
simultaneously. An intrinsic disadvantage associated to most of the passive sounding systems is
related to their strong dependency on complex radiative transfer models. To obtain range-resolved
information first guess (model) profiles are required. Moreover the difficult error structure of passive
instruments does not allow to separate the random error from systematic error sources.
In contrast, active sensors comprise a direct measurement method which offers the possibility to
obtain range-resolved information without need for any radiometric calibration. By selection of
appropriate wavelengths low sensitivity to other parameters ( e.g. surface emissivity, temperature
profiles, concentration of other gases, aerosols ) can be easily achieved. Active sensors can be
operated above clouds and in broken cloud fields by keeping only the data from clear spots. The
simple retrieval technique allows for discrimination between random and systematic errors. The main
disadvantage of active sensors are associated to the technical realisation of an cost-effective instrument
for space-borne applications. This includes also the life time of a mission typically limited by the life
time of the radiation source. Because of the 1/r2 dependency of the backscattered signal only LEO
satellite platforms are of practical interest. In addition a dawn/dusk orbit could help to avoid
interference from solar background radiation. The latter can have a strong impact on the measurement
performance. Other limitations are related to the eye safety requirement which need particular
attention for wavelengths in the visible spectral range.
2.1
Passive Sensors
In this study current and planned passive instruments have been exhaustively analysed by literature
search over a broad frequency range spanning the ultraviolet up to the microwave spectral region.
There performance has been critical assessed in the light of the observational requirements introduced
in the previous paragraph.
2.1.1
Temperature Profiling
The sensors that measure temperature profiles in the troposphere and stratosphere are nadir sounders
operating in the microwave and thermal infrared spectral region. Temperature sounding from space
has a relatively long history for NWP and other applications (e.g., the microwave instrument series
MSU/AMSU on NOAA satellites and SSM/T / SSMIS series on DMSP satellites). SSM/T has a
precision of 0.3 K, an accuracy of 2-3 K, a vertical resolution of 8-10 km, and a horizontal resolution
of 175 km at direct nadir (~260 km at extreme swath) [Grody, 1993; Goodrum, 2000; Redmann, 1992;
Deblonde and English, 2002]. AMSU-A/NOAA has a better performance, namely a precision better
than 0.27 K, an accuracy of 1.5 K, a vertical resolution of 3 km in the height range 0-48 km and a
horizontal resolution of 50 km at direct nadir (85x172 km2 at extreme swath) [Grody, 1993; Diak et
al., 1992; Kidder et al., 2000; Goodrum, 2000; Goldberg, 2002; Goldberg, 2002b; Goldberg, 1999;
Deblonde and English, 2002]. An advantage of the microwave sounders is that they are quite
insensitive to clouds and measure during day and night.
31
The new generation of high resolution thermal infrared (TIR) nadir sounders such as AIRS/AQUA and
IASI/METOP have a vertical resolution of 1 km and a (required) precision of 1 K, and a ground pixel
size of 14 x 14 km2 (AIRS) or 12 x 12 km2 (IASI) [Aumann et al., 2003; Susskind et al., 2003; Fetzer
et al., 2003; Aires et al., 2002]. The TIR measurements are also performed during day and night but
are affected by clouds which reduces the number of useful measurements. 1 km / 1 K is also the
performance of the sensor combination AIRS/AMSU-A onboard AQUA [Grody, 1993; Diak et al.,
1992; Kidder et al., 2000; Goodrum, 2000; Goldberg, 2002; Goldberg, 2002b; Goldberg, 1999;
Deblonde and English, 2002]. The predicted performance of the nadir/limb sounder TES/AURA is 1-2
K with a vertical resolution of 2-6 km in the height range 0-34 km (horizontal resolution 5 x 8 km2, no
scan) [TES web page].
Main conclusion: There are several satellite sensors with global coverage with a performance or
predicted performance close to or better than the required performance (e.g., AIRS/AMSU-A/HSB on
AQUA and IASI on METOP both with 1 K accuracy, 1 km vertical resolution, < 50 km horizontal
resolution). The target performance of 0.5 K accuracy and precision, 0.5 km vertical resolution, and 15
km horizontal resolution as indicated in Table 1.1-1 is, however, not achieved.
2.1.2
Pressure Soundings
Surface pressure:
In principle, the (dry air) surface pressure can be determined by measuring the total column of a well
mixed gas, for example O2 or CO2, from which the (dry) air column can be computed and hence
surface pressure. O’Brian, 2002, has reviewed recent and past attempts to measure surface pressure
(or, basically equivalent, the geopotential height of pressure levels in the troposphere, e.g., the 500 hPa
level) from space. He summarizes his findings as follows: “While satellites now provide routine
measurements of many important meteorological and geophysical parameters, measurements of
surface pressure have remained elusive, despite its importance for … NWP. The difficulty is
principally one of accuracy …”. The review of O’Brian, 2002, covers various techniques, including
microwave, thermal infrared, and visible/near-infrared passive sensors as well as active sensors (GPS
occultations, DIAL, millimetre radar).
The main part of his study is devoted to surface pressure retrieval using oxygen A band measurements
from a geostationary orbit. As discussed in, e.g., O’Brian, 2002, the O2 column can be determined, for
example, from nadir radiance measurements in spectral regions corresponding to its visible/near
infrared absorption bands. The band that has been investigated in most studies is the O2 A band
located at 760 nm. The problem in retrieving the O2 column is not a principal one but one of accuracy
due to the demanding accuracy requirements for NWP applications. A 1 hPa surface pressure change
corresponds to a 0.1% air column change which corresponds to a 0.1% O2 column change. This
means that the O2 column (or the column of any other well mixed gas) needs to be determined with an
accuracy of better than 0.1%. This is (at least) one order of magnitude better than what has been
achieved for other gases which have been extensively studied over many years (e.g., ozone). The main
problem in accurately determining the O2 column using, for example, the O2 A band, are uncertainties
resulting from highly variable atmospheric scatterers (aerosols and clouds) which influence the path
length of solar photons in the atmosphere. In addition, there are many other radiative transfer effects
that affect accuracy such as the dependence of the retrieved column on surface reflectivity. There are
also demanding requirements on the instrument: high stability, high signal to noise ratio, high spectral
resolution, high spatial resolution, to mention the most important ones.
32
3-D pressure field:
There are a number of limb viewing instruments that measure or aim at measuring pressure as a
function of altitude in the stratosphere and above. For all these systems the horizontal resolution (in
the line-of-sight direction) is ~200-250 km basically for geometrical reasons. SAGE III, for example,
aims at measuring pressure with a precision and accuracy of 2% (vertical resolution ~1 km).
MIPAS/ENVISAT determines pressure levels from its CO2 measurements with a precision of ~2%
(vertical resolution ~3 km).
Pressure as a function of altitude is a planned operational data product from SCIAMACHY (vertical
resolution ~3 km). The operational algorithm is, however, currently only in its initial stage. In
addition, the (platform) pointing knowledge uncertainties are quite large (currently < 3 km, to be
improved to ~1 km) which affects both MIPAS and SCIAMACHY. EOS-MLS has a Level 3 data
product “Daily map of geopotential height”. The expected precision (for 3 km vertical resolution) is <
30 m in the height range 5-30 km (~ 500-10 hPa) [EOS MLS Science Objectives]. The EOS-MLS
geopotential height retrieval is based on the assumption of hydrostatic equilibrium (i.e., pressure and
temperature are not retrieved independently). For a standard profile a 100 m increase in altitude
corresponds to a 1.2% pressure decrease, i.e., 30 m correspond to a 0.36% pressure change. This
means that the expected EOS-MLS retrieval precision is 1.8 hPa at 500 hPa (required 0.2 hPa), 0.36
hPa at 100 hPa hPa (required 0.1/0.2 hPa), and 0.036 hPa at 10 hPa hPa (required 0.1 hPa). If EOSMLS reaches its expected performance it might be able to provide data in the middle stratosphere (30
km, 10 hPa) close to the required values.
Main conclusion: In summary, no passive satellite systems exist for which it has been demonstrated
that the target or threshold requirements given in Table 1.1-2 can be met. Furthermore, we are not
aware of any such systems planned for the future (see also Eyre et al., 2002, and O’Brian, 2002).
2.1.3
Ozone Profiling
Ozone vertical profiles are retrieved from nadir looking instruments measuring in the UV/visible such
as SBUV/2 and GOME. The vertical resolution is about 6 km in the middle stratosphere (20-25 km)
and worse (about 10-12 km resolution) above and below. The precision and accuracy of the
stratospheric profiles is in the range 5-15%. Tropospheric information from SBUV and GOME is,
however, limited. According to Bhartia et al., 1996, SBUV profiles are strongly influenced by a-priori
assumptions outside the range 1-20 hPa (~26-50 km). For the GOME instrument more information is
available as (continuous) spectra are recorded [Munro et al., 1998; Hoogen et al., 1999]. Nevertheless,
the errors on the retrieved tropospheric ozone columns are quite large. According to Siddans et al.,
1997, agreement with ozone sonde measurements is within 30-40%.
Similar results can be expected from their successor instruments SCIAMACHY (nadir mode),
OMI/AURA and GOME-2/METOP but with faster global coverage (one day for OMI and GOME-2)
and smaller ground pixel size (e.g., 13 x 24 km2 for OMI).
Maps of tropospheric ozone have also been generated from IMD/ADEOS measurements [Clerbaux et
al., 2003]. These data have been compared with ozone sondes and model results. A high correlation
has been found but also biases (mean bias: ~15 DU) which might result from the decreasing sensitivity
of IMG in the boundary layer.
Tropospheric ozone has also been determined using a combination of sensors (e.g., TOMS / SAGE or
TOMS / SBUV). Fishman et al., 2003, used a method, which essentially subtracts the stratospheric
33
column measured by SBUV from the total column as measured by TOMS. Fishman et al., 2003,
report on agreement with ozone sonde measurements within 13-20%. De Laat and Aben, 2003,
however, point out that the method used by Fishman et al., 2003, is not unproblematic. De Laat and
Aben, 2003, report that they can produce similar maps of tropospheric ozone using (mainly)
tropopause height information.
Tropospheric information can also be derived from SCIAMACHY due its quasi simultaneous limb
and nadir observation of the same air mass. The theoretical retrieval precision has been estimated to
~10% [Bovensmann et al., 1999]. At present, however, no detailed information based on
comprehensive theoretical analyisis or validation of real data is available.
In summary, retrieval of tropospheric ozone information from space is still in its initial stages and
uncertainties are large. The best that has been obtained so far is one piece of information for the
troposphere (i.e.,no vertical resolution within the troposphere).
Many limb viewing instruments exist that measure vertical profiles of ozone in the stratosphere and
mesosphere by solar, lunar or stellar occultation (e.g., SAGE, POAM, SCIAMACHY and GOMOS),
measurements of scattered limb radiance (e.g., SCIAMACHY) or limb thermal emission (e.g., the TIR
measurements of MIPAS/ENVISAT or the microwave measurements of MLS/UARS). Solar
occultation measurements may have high precision and vertical resolution (SAGE III: ~6% precision,
~1 km vertical resolution) but cannot provide global coverage.
Stellar occultation: GOMOS/ENVISAT has a field of view of ~1.7 km in the vertical direction but the
resolution of the operational products has a resolution of ~3 km due to regularization. Preliminary
validation of selected GOMOS ozone profiles showed agreement with independent data within 5-15%
in the altitude range ~18-50 km [ACVE-2/GOMOS, 2004].
Limb microwave emission sounding: MLS/UARS ozone profiles cover the pressure range from 100
hPa to 0.22 hPa with a resolution of 4-6 km. Precision is best in the range 2-20 hPa (~4%; accuracy
~6%). At 100 hPa the accuracy is ~15% and the precision > 50% [Livesey et al., 2003].
Limb TIR emission sounding: MIPAS/ENVISAT has a vertical resolution of ~3 km. Preliminary
validation of the MIPAS ozone operational data product indicates that there are no obvious biases and
that the precision is ~10-15% in the altitude region ~20-55 km [ACVE-2/MIPAS, 2004].
Using SCIAMACHY limb observations of scattered light stratospheric (and mesospheric) ozone
profiles have been retrieved with a vertical resolution of 3 km and an accuracy of ~10% [von Savigny
et al., 2004]. The lowest altitude that can be observed is determined by clouds.
TES/AURA (launch mid 2004): TES will operate in nadir and limb mode. For the nadir measurements
Clough et al., 1995, estimate the ozone profile retrieval precision to approximately 5% (1 sigma) for a
vertical resolution of 5 km in the middle and upper troposphere (corresponding to 2-3 independent
pieces of information, see also Luo et al., 2002). Whether boundary layer ozone can be retrieved
depends on the (thermal) contrast between the boundary layer and the surface. The vertical resolution
in limb mode is about 2-6 km.
OMI/AURA (launch mid 2004) and GOME-2/METOP (launch 2005): see GOME/ERS-2 discussion
given above.
MLS/AURA (launch mid 2004): Similar as MLS/UARS (see above).
34
IASI/METOP (launch 2005): The required ozone total column precision is 5-10% and for ozone in the
troposphere 10-20% [Clerbaux et al., 1998; Hadji-Lazaro et al., 2003].
Main conclusion: No passive satellite systems have been identified for which it has been
demonstrated that the target requirements given in Table 1.2-1 can be met (especially 1 km vertical
resolution with global coverage and a horizontal resolution of 10 km). No passive satellite systems
have been identified for which it has been demonstrated that the threshold requirements given in Table
1.2-1 can be met (troposphere: especially 3 km vertical resolution with global coverage and a
horizontal resolution of 20 km, UTLS: especially 2 km vertical resolution with global coverage and a
horizontal resolution of 50 km, stratosphere: especially 2 km vertical resolution with global coverage
and a horizontal resolution of 100 km).
2.1.4
Greenhouse gases
Carbon dioxide (CO2)
Chedin et al., 2003a, analysed TOVS/NOAA-10 data to retrieve atmospheric CO2 concentrations.
They report that they have achieved (“rough estimate”) a precision of 3.6 ppmv (~1%) for midtropospheric CO2 concentrations in the tropics. It is, however, unclear whether the method used by
Chedin et al., 2003a, can be extended to extra tropical regions as the method requires low variability of
atmospheric temperature (the measurements are very sensitive to temperature and significantly less
sensitive to CO2 concentration changes).
Simulations performed by Chedin et al., 2003a, indicate that ~2 ppmv (~0.5%) may be achievable with
the new generation of high resolution thermal infrared (TIR) nadir sounders such as AIRS/AQUA.
This is similar to what has been found in other studies for AIRS and IASI/METOP, e.g., 1-2 ppmv
reported by Engelen and Stephens, 2004. These TIR nadir measurements have their maximum
sensitivity in the middle troposphere. They are insensitive to the lower troposphere and contain only
little information about the stratosphere (see, e.g., Chedin et al., 2003b; Engelen et al., 2001). CO2
total columns from AIRS are not an operational Level 2 core product of AIRS but a scientific data
product under development by members of the AIRS science team (we are not aware of any published
results using real AIRS data).
The near-infrared (NIR) nadir measurements of SCIAMACHY are sensitive to CO2 concentration
changes at all altitudes including the boundary layer [Buchwitz and Burrows, 2004; Buchwitz et al.,
2004c]. So far only column measurements have been investigated. Preliminary analysis of
SCIAMACHY data indicates that the precision of the column measurements is in the range 1-4% for
single cloud free measurements over land [Buchwitz et al., 2004c; Buchwitz et al., 2004d]. Assuming
that the precision improves with the square root of the number of measurements added the following
precisions can be achieved (over land): 0.2-0.7 % to be compared with the target requirement of 0.2 %
(threshold requirement of 1%). More analysis is needed to confirm these estimates. In addition, biases
need to be considered.
TES/AURA (launch: mid 2004): CO2 is not a standard operational TES data product. We are not
aware of any publications where (scientific) CO2 retrieval from TES has been investigated and
accuracy/precision values are given.
Future systems: IASI/METOP (launch 2005): The performance of the high resolution thermal infrared
nadir sounder IASI is expected to be similar as the estimated performance of AIRS/AQUA (see
above), i.e., ~1-2 ppmv [Engelen and Stephens, 2004]. Chedin et al., 2003b, also have conducted a
35
study dedicated to IASI concluding that changes at the level of 1% or less for 500 x 500 km2 and 2
weeks averages are expected to be possible for the total column carbon dioxide amount. CO2 is not a
standard operational IASI data product.
NASA/JPLs Orbiting Carbon Observatory (OCO) satellite missions [Crisp et al., 2004] due for launch
in 2007 aims at achieving a precision of 1 ppmv (0.3%) in 16 day intervals with a spatial resolution
corresponding to regional scales (4° x 5°). This is similar as the values given above for AIRS and IASI
but OCO’s near infrared nadir measurements are sensitive also to the boundary layer where the signal
variation due to CO2 sources and sinks is largest.
The Japanese space agency also plans to build a satellite for CO2 measurements accurate enough for
source/sink quantification, namely GOSAT which is due for launch in 2007 [Ogawa et al., 2004;
Shimoda, 2003]. GOSAT shall make high spectral resolution Fourier transform nadir measurements in
the NIR and TIR spectral regions. These measurements shall allow discrimination between boundary
layer and free tropospheric CO2 concentrations.
Main conclusion: Currently no passive satellite systems exist for which it has been demonstrated that
the requirements given in Tables 1.3-1 and 1.3-2 can be met. There are, however, satellite systems
which have the potential to meet at least the threshold requirement for the column measurements in the
near future (e.g., SCIAMACHY over land (requires more studies), the TIR nadir instruments AIRS,
TES, and IASI, and the NIR nadir instrument OCO (launch in 2008)).
Methane (CH4)
Methane total columns can also be retrieved from AIRS/AQUA. CH4 total columns from AIRS are
not an operational Level 2 core product of AIRS but a scientific data product under development by
members of the AIRS science team (we are not aware of any published results based on real AIRS
data). The TIR nadir measurements (IMG, AIRS, IASI, etc.) have their maximum sensitivity in the
middle troposphere and are insensitive to the lower troposphere.
Currently, there are two instruments in space which measure reflected sun light in spectral regions
which correspond to the near-infrared (NIR) absorption bands of CH4, namely MOPITT onboard
EOS-Terra and SCIAMACHY on ENVISAT. These NIR nadir measurements are sensitive to CH4
concentration changes at all altitude levels, including the boundary layer (see, e.g., Buchwitz and
Burrows, 2004). Both instruments have a theoretical single pixel CH4 column retrieval precision of
~1% [Deeter et al., SPIE; Buchwitz et al., 2000a]. Due to problems with the near-infrared retrievals
MOPITT has not delivered any CH4 column data products. Preliminary analysis of SCIAMACHY
data [Buchwitz and Burrows, 2004; Buchwitz et al., 2004c; Buchwitz et al., 2004d] indicates that a
precision of 1-6% has been achieved for single cloud free measurements over land using the WFMDOAS Version 0.4 retrieval algorithm. Assuming that the precision improves with the square root of
the number of measurements added the following precisions can be achieved (over land): 0.25-1.5% to
be compared with the target requirement of 0.6%, 0.25-1.5% to be compared with the threshold
requirement of 2%. More analysis is needed to confirm these estimates. In addition, biases need to be
better quantified.
TES/AURA (launch mid 2004): Methane total columns are a standard operational data product from
TES. The predicted precision for methane total columns and vertical profiles (vertical resolution: 2-6
km) to be retrieved from TES is ~3% for (quasi) single measurements (5.3 x 8.5 km2 ground pixel
[TES web page].
36
Future systems: IASI/METOP (launch 2005): Methane total columns are a standard operational data
product from IASI. The required precision for methane total columns to be retrieved from IASI is < 510% for single measurements (12 km pixel size at direct nadir) [Diebel et al., 1996; Clerbaux et al.,
1998; Hadji-Lazaro et al., 2003]. If this will be confirmed using real flight data and assuming that the
precision improves with the square root of the number of measurements averaged only a small number
of measurements have to be added to get a precision better than the required threshold precision of
2%.
Main conclusion: Currently, no passive satellite systems exist for which it has been demonstrated
that the requirements given in Table 1.3-1 and 1.3-2 can be met. There are, however, satellite systems
which have the potential to meet at least the threshold requirement for the column measurements in the
near future (e.g., SCIAMACHY over land (requires more studies); AIRS, TES, and IASI (launch
2005)).
Nitrous oxide (N2O)
Until now only a limited amount of IMG/ADEOS data have been processed to retrieve N2O columns
[Lubrano et al., 2003]. Based on simulated IMG retrievals Lubrano et al., 2003, conclude that a
measurement precision of 1-3% should be possible (single 10 measurements, 8 x 8 km2). The
sensitivity maximum is between 300 – 700 hPa with no information from the boundary layer. Lubrano
et al., 2003, estimate that the single pixel retrieval precision for AIRS/AQUA and IASI/METOP is
similar as for IMG but with more data points available for averaging.
Chedin et al., 2002, present first results concerning the retrieval of N2O atmospheric mixing ratios
from space. Their method relies on the analysis of the difference between NOAA/TOVS observations
(radiances) and simulations using collocated radiosondes data. The collocated radiosondes were
necessary as not all the necessary informations can be derived from TOVS. No precision/accuracy
values are given in Chedin et al., 2002.
The near-infrared (NIR) nadir measurements of SCIAMACHY are sensitive to N2O concentration
changes at all altitude levels, including the boundary layer. Preliminary analysis of SCIAMACHY
data [Buchwitz et al., 2000; Buchwitz and Burrows, 2004; Warneke et al., 2004; Buchwitz et al.,
2004d] indicates that a precision of 10-20% can be achieved for single cloud free measurements over
land. Assuming that the precision improves with the square root of the number of measurements added
the following precisions can be achieved (over land): 2.5-5% to be compared with the target
requirement of 0.03%; 2.5-5% to be compared with the threshold requirement of 0.3%. More analysis
is needed to confirm these estimates. In addition, biases need to be better quantified.
TES/AURA: N2O is not a standard operational TES data product. We are not aware of any
publications where (scientific) N2O retrieval from TES has been investigated and accuracy/precision
values are given.
Future systems: IASI/METOP (launch 2005): N2O is not a standard operational IASI data product.
For scientific retrievals see statement given above from Lubrano et al., 2003.
Main conclusion: Currently, no passive satellite systems exist for which it has been demonstrated that
the requirements given in Tables 1.3-1 and 1.3-2 can be met. For existing systems it is unclear whether
the threshold requirement can be met. Concerning future systems such as IASI more studies are
needed to reliably assess if the requirements can be met.
37
2.1.5
Summary on Passive Systems
The following Table 2.1-1 summarises the findings of this literature research. The indication “no” in
this table indicates that no system has been identified which meets the requirements; the requirement
that is not met is given in brackets (prec. = precision; vert. = vertical; horiz. = horizotal; res. =
resolution):
Table 2.1-1: Summary of performance assessment of passive sensors taken from [TN 210]
Parameter
CO2 column
Threshold requirement met?
Target req. met?
SCIAMACHY, AIRS, TES?, IASI,
no (prec.)
OCO
CO2 range resolved
no (vert.res.)
no (vert.res.)
GOSAT?
SCIAMACHY, AIRS?, TES, IASI
SCIAMACHY, AIRS?, TES, IASI
CH4 column
CH4 range resolved
no (vert.res.)
no (vert.res.)
N2O column
no (prec.)
no (prec.)
N2O range resolved
no (vert.res.)
no (vert.res.)
O3 troposphere
no (vert.res.)
no (vert.res.)
O3 UTLS
no (vert.+horiz.res.)
no (vert.res.)
O3 stratosphere
no (vert.+horiz.res.)
no (vert.res.)
Pressure @ surface
no (prec.)
no (prec.)
Pressure LT
no (prec.)
no (prec.)
Pressure UT
no (prec.)
no (prec.)
Pressure LS
no (prec.)
no (prec.)
Temperature LT
AIRS/AMSU/HSB, IASI
no (prec.)
Temperature UT
AIRS/AMSU/HSB, IASI
no (prec.)
Temperature LS
AIRS/AMSU/HSB, IASI
no (prec.)
Temperature US
AIRS/AMSU/HSB, IASI
no (prec.)
GREEN: Requirement met (demonstrated by comprehensive analysis of real data)
BLUE: Potential exists that requirement can be met (verified at least partially by simulation)
BLACK: Potential exists but detailed analysis not available
Note on p and T: Threshold req. is global NWP req.; Target req. is regional NWP req.
38
2.2
Active optical sensors
In contrast to passive sensors which are routinely operated from space over more than three decades,
deployment of an active optical sensor in space is still an extreme rare event. At present no active
optical instrument exists or is planned for the measurement of the target species CO2, CH4, N2O, O3,
pressure and temperature. Only few attempts have been made for ozone (ORACLE) and CO2
(CARBOSAT, CELSIUS, SPARCLE) but none of these proposals have reached Phase A level yet.
Furthermore, from extended literature study we can conclude that there is still enormous lack of
knowledge on what can be expected from a space-borne active instrument in terms of measurement
precision, accuracy, coverage and spatial resolution. To close this gap detailed performance analyses
have been carried out in this study which includes computer simulations of different active optical
sensors for the various target species.
2.2.1
The Physics of Measurement
Similar to passive systems which operate in the solar backscatter region the concentration of
atmospheric trace gases as well as atmospheric temperature and pressure can in principle be derived
from differential absorption of electromagnetic radiation at specific wavelengths. However, in contrast
to passive instruments where the sun is the primary radiation source active optical instruments carry
their own radiation source. This instruments use powerful laser systems which emit their total
radiation energy in a small atmospheric volume, only. As a result the light path of active optical
instruments is well known thus no complex radiative transfer formalism is required to retrieve the
trace gas concentration from absorption measurements. In addition, laser sources can be operated
single-frequency which helps to discriminate against other trace gases, particular in spectral regions
where the absorption features at coarse resolution strongly overlap.
The optical depth at specific wavelength is proportional to the path integrated product of the trace gas
concentration and the absorption cross section along the measurement path. Knowing the absorption
cross section this allows deriving the column number density of the trace gas (Hinkley 1976, Measures
1984). If vice versa the concentration is known, like for example for O2, it is possible to calculate the
absorption cross section. For specific absorption lines the temperature and pressure dependence of the
cross section can be inverted which enable the retrieval of T and p.
A suitable approach to infer information on the optical depth is differential absorption Lidar where the
Lidar signals originate from scattering or reflection of the laser radiation in the atmosphere or on cloud
and ground surfaces. At least two wavelengths one with high and the other with low absorption are
required. Some information about the vertical distribution of the trace gas can be obtained from
pressure broadening effect of the absorption cross section. Lidar measurements in the centre of line are
more sensitive in the upper troposphere, whereas wavelengths tuned slightly off line-centre would
weight low altitudes.
For profile information the return signals from the atmosphere at each height level are required. In this
case, the trace gas profile can be calculated from applying the simple differential-absorption lidar
(DIAL) equation (1976, Measures 1984). The potential of depth-resolved concentration measurements
using DIAL is most appealing; however, this approach would require a comparatively large instrument
carrying a large telescope and powerful laser system to obtain a low error in the measurement.
Following advantages are associated to active remote sensing:
39
•
•
measurements can be performed at day and night time,
tailor the wavelength spectrum of the light source to the absorption features of the investigated
trace gas,
•
greatly reduce measurement biases by the well-known light path as opposed to passive techniques
where the light path is only tentative known (air mass factor),
•
•
range-resolved measurements are possible by applying pulsed or modulated light source,
simple radiative transfer equation allows direct measurement of the trace gas concentration
without model assumption.
Pulsed Lidar
The optical analogue of short-pulse radio frequency radar is called LIDAR, from the acronym Light
Detection and Ranging. Compared to radio-waves, optical wavelengths are several orders of
magnitude shorter and interact also with scatterers as small as air molecules. Such an instrument in its
simplest form consists of a pulsed laser transmitter, a receiving telescope and a detector. Typically, a
Q-switched laser is used which produces light pulses in nano-second regime which enables a high
range resolution in the measurement.
CW Lidar
If the laser power is constant over time no range information can be extracted from return signals.
Even if only lidar returns from clouds or ground would dominate the signal it is not possible to derive
the exact length of the measured column without auxiliary data. CW-systems require heterodyne
detection principles. This allows for small systems due to the fact that the effective receiver bandwidth
is very low and the mixing with the local oscillator results in an effective amplification of the incoming
radiation above the dark noise level of the detector. The major performance limiting factor of a
heterodyne system is the speckle effect which limits the SNR of a single measurement to about one.
This means that about 106 statistically independent measurements are necessary to achieve a
radiometric resolution of 0.1% as required for high precision measurements.
Random-Modulated (RM) CW Lidar
Range information like for pulsed systems can also be achieved for the CW system by modulating the
output power of the laser source with a special time sequence. In general the received signal is
generated by a convolution of the atmospheric profile with the time signal of the laser power. For
special quasi-random time sequences it can be shown that they are approximately self-inverse with
respect to the convolution. If one thus modulates the output power of a cw-laser with such a sequence
and afterwards applies a numerical convolution with the same sequence to the measured backscatter
signal, it is possible to extract the atmospheric profile.
A significant disadvantage of a direct detection RM CW Lidar system comes from the fact that the
receiver bandwidth has to be equal to a pulsed system of the same vertical resolution. But the peak
signal levels are orders of magnitude lower and thus the radiometric resolution will not be limited by
the quantum statistics of the backscatter light but by the dark noise of the receiver and the solar
background radiation. This leads to a linear scaling of the statistical error with laser power instead of
the usual square root law. The lower peak power of the RM-CW-laser hence will not be compensated
by the longer (i.e. continuous) measurement interval as compared to a short pulse system of the same
mean power.
40
In contrast in a heterodyne detection RM CW Lidar system the background noise is mainly LO
noise and hence for this type of receivers there is no principle difference between pulsed, CW and
RM-CW systems as long as the radiometric resolution at equal power-aperture product is concerned.
Based on this obvious advantage of the latter technique we focused on the analysis of a heterodyne
detection RM CW Lidar in this study.
Simple two-wavelength DIAL
Differential Absorption Lidar (DIAL) uses atmospheric backscatter from short laser pulses at two
wavelengths called on- and offline to detect trace gases with range resolution. In this approach, the
online wavelength is set to an absorption feature of the trace gas under study whereas the offline
wavelength is close but off any absorption line. As the online return is more attenuated than the
offline, the number density of the trace gas molecule can be measured along the laser beam direction.
This method is free from calibration needs and requires only knowledge about the molecular
absorption cross section with appropriate accuracy. DIAL instruments deliver the range-resolved trace
gas molecule number density profile ngas [m-3].
Integrated Path Differential Absorption IPDA
Integrated Path Differential Absorption IPDA uses backscatter signals from target reflections, only.
The hard target can be a cloud surface or the ground. Similar to the conventional DIAL, the online
target return is more attenuated than the offline, so that the trace gas molecule column content can be
measured as integral value along the laser beam. This method is not calibration-free. It requires the
knowledge on the relative on- and offline laser pulse energies as well as the relative receiver
transmission at both wavelengths. In addition the molecular absorption cross section has to be to be
known with appropriate accuracy. It also requires that the distance to the target be measured with
sufficient accuracy. In addition, care must be taken that target albedo differences between the on- and
offline pulses keep below an appropriate value. IPDA delivers the trace gas molecule column content
Ngas [m-2] between the lidar and the target.
Classes of Active Optical Instruments
With respect to the notation above the measurement performance of three classes of active optical
instruments have been analysed in this study. The first class utilises conventional atmospheric
backscatter DIAL with the following characteristics:
•
Determines altitude-resolved number density measurements by differential-absorption Lidar.
•
Number density achieved by the return signal at two adjacent wavelengths without the need for
any radiometric calibration.
•
Height resolution achieved by use of pulsed light source.
•
Requires reception of many photons scattered back by aerosols or molecules in clear air for a high
SNR.
•
A relative high power aperture product required.
•
Basically immune against accurate knowledge of the state of the probed atmosphere (e.g.
temperature profile, pressure, aerosol loading, etc).
41
The second class denoted as pulsed IPDA is based on Integrated Path Differential Absorption
measurements using pulsed radiation sources. These sensors can be characterised as follows:
•
Determines total column gas concentration by differential absorption measurement of laser light
backscattered from a surface (ground or dense cloud) or the atmosphere at two adjacent
wavelengths.
•
Allows direct measurement of distance to the reflecting surface (ground/cloud).
•
Clear sky conditions provide no height information. Height information can only be achieved from
reflection at two adjacent cloud layers.
•
Weighting functions are very well defined by selection of the on/off-line wavelength. No model
assumption for trace gas retrieval required.
•
Strong echo signal leads to moderate power aperture product.
•
Measurement method independent of sun illumination, no additional error contributions, global
coverage capability.
•
Allows direct suppression of error contributions resulting from scatterers along the propagation
path, e.g. cirrus clouds.
The third class denoted as RM-CW-IPDA uses random modulated continuous wave (CW) lasers and
heterodyne detection. These instruments can be characterised as follows:
•
Determines column gas concentration by differential measurement of laser light backscattered
from a surface (ground or dense cloud) at two wavelengths.
•
Height information obtainable from decoding of random modulated CW transmitter signal
•
Strong echo signal, continuous operation and very low bandwidth result in low power aperture
product.
•
IR wavelengths preferred for heterodyne measurement method.
•
Achievable measurement accuracy limited by speckle constraints and number of independent
looks.
•
Measurement method independent of sun illumination, global coverage capability.
All above instruments can further be distinguished by their mode of detection which uses direct or
heterodyne detection principles.
42
2.2.2
Instrument Performance Analysis
For the investigation of the measurement performance appropriate performance models for the classes
of instruments under consideration have been established and implemented as software modules at the
computer for parametric studies. The performance models established in this study describe both
random and systematic error components for the individual parameter. In general, all these errors
strongly depend on the technical specification of the individual sensor, the selected orbit geometry, the
assumed state of the sounding atmosphere, and the surface reflectivity in case of the IPDA sensor.
Detailed analyses can be found in the Technical Notes TN 220-240.
The simulation tool
In recent years DLR has developed several performance analysis modules for space-borne and
airborne DIAL systems. Its core component is a simulation tool which can be used for parametric
analyses on the base of technical instrument specifications, standardised or real atmospheres and user
requirements. The code originally only included error propagation equations for atmospheric
backscatter differential absorption lidar (DIAL). It has been extensively used and refined in the frame
of the ESA space borne water vapour DIAL study and all subsequent activities dealing with
performance analysis with respect to the WALES mission (ESA ITT AO/1-3654/00/NL/DC, WALES
PHASE A Study). It is currently used for the assessment of airborne water vapour, ozone and carbon
dioxide DIAL performance, as support for campaign preparation and design of new systems in the
DLR lidar group.
The performance model has been implemented in the high-level, graphics-oriented programming
language IDL (Interactive Data Language, Version 5.4, Research Systems, Inc., 2001). A typical
parametric analysis run will first invoke an initialisation routine to set up the default parameter
configuration. After modification of a certain parameter subset, the trace gas number densities and
atmospheric situations are selected, and the lidar and DIAL or IPDA error propagation equations are
invoked to compute the statistical error as a function of altitude. With the various plot routines,
specific intermediate results like the atmospheric extinction coefficients, the power on the detector or
the signal-to-noise ratio can be displayed. As result, vertical profiles of the trace gas measurement
statistical error are generated as function of the varied parameters. This process is rerun for each
parameter to be varied.
For the present study the performance model has been enlarged with the following main features:
1. Implementation of error propagation terms for the assessment of different lidar techniques such as
IPDA and heterodyne detection,
2. Implementation of error propagation terms for the assessment of atmospheric pressure and
temperature measurements using the p and T sensitivity of oxygen lines,
3. Assessment of cross-sensitivities between the different trace gas absorption lines that may induce
systematic errors.
The performance model flow chart for DIAL simulation illustrates this process and gives an overview
of the program sequence.
43
Figure 2.2-1: Schematic view of DLR-performance model for parametric studies of Lidar
Instruments; taken from [TN 240]
Instrument Model:
Orbit altitude,
pulse energy,
repetition rate,
telescope size,
optical efficiency,
filter and detector
parameters.
Atmospheric trace gas
profiles from climatological data
or real measurements;
absorption cross sections
from HITRAN database
or lab measurements;
pressure and temperature.
Atmospheric Model:
aerosol, cloud and molecular
backscatter and extinction profiles;
surface albedo,
background radiation.
Retrieval Model:
detector bandwidth,
range resolution,
hor./vert. resolution,
differentiation
method
Lidar equation
DIAL error propagation equation
Trace gas statistical (RMS) error profiles
Orbit selection
For all approaches a realistic scenario was assumed where the instrument is embarked in nadir
sounding mode on a LEO satellite at orbit height of 450 km. Since no strong requirements exist for the
orbit geometry or equator crossing time, a polar orbit at dawn/dusk equator crossing time has been
selected. Such an orbit is considered the best compromise with respect to a low level of background
radiation and sufficient solar radiation emerging the solar panels to run both instrument and satellite
without further restrictions.
Atmospheric model
The Reference Model of the Atmosphere (RMA) provided by ESA was used (Appendix A to the
study’s statement of work EOPP-FP/2003-09-828) for modelling the lidar signals which are influenced
by the atmosphere. It comprises the following parts:
•
•
•
US standard atmosphere for pressure and temperature profiles.
The US standard atmosphere concentration profiles for the four trace gases under investigation:
CO2, O3, CH4, and N2O.
The median aerosol model.
44
•
•
Background radiation from the sun and the earth.
Cloud optical thickness and albedo.
Background radiation
The impact of background radiation on the performance of lidar systems is an important issue in the
wavelength range 300 – 1000 nm, because solar emission is high in this range, and the solar radiation
is an additional source of noise for daylight or dawn/dusk measurements. To avoid serious
performance degradation in the visible und near infrared spectral region narrow-band filters and a
small field-of-view of the receiving telescopes are mandatory requests for the instrument. Above 1 µm
however, solar irradiance decreases rapidly, and at approximately 2.5 µm equals the terrestrial thermal
emission which from then on dominates. In the infrared spectral region background radiation
suppression is of less concern.
The libRadtran package was used for all background radiation calculations. LibRadtran has been
developed by Arve Kylling and Bernhard Mayer over the last 12 years (http://www.libradtran.org).
Recently, the software package was extended to do line-by-line calculations in the infrared spectral
range. To be most flexible in line selection, a fast “pseudo-spectral” method has been applied, based
on a parameterisation of low-resolution band models developed for the LOWTRAN 7 atmospheric
transmission code. These models include the effects of all radiatively active molecular species found
in the Earth's atmosphere. The models are derived from detailed line-by-line calculations that are
degraded to 20 cm-1 resolution for use in LOWTRAN. This translates to a spectral resolution of about
5 nm in the visible and about 200 nm in the thermal infrared (B. Mayer, personal communication). A
Sun zenith angle of 75 degrees is assumed, corresponding to the “worst case” on a typical dawn/dusk
LEO orbit.
Surface Reflectance
In case of the IPDA sensor the reflectivity at the surface largely determines the signal-to-noise ratio of
the backscattered signal and hence size and cost of the instrument. The amount of backscattered
radiation is determined by the bi-directional reflectance factor (BDRF) which spans about two orders
of magnitude over ocean and land surfaces. In addition this value shows a strong wavelength
dependency. In the visible spectral region the values have been taken from the ESA-RMA. For the
near infrared at 1.6 and 2 µm MODIS data have been used for land surfaces (~ 0.2) and Polder data for
the ocean (0.08) (source: E. Dufour, Thesis 2003).
It should be noted that water is highly absorbing at those wavelengths and the solar light returned by
the sea is only caused by specular reflection on the small portion of capillary waves whose slope is
oriented in the appropriate direction. The BDRF is strongly dependent on the wave slope distribution
function, itself dependent on the surface wind speed. The variability of the BDRF can be represented
by its lower decile (0.025), lower quartile (0.05) higher quartile (0.14) and higher decile (0.25) values
(source: E. Dufour, Thesis 2003). The lidar reflectivity can be considered constant over approximately
5 degrees around nadir. Besides, the range extent of the sea return depends on the wave amplitude. We
can consider that, in averaged oceanic conditions this amplitude is δRS ≈ 10 m.
In conclusion we have to note that the surface reflectance is of primordial importance to IPDA system
performance. On the other hand we found no literature dealing with specular reflection at strict nadir
viewing geometry for lidar application. Hence there is serious demand for both experimental and
model confirmation of the above albedo values, especially over sea.
45
2.2.3
Overview on Sources of Random and Systematic Errors
Random error
The random error has been evaluated analytically by error propagation means using the DIAL and/or
hard target Lidar equation. This error is generally driven by the low number of photons backscattered
from far distant target, the speckle noise and noise from solar background radiation. Further noise
sources which can have a significant influence on the performance of the random error arise from
specific detection technique used in the measurement approach. Direct detection principles are
generally plagued by noise of the dark current from photo detector which converts the collected
photons into a photo current and noise from the subsequent current amplification. Vice versa coherent
detection principles are limited by speckle noise because of the diffraction limited observation
geometry and noise induced by the local oscillator. Generally, the random error can be minimised by
using a large area telescope for collecting more photons and a powerful laser source for emitting more
photons, which is true for all measurement approaches. On the other hand, the power area aperture
product in the following denoted by PA mostly influence cost and size of an instrument. The
following relationships (see Table 2.2-1) derived analytically from the basic equations and verified by
parametric analyses characterise the instrument’s performance in terms of the required PA.
Table 2.2-1: Simplified scaling laws of the random error for pulsed IPDA and DIAL instruments;
taken from [TN 240]
Pulsed System
IPDA
DIAL
⎛ δN ⎞
⎜
⎟ ≈
⎝ N ⎠ gas
col
Direct detection,
low background noise,
eye safety limited FOV.
1
P ⋅ A ⋅ nshots
Direct detection,
high background noise.
1
⎛ δN ⎞
⎜
⎟ ≈
⎝ N ⎠ gas P ⋅ A ⋅ nshots
Heterodyne detection.
⎛ δN ⎞
⎜
⎟ ≈
⎝ N ⎠ gas
col
col
1+
BIPDA
η⋅P⋅ A
nshots
δn gas
n gas
δngas
ngas
δn gas
n gas
Valid for
≈
≈
1
P ⋅ A ⋅ n shots
1
N2O
P ⋅ A ⋅ nshots
1+
≈
p, T, CO2, CH4, O3
B
η⋅E⋅A
CO2, CH4, N2O
B ⋅ ΔR 3 ⋅ n shots
⎛ δN ⎞
is the random error of the total column number density for IPDA
⎜
⎟
⎝ N ⎠ gas
col
δngas
ngas
is the random error of the number density for DIAL
PA is the power aperture area product, n is the number of shot-pairs on- and off-line, η is the overall optical
efficiency, E is the pulse energy, ΔR is the range-resolution, and B is the electrical bandwidth in case of
heterodyne detection
Note that these relationships between the random error in the measurement and the most important
instrument parameters are simplified derivations of the basic equations. The high/low background
46
noise limited relationships for direct detection represent extreme cases; the performance of real
systems scales in between.
Sources of systematic errors
The following list addresses the most important sources of possible systematic errors connected to the
classes of optical instruments which have been identified in the previous paragraph.
1. Uncertainties in the trace gas absorption coefficient due to errors in line parameter
data or model.
2. Uncertainties in the pressure and temperature field used to calculate the absorption
cross sections.
3. Interference with other trace gases having absorption lines in the same wavelength
region, especially the highly variable H2O.
4. Uncertainties in the trace gas absorption coefficient due to errors in the wavelength
and bandwidth determination of the laser source.
5. Unknown spectral impurity of the laser source (only critical for direct detection instruments)
6. Wavelength shift due to non nadir pointing (Doppler shift).
7. Unknown aerosol scattering in the case of a large on/off-wavelength separation.
8. Non precise alignment of the overlap between laser beam and the telescope field
of view.
9. Statistical biasing by application of the non-linear DIAL equation to noisy input data.
10. Quantisation error and detector/analogue chain non-linearities.
11. Pickup of temporary correlated electromagnetic noise by the analogue chain.
Absorption cross section: Since in the DIAL equation the number concentration is inversely
proportional to the absorption cross section it is clear that a relative error in the knowledge of the cross
section translates to the same amount of error in number concentration. In this study absorption cross
section have been calculated by adopting line parameters from the HITRAN 2002 data base
(http://www.hitran.com). The line shape was modelled by the Voigt profile approximation which takes
into account both effects Doppler broadening (caused by thermal motion of the air molecules) and
pressure broadening (caused by collisions with air molecules; Ehret et al., 1993). It should be noted
that an error introduced by uncertainties in the line strength is just a scaling factor and not subject to
any long term drift. This error may be corrected by re-processing of the final data-products. However,
it should be noted that uncertainties associated with the line shift and line broadening parameters as
well as the model used for the cross section calculation can cause a measurement bias particular in
case of greenhouse gas observations where biasing is extremely demanding.
Pressure and temperature variability: Since the absorption cross section generally depends on
temperature and pressure a certain a priory knowledge of these atmospheric parameters is required.
Former sensitivity studies [e.g. Cahen et al., 1982; Wilkerson et al., 1979] showed that errors due to
temperature uncertainty can be minimised by selection of absorption lines with proper ground state
energy levels E”.
Interference with other trace gases: The DIAL method requires the selection of wavelengths where
the differential optical depth is dominated by the probed species. Especially the highly variable H2O
may cause problems for finding appropriate absorption lines for the trace gases under study. An
extensive search for interference free spectral regions for all parameters was subject to this study.
47
Laser line width: The absorption cross section calculated by the Voigt profile model is only valid for a
monochromatic laser line width. If the width of the laser spectral profile is of the same size as the
molecular line width this can introduce large measurement errors if not taken into account in the data
reduction. (Megie et al., 1982). Cross section correction by the finite laser line width can only be done
in an iterative manner which is time consuming and erroneous if the laser line shape is not well
known. This source of error can largely be avoided if a frequency stabilised single mode laser source
is used with a the laser band width which is significantly smaller than the spectral width of the
molecular line. In case of pulsed laser sources operation close the Fourier limit is recommended.
Frequency stability: It is also clear from the discussion above that a frequency jitter can pose
significant measurement errors particularly when the laser wavelength is locked to the edge of an
absorption line. For soundings in the line centre this source of error is more relaxed.
Laser spectral purity: In general the spectral shape of the emitted laser pulse has an unwanted broad
band component or additional side modes if not running exactly single frequency. This unwanted
radiation is not substantially absorbed by the probed trace gas and gives rise to a modification of the
effective absorption cross section seen by the laser source. The magnitude of this part often depends
strongly on the internal laser adjustment and cannot be easily obtained from simple in orbit monitoring
devices. To avoid errors by insufficient spectral purity, either spectral filtering of the outgoing pulses
or the incoming radiation are required. We note that heterodyne detection principles are not affected
by the spectral impurity of the laser line width which is a big advantage of the coherent detection
principle over the direct detection one.
Doppler Effect and Viewing Geometry: The rapidly moving space craft (v ~ 7000 m/s leads to a
Doppler shift of the laser centre frequency if the line of sight is not orthogonal to the platform
velocity. If the pointing is known the induced frequency shift may be corrected within the retrieval
procedure. Also the movement of the atmosphere (up to v ~100 m/s) leads to a range dependent
Doppler shift of the laser centre frequency if the line of sight is not orthogonal to the wind velocity.
However the pointing requirements along cross-track are considerably relaxed compared to those
along track. This allows for a constant offset along cross rack direction by a few degrees to avoid large
signal fluctuations due to specular reflection from cirrus cloud particles or the sea surface if the target
is the atmosphere. In contrast to the Doppler shift caused by the moving platform a proper correction
within the retrieval algorithm needs knowledge about intrinsic system parameters like pointing and
platform velocity and the atmospheric wind field.
Overlap of Receiver and Transmitter: If the overlap function for the on-line signal deviates from the
off-line one, then an additional term occurs in the DIAL equation. This accounts for an additional
pseudo optical thickness which gives rise to a measurement error. For a laser beam divergence which
is much smaller than the field of view of the receiving telescope, this sensitivity can be made very
small. On the other hand if a high background light rejection is required it is necessary to chose the
field of view as narrow as possible. Hence a careful is required for trading the different error sources.
Statistical biasing: The non-linear character of the logarithm in the DIAL equation may lead to a bias
in the retrieved trace gas concentration if the input signals are very noisy. We recommend summing
raw data profiles until it is possible to calculate trace gas profiles with a statistical error of better than
20%. The statistical variation of the input signal can be used to correct the effect of statistical biasing.
In this case care has to be taken that the raw signal variations can be determined precisely.
Quantisation error: The quantisation error of an analogue-to-digital converter (ADC) in standard
configuration is > ±0.5 LSB even for a ideal device. A relative measurement accuracy of 10−3 is
needed for a low bias trace gas retrieval it is immediate that this would require an input signal of
48
several thousand (LSBs). Since the total signal dynamics including thin cirrus clouds is about three
orders of magnitude we would need at least a 20-bit ADC. But such a high resolution is not available
at the high data rates (> 3 MHz) needed. An alternative approach is to cascade several digitisers of
lower resolution, but this may require some additional calibration procedures for a proper treatment of
signals in the overlap region of the individual devices within the cascade.
Analog Non-Linearity: As long as there are no saturation effects the linearity of most high bandwidth
operational amplifiers is good enough to ensure a relative error below 10−3. The total harmonic
distortion is the typical measure of the non-linearity and approx. equal to the integral nonlinearity.
There is a relatively broad number of devices with distortions below 0.01% (80 dB), so it should not
be a specific problem to find adequate parts for the analog chain. A further critical parameter is the
long term settling after large, (maybe even saturated pulses). If shortly after a thin cirrus with a
backscatter ratio of 100 the signal has to be determined to a accuracy of 10−3 we need a fast settling of
the amplifier as well as the ADC to a level of 10−5 of the pulse maximum. Based on our experience
with our ground based and airborne systems we know that this criterion drastically limits the number
of useful devices. If the settling characteristics are reproducible it may be possible to use a
deconvolution scheme to reduce this error to the required level. Of course the non-linearity of the
detector (APD or ACCD) also has to be checked to confirm with the 10−3 bound.
Noise pickup: Pickup of noise that is correlated with the laser pulse has to be kept below 10−6 of full
scale to guarantee a relative signal error of 10−3 over a signal dynamic range of 103. By a careful
design of the analog chain this is possible, but has to be verified for the complete system assembly.
Sources of systematic errors specific to atmospheric backscatter DIAL
Systematic errors which are common to the atmospheric backscatter DIAL are:
1. Rayleigh-Doppler error caused by broadening of the emitted laser radiation on the way back due to
molecular scattering.
2. Too low vertical raw data resolution.
3. Multiple scattering by optically thick clouds.
4. Horizontal averaging of the raw signals over inhomogeneous regions
Rayleigh Doppler error: There are mechanisms that can alter the spectral profile of the laser beam as
it penetrates the atmosphere. Doppler broadening caused by backscattering from randomly moving
molecules in the atmosphere influences the effective absorption cross section on the way-back from
the scattering volume to the receiving telescope. In combination with large gradients from aerosol or
clouds systematic errors up to 20% have to be accounted for [Ansmann and Bösenberg, 1987]. This
effect is particular important for DIAL measurements in the upper troposphere where molecular
scattering dominates and the molecular absorption line width is comparable to the spectral width of the
laser radiation on the way-back from the scattering particles and molecules. This effect in principle
may be corrected if the exact splitting of the return signal into parts coming form aerosol particle
scattering from molecules can be performed. But given our present experience with the H2O retrieval
it is questionable if a residual error below 1% can be reached. Another broadening mechanism is
rotational Raman scattering of the air molecules (i.e. N2 and O2). These contributions are relatively
easy to suppress by a narrow band interference filter. A filter FWHM of less than 0.5 nm is typically
sufficient.
49
Raw data resolution: For an exact cancellation of the atmospheric on- and off-line backscatter
properties in the DIAL retrieval it is necessary to sample the gradients of aerosol layers with a
resolution that is much higher than the desired resolution of the trace gas retrieval.
Multiple scattering: At larger optical thickness (> 0.9) multiple scattering has to be taken into
account in the single scattering Lidar approximation [Ruppersberg 1997]. Fortunately, the effect of
enhancement of the backscatter coefficient by multiple scattering exactly cancels in the DIAL
equation. Furthermore, the second order scattering effect which elongates the path length by a nonstraight light path can be neglected for the very narrow field of view expected for a space-borne DIAL
systems (< 100 µrad).
Horizontal averaging: To avoid numerical problems it is necessary to reduce the statistical noise by
summation over several lidar returns. A temporal and/or spatial miss-alignment of the volume probed
by on-/off-line pairs in the order of 10% will result in tolerable errors (below 1%) for normal free
atmospheric conditions. But highly non-uniform backscattering regions (boundary layer/cirrus clouds)
may lead to a considerable error. The possibility for miss-alignment can be reduced by using a
common FOV defining aperture for all receiver channels (and a relative divergent laser beam).
A further effect of horizontal inhomogeneity is that pulse energy fluctuations between corresponding
on-/off-line pairs lead to a different weighting of the backscatter coefficients. This gives rise to a
coupling of laser output energy fluctuations with horizontal atmospheric inhomogeneities. This error
scales linearly with the energy fluctuations. A worst case estimation leads to huge errors, but for
normal clear air conditions (only smoothly ascending/descending of growing aerosol layers) the effect
can be tolerated if the laser energy fluctuations are in the range of 3%. This effect may be reduced by
energy corrected summing of the backscatter signals.
Sources of systematic error specific to IPDA systems
Systematic errors which are specific to IPDA systems are:
1. Difference in the surface reflectance between on- and off-line.
2. Errors in the power measurement which is used as the upper backscatter signal point.
3. Uncertainties in the relative sensitivity of the receiver.
4. Uncertainties in determination of the distance to the ground.
5. Horizontal averaging over structured terrain.
Difference in surface reflectance: For column measurements any difference in the apparent ground
reflectance must be kept to a minimum. In the extreme case of a large reflectance step this means that
the illuminated ground spots for on- and off-line pulses have to be equal within this accuracy. This
translates into a relative on- to off-line pulse pointing accuracy of 0.2% of the full divergence angle.
For a system with 100 µrad divergence this means a pointing stability of 0.2 µrad. This extreme case
of a surface reflectance step from a value near zero (say water) to a value near one (say sand) is of
cause extremely rare, but may give rise to out-layers in the column retrieval. Also for regular overpasses of the satellite over a stationary albedo feature (e.g. coast line) this may show up in the
climatological average.
For a proper numerical estimate of this effect one needs to know the horizontal autocovariance
function of the earths surface-reflectance on the length-scale of the laser footprint, i.e. in the range of
1-3m for heterodyne detection systems and in the range from 50-200m for direct detection IPDA. To
our knowledge there is no data available which can be used to estimate the auto-covariance function at
50
the desired level of resolution. If the earth surface would be a random pattern this error source could
be subsumed under the statistical error. In order not to significantly raise the single shot statistical
error which is in the order of 1% for a direct detection LPDA the relative on/off-line alignment should
be better than a few % of the laser beam divergence assuming an average albedo variation of about
10% on the length-scale of the ground-spot. For heterodyne detection systems the speckle noise limits
the single shot SNR to about 1, so the alignment requirements are also relaxed considerably, as long as
only the effect on the statistical error is considered.
Pulse energy measurement and relative receiver sensitivity: Total column measurements require
additional information about the relative pulse energy of the laser pulses which are transmitted on- and
off-line in the atmosphere. If the power measurement for on- and off-line pulses is not exactly known
this leads to an error in the retrieved optical thickness which is equal to the deviation of these relative
calibration.
Furthermore, the relative sensitivity of the receiver channels do not cancel. Even if only one detector is
used the transmissions of the optical filter system may differ. In summary there is one calibration
constant left which is the ratio of the relative sensitivities of the backscatter-signal receiver and the
pulse energy monitors.
One method for partially determination of the calibration may be the use the receiver channel to
monitor the laser pulse energy. But this requires a very thorough optical layout for the alignment of the
outgoing laser beam relative to the receiver. Especially the spatial beam characteristics of the light
received from the earth and form the transmitter have to be the same to guaranty the same transmission
though the optical filter.
A better approach for an in flight calibration procedure may be to use cases with broken cloud fields.
This would allow for applying the full DIAL method which in the other hand helps for determining the
calibration constant (which may not be constant but drift slowly during the mission duration) for the
column measurements.
Path length determination: Proper interpretation of integrated path measurements requires knowledge
of the exact path length. Otherwise measurements of homogeneously mixed gases like CO2 will
mainly map the orography. For a trace gas with an exponential decreasing number density as it is
approximately the case for the greenhouse gases CO2, CH2, and N2O it is easy to show that the
relative column error for an error in the path length measurement is just the inverse of the atmospheric
scale height of ~ 7 km. Hence a 10 m error in path length determination results in a trace gas column
error of about 0.14%.
Horizontal averaging over structured terrain: For IPDA sensors which use atmospheric backscatter it
may be necessary to sum a certain number of returned signals to avoid numerical problems when
applying the DIAL equation. This is especially true for heterodyne detection systems where a single
return signal measurement has a relative error of 100% due to speckle noise. In case this averaging is
done over inhomogeneous terrain, this results in the summation of signals which belong to different air
columns. Due to the non-linearity of the transmission this may result in a possible bias as compared to
the mean trace gas column. From analysis found in this study this error is in the order of 0,083% for an
altitude variation of 1 km. Hence we conclude that this effect may be neglected, or can easily be
compensated for, if the altitude variations are tracked at high enough rate to calculate the variances
and possible higher order moments of the ground height variability.
51
2.2.4
Temperature-DIAL
Temperature profiles can be measured by differential-absorption Lidar (DIAL) applied in the oxygen
A-band around 0.76 µm (see Figure 2.2-2). As already showed by others (Korb et al, 1995) the idea
here is to select temperature sensitive oxygen absorption lines with ground state energies E’’ > 1000
cm-1. The feasibility of this technique has already been demonstrated by airborne measurements (Korb
et al., 1995). In this study we investigated the performance of a nadir viewing oxygen DIAL
instrument which is assumed to fly on a low-orbit satellite of 450 km orbit height. The simulation
indicate that the observational requirements can only be met on the expense of a big instrument with a
large power*aperture product of ~ 400 Wm2. In particular, the solar background radiation in
combination with eye safety are the basic limitations of a temperature DIAL at 0.76 nm.
Another difficulty arises from the fact that the derived temperature profile will only partly be
compliant with the observational requirements and gaps will exist particularly in the tropopause
strong online
weak online
offline
offline
Figure 2.2-2: Suitable oxygen line troughs at 760 nm for temperature measurements with the DIAL
technique; taken from [240]
region. Nevertheless, the simulations reveal that temperature soundings from ground up to 20 km are
possible. Figure 2.2-3 illustrates these findings, displaying the relative statistical (RMS) temperature
measurement error in percent as a function of altitude. The most important temperature DIAL baseline
system parameters are listed below the plot. Like for pressure, a PMT detector with 24% quantum
efficiency and negligible dark current noise density was assumed.
The vertical dashed line in the plot indicates the minimum precision required for global NWP, from
Table 1.1-1. There is a jump at 15 km due to the jump in required precision from 1 K to 2 K. As a
result of the varying horizontal and vertical sampling requirements, the error profiles for both oxygen
absorption lines exhibit jumps at 5 at 15 km. Their shape is also influenced by different optical depths
and the impact of the ESA RMA median aerosol profile.
52
Figure 2.2-3: Results of temperature DIAL simulation from Space for global NWP requirements.
Concerning the systematic error, the required spectral purity of 99.99% will be the limiting factor on
the instrument side. Furthermore a laser band width of 120 MHz and a frequency stability of 8 MHz
would be needed to satisfy the observational requirements for the bias which is < 0.5 °K. Although,
not evaluated in detail in this study, other critical error sources such as Rayleigh-Doppler broadening
need particular attention in order to avoid systematic errors.
A possible solution of above drawbacks could be the use of a high-spectral resolution filter in the
receiver channel as already foreseen in the AEOLUS and ATLID instrument concepts. Both, solar
background radiation and the error caused by spectral impurity could then be reduced to acceptable
values. However, such a modification could also have a significant impact on the overall efficiency of
the system which would probably lead to a further increase of the required power aperture product.
Main conclusion: DIAL temperature measurements from space would require an unrealistic large
instrument incorporating a power aperture area product of about 400 Wm2 in order to meet the
observational requirements for global NWP. The solar background radiation in connection with the
eye-safety requirement have been identified as the most significant performance limiting factors for
the random noise of a space-borne oxygen-DIAL instrument. To fulfil the systematic error
requirement would require a narrow-band filter in the receiver channel to avoid errors caused by
unsufficient spectral purity of the outgoing laser radiation Rayleigh Doppler broadening of the
received radiation.
53
2.2.5
Pressure-DIAL
The DIAL technique applied in the oxygen A-band at 0.76 nm can also be used for the measurement
of the atmospheric pressure profile (Korb et al., 1995). A good performance can be achieved when the
measurements are conducted in the trough region which is formed by two adjacent strong oxygen
absorption lines (see Figure 2.2-4). Here the measurement precision scales to the square of the
pressure variable. By selection of temperature insensitive lines (e.g. trough regions) the temperature
error in the pressure retrieval caused by the unknown temperature profile can be minimised.
on 1
offline for on 1, 3
on 3
on 2
offline for on 2
Figure 2.2-4: Suitable oxygen line troughs at 760 nm for pressure measurements; taken from [TN 240]
The feasibility of this technique for the pressure profile in the troposphere has already been
demonstrated by airborne measurements (Korb et al., 1995). Here we investigated the measurement
performance of such an instrument which is assumed to fly on a low-orbit satellite of 450 km orbit
height. The simulations reveal that lidar signals arising from atmospheric backscatter are too weak for
precise measurements of pressure profiles from space unless a large instrument is assumed. Therefore
we concentrated our investigation on the analysis of the measurement performance of an IPDA sensor
which uses much stronger lidar echoes from hard target reflection of the Earth surface and clouds. This
technique enables the measurement of the total oxygen column above the target. In particular, the
surface pressure can be derived from target reflection on ground. Lidar signals from cloud reflection
could provide useful information on the pressure profile.
Regarding IPDA the following performance was found from parametric analyses: To satisfy the
breakthrough observational requirements for global NWP according to the EUMETSAT study from
Table 1.1-3 (1 hPa pressure accuracy and 100 km horizontal resolution) over land (sea) surfaces would
require a power aperture area product of 5 (10) Wm2. However, to fulfil the stringent target
requirements (0.5 hPa pressure accuracy and 15 km horizontal resolution) would require an IPDA
instrument offering a power aperture area product as large as 100 (240) Wm2 over land (sea).
54
These findings are illustrated in the following plot which displays the relative statistical or RMS
instrumental noise error of pressure DIAL using the previously defined baseline parameters (Fig. 2.25). The error is given as a function of altitude, whereby the surface is located at an arbitrary altitude of
500 m. Above 500 m, atmospheric backscatter DIAL performance is displayed, using the ESA median
aerosol profile and a height independent pressure sensitivity factor of 2.0. The horizontal (vertical)
resolution of the measurements is 100 (1) km below 5 km altitude and 200 (2) km above, as specified
in the EUMETSAT target requirements.
Below 500 m, surface pressure IPDA performance is plotted for measurements over land using an
albedo of 0.2 specified by ESA, and 15 km horizontal resolution as specified in the EUMETSAT
target requirements. The EUMETSAT surface pressure target precision (or RMS error) requirement
from TN 110 (0.5 hPa) is the vertical dashed line in the plot, in order to have a quick overview of
whether and where the requirements are fulfilled. The log/log scale enables a better general
performance overview and puts emphasis into the most important low altitude and low error range.
Figure 2.2-5: Results of pressure DIAL simulation from Space for global NWP target requirements.
The overall systematic error for the surface pressure measurements has been estimated to 0.45 hPa.
The most significant driver of the systematic error is the path length uncertainty: 2 m path uncertainty
55
translates to 0.25 hPa surface pressure uncertainty. Second importance have temperature and water
vapour biases: the temperature (water vapour mixing ratio) profile needs to be known with an accuracy
of 1 K (0.12 g/kg) on a global basis with a spatial resolution which fits the horizontal sampling length
of the IPDA sensor. Fortunately, if pressure IPDA were used to support CO2 IPDA measurements, the
water vapour concentration is irrelevant, because CO2 IPDA has the same humidity sensitivity. In this
case the dry air CO2 mixing ratio would be provided.
Regarding other systematic errors, the requirements on the laser spectral performance (laser line width,
frequency stability, and spectral purity) and of a space-borne IPDA sensor for surface pressure are
moderate and quite similar to the WALES laser instrument definition. This is a direct consequence of
sounding in the trough region where the absorption signature is rather smooth.
In reference to the required large power aperture area product for the direct detection instrument we
expect that a modulated cw heterodyne instrument could lead to some relaxation on this issue, but
probably with the expense of a less accurate path length resolution. Nevertheless trades for this option
should be further investigated.
Main conclusion: IPDA sensors using hard target returns are suited for surface pressure
measurements from space-borne platforms. An instrument with an power aperture area product of 5/10
Wm2 would fulfil the EUMETSAT breakthrough observational requirements for the random error for
global NWP over land/sea. In order to meet the target requirements for regional NWP, however,
would require an unrealistic large instrument. The overall systematic error for surface pressure
measurements by IPDA is estimated to be about 0.45 hPa.
2.2.6
Ozone-DIAL
The performance of space-borne ozone DIAL operating in the ultraviolet spectral region around 308
nm has been investigated in detail in this study. In contrast to active remote sensing of pressure and
temperature with the DIAL technique, where only a few applications are reported, UV-DIAL for
ozone is a well established technique and routinely used from ground and airborne platforms.
Despite this strong heritage it was found that the measurement of the ozone profile in the troposphere
from space-borne platform is challenging and requires a big instrument. This is based on the fact that
the stratospheric ozone layer which contains about 90% of the total ozone column has to be penetrated
twice. The simulations reveal that the threshold requirements for the random error in the troposphere
and UTLS can only be met by a large size instrument offering with a PA of ~125 Wm2.
In contrast to the troposphere the situation of a space-borne ozone DIAL for stratospheric research is
more favourable. Here a relatively small instrument (PA ~ 7.2 Wm2) could satisfy the threshold
requirements. An interesting alternative for stratospheric ozone measurements would be to use three
online wavelengths instead of one, even needing less average power (PA ~ 5 Wm²) in total for this
four wavelength system. It would fulfil the threshold requirements between 16 – 45 km, using
appropriate online wavelengths at 270, 290 and 306 nm. See Fig. 2.2-6 which illustrates these
findings.
It was found that the solar background radiation for the offline wavelength is the dominant noise
source for measurements in the stratosphere. To meet the target observational requirements in the
stratosphere would need a comparably large instrument as it would be required for measurements in
the troposphere and UTLS.
56
Due to the relatively large wavelength separation which is required for achieving sufficient sensitivity,
systematic errors may be introduced from unknown scattering by atmospheric aerosols. Systematic
errors introduced by aged background aerosol in the stratosphere are only ~1% and can be negligible.
Strong aerosol layers originating from fresh volcanic eruption, however, can cause significant errors
up to 100% particularly in the vicinity of aerosol gradients. In case of ozone profiling in the
troposphere the errors caused by the aerosol is 10% for the US standard atmosphere profile and the
ESA median aerosol. Here corrections for the extinction and backscatter terms in the DIAL equation
are mandatory.
For the calculation of the ozone dry air mixing ratio the required temperature and pressure profiles can
be adopted from operational weather services. As a consequence of the continuous ozone absorption
band shape in the UV no strong requirements exist for the spectral performance and frequency stability
of the laser source which is a big advantage of an UV-DIAL over the near IR ones.
In the absence of strong aerosol gradients from fresh volcanic eruptions, the overall systematic error of
a space-borne ozone DIAL in the stratosphere is estimated to be < 2% dependent on the accuracy of
the ozone absorption cross section and the temperature and pressure profiles obtained from NWP
centres.
Figure 2.2-6: Relative statistical ozone number density (RMS) error in percent as a function of altitude.
The threshold precision (or RMS error) requirement from Table 1.2-1 is the vertical dashed line and
gives a quick overview of whether and where the requirements are fulfilled. The most important
system parameters are listed below the plot.
57
Main conclusion: DIAL performed in the UV can be regarded as a potential candidate instrument for
the measurement of stratospheric ozone from space-borne platform. An instrument of moderate size (5
Wm²) would fulfil the threshold observational requirements over a height range from 16 km up to 45
km using a four wavelength instrument. Measurements in the UTLS and troposphere in all climates,
however, would require a relative large instrument which is not available from current technology. A
big system is also required for fulfilling the target requirements in the stratosphere. In the absence of
strong aerosol gradients the overall systematic error is < 2% for measurements in the stratosphere. For
Lidar measurements in the troposphere aerosol correction need particular attention.
2.2.7
Greenhouse Gases
Line selection criteria
Active remote sensing in the near infrared is potentially suited for the measurement of the greenhouse
gases CO2, CH4, and N2O. In this study promising candidate lines have been identified around the
wavelengths 1.6 µm and 2.0 µm for CO2, 1.65 and 2.3 µm for CH4 and 3.9 and 4.5 µm for N2O (for
details see Figures 2.2-7 – 2.2-10. For theses lines errors caused by the unknown temperature profile
or interference from water vapour and other trace gases can be minimised. Another selection criterion
is related to the optical thickness, which should be close to unity across the total atmosphere for a
nadir viewing sensor. In case of IPDA measurements soundings in the line wing have been preferable
selected in order to make use of the more favourable weighting function which peaks near ground
where the sources and sinks reside. It was found that the heritage of greenhouse gas monitoring with
active sensors is sparse. Only a few measurements of CO2 at 2 µm, CH4 at 1.65 µm and 3.3 µm, and
N2O at 3.9 µm have been reported in the literature.
The systematic error due to different on-/offline wavelengths in combination with the aerosol
extinction dependency on wavelength is negligible for IPDA using the ESA median aerosol profile
due to the very low on-/offline separation.
The wavelengths selected in this study are mainly a compromise between temperature and water
vapour dependency and total optical thickness on one hand and the proper weighting function on the
other hand. Note that initial simulations for IPDA in line wing positions have shown that the optimum
one-way online optical thickness is about 1.2 (including the ESA median aerosol) but that there is no
significant performance loss for values between 0.8 and 1.6. This is because a higher optical thickness
towards line centre positions is counterbalanced by a worse weighting function.
The following plots give an overview as well as specific details on the absorption lines selected. The
trace gas optical depth of the total atmospheric column up to 65 km altitude is displayed for the
different trace gases as function of wavelength and wave number. The total optical depth (sum of all
trace gases) is the black line. The plots give a quick overview of the strength of the absorption lines
and possible interference by other gases.
The computations were performed using a high spectral resolution radiation transfer program under
US standard atmosphere conditions. Atmospheric absorption was computed with Voigt line profile
assumption using line parameters from the HITRAN’96 (HITRAN 2001 for H2O) database as input
parameters.
58
online
online
CH4
CO2
CO2
H2O
H2O
offline
offline
Figure 2.2-7: CO2 at 1610 nm.
Figure 2.2-8: CH4 at 1650 nm
online
CO2
online
N2O
H2O
offline
offline
Figure 2.2-9: CO2 at 2050 nm.
CH4
H2O
Figure 2.2-10: N2O at 3880 nm.
Specific to Carbon Dioxide
As a major result of this study it was found that active remote sensing is regarded an appropriate
means for CO2 monitoring from space-borne platform. Favourable candidate instruments are based on
IPDA incorporating coded cw lasers or IPDA sensors which use pulsed laser sources.
Both sensors are expected to meet the stringent target observational requirements for the random error
even over the ocean where a ground albedo of 0.08 has been assumed. The target requirements for
CO2 can also be fulfilled by a worst case scenario of an albedo of 0.02 over the ocean. Moreover the
instrument is expected to be of moderate size. As an example, the required power aperture area
product for a pulsed, direct detection IPDA system operating at 1. 6 µm is expected to be in the order
of 7 Wm2 for measurements over the ocean at 0.08 ground albedo. For the coded cw system the
requirements on the size of the instrument are significant less. Here an instrument of the 1 Wm2 class
would satisfy the target observational requirements.
59
Some information on the CO2 profile can be obtained from hard target reflection on cloud surfaces at
different height levels. Range-resolved measurements using the DIAL are only possible in the
boundary layer, but at the expense of a significantly bigger instrument (PA > 50 Wm² for pulsed,
direct detection DIAL). Moreover, DIAL would only satisfy the threshold observational requirements.
It is important to note that several constraints are associated to the measurement of the dry air CO2
column by the above reported IPDA lidar approaches. Most important, the path length of the total
column has to be adequately known (< 10 m) and scattering from aerosols and clouds which can alter
the effective path lengths have to be discriminated by the measurement process. Both issues can easily
be accomplished for pulsed laser transmitters providing pulses in the order of several ns.
In case of a cw system the transmitted signal must be modulated for achieving the required rangeresolution and suppression of scattering from aerosols and optically thin clouds. Both types of sensors,
the pulsed and the cw modulated one, require auxiliary data about surface pressure (< 1 hPa) and the
temperature profile ( ~1 °K) to meet the stringent bias criteria for greenhouse gas observations as
shown in Table 1.3-4.
Strong requirements exist for the spectral characteristics of the radiation source. It was found that the
measurement accuracy critically depends on frequency stability and spectral impurity. The latter is
only a constraint for pulsed lasers if the direct detection principle is applied. A heterodyne instrument
does not suffer from spectral impurity of the radiation source. The recommended spectral
characteristics for pulsed laser sources are 60 MHz (FWHM) for the laser bandwidth, 6 MHz for the
laser bandwidth variability, < 1 MHz for the frequency control. In case of direct detection a spectral
purity value of > 99.97% is needed in order to meet the observation requirements.
Other requirements exist for pointing to compensate for the Doppler shifts (< 0.070 mrad along track
and < 10 mrad cross track) and the determination of the relative pulse energy of the transmitted beams
(~0.1%). The timing between on- and off-line pulses needs also attention. This issue encounters
twofold if pulsed laser are used. To avoid ambiguity in the lidar returns from ground and clouds at 18
km height in the tropics, the time duration between on- and off-pulses should be > 120 µs. On the
other hand to avoid errors encountered from albedo change at the moving platform the time separation
between the two subsequent pulses should be as small as possible.
60
Figure 2.2-11: Relative statistical CO2 column number density (RMS) error in percent as a function of
target altitude. The threshold precision (or RMS error) requirement from table 1.3-6 is the vertical
dashed line and gives a quick overview of where the requirements are fulfilled. The most important
system parameters are listed below the plot.
In a first approach, the overall systematic error for the CO2 column content measurements has been
estimated to be ~ 0.3 ppmv for the direct detection IPDA instrument and is slightly degraded to 0.45
ppmv for the coded cw IPDA system.
Main conclusion: Active remote sensing is regarded an appropriate means for CO2 monitoring from
space-borne platforms. Favourable candidate instruments are based on IPDA incorporating coded cw
lasers or IPDA sensors which use pulsed laser sources. Both sensors are expected to meet the
stringent target observational requirements for the random error even over the ocean. In addition the
systematic errors can be kept well below 1 ppmv for both sensors using realistic system parameters.
Specific to Methane
Compared to CO2 and N2O, the scientific requirements for CH4 measurements are more relaxed:
Compared to CO2 the precision requirements are a factor 2 less demanding, which in turn decreases
the required power aperture product by a factor of 4 for IPDA and by a factor of 2 for atmospheric
backscatter DIAL measurements. At 1650 nm the target requirements would be reached over land with
a power aperture product of 0.2 Wm², over ocean with a power aperture product ten times higher.
61
Due to worse detector performance at 2.3 and 3.4 µm, these wavelength ranges are less suitable for
direct detection, despite somewhat better weighting functions. The direct detection system is detector
noise limited. Heterodyne detection would lead to a smaller system here. A high repetition rate would
ensure that speckle noise can be kept under control. It is the performance determining factor for
heterodyne detection IPDA. However, the reprate is limited by the fact that if the individual field of
views overlap too much, there is ambiguity in the lowest 20 km of the atmosphere. Therefore, a
maximum of 7.5 kHz is the limit. The heterodyne performance is listed in the subsequent summary
section.
Figure 2.2-12: Relative statistical CH4 column number density (RMS) error in percent over land
(albedo 0.2) for direct detection as a function of target altitude. The threshold precision (or RMS error)
requirement from TN 120 is the vertical dashed line and gives a quick overview of where the
requirements are fulfilled. The most important system parameters are listed below the plot.
Main conclusion: Due to similar absorption line features, basically the same rules apply to CH4 as to
CO2. However the precision requirements are a factor 2 less demanding than for CO2, which in turn
decreases the required power aperture product by a factor of 4 for IPDA and by a factor of 2 for
atmospheric backscatter DIAL measurements. At 1650 nm the target requirements would be reached
over land (albedo 0.2) with a power aperture product of 0.2 Wm², over ocean with a power aperture
product ten times higher.
62
Specific to Nitrous Oxide
Due to both the strong scientific requirements and the decreasing atmospheric backscatter at longer
wavelengths around 4 µm DIAL profiling measurements are out of reach. Only hard target IPDA can
be considered as a realistic technique. At 4 µm the surface albedo is about 0,02 and rather
independently of ocean or land. Background radiation at 4 µm is dominated by the Earth’s thermal
emission. It plays a minor role in the overall performance analysi thus the filter is not a critical
element. The most important performance determining factor which limits the measurement accuracy
of a direct detection IPDA system at 4 µm is the noise arising from dark current of the detector.
For heterodyne detection a high PRF ensures that speckle noise can be kept under control.
Nevertheless, this is the performance determining factor of such type of sensor. To avoid signal
ambiguity from ground return and top of the troposphere in the tropics the maximum PRF is limited to
7.5 kHz in case of a pulsed heterodyne instrument. While for the RM CW system the maximum PRF
is 15 kHz.
Main conclusion: For monitoring of the greenhouse gas N2O the situation is worse compared to CO2
and CH4 as here only the threshold observational requirements can be fulfilled from space-borne
instrument. In addition a bigger system (PA ~10 Wm² for pulsed, direct detection IPDA) would be
required.
2.2.8
Performance Summary on Active Systems
The performance overview Table 2.2-2 summarises the results of the simulations for all classes of
active optical instruments investigated in this study. It focuses one of the most critical system
parameters, the power aperture area product (PA), which is required to meet the observational
requirements for the relative random error. This PA factor largely determines size and cost of the
instrument. This table enables a direct inter-comparison between the different atmospheric species
under investigation, and between different system solutions designed such as to fulfil the respective
user requirements. Note that the surface albedo values (blue means over sea) are different for CO2.
This is due to an adjustment after the MTR. Also the presence of clouds was taken into account only
for CO2 in the form of stronger target/threshold requirements.
The power aperture product is the product of the average transmitter power including the offline signal
power, and the receiver (telescope) aperture area. Green colours indicate feasible instruments with low
or moderate size, yellow feasible with big size and red not feasible for Space application. The WALES
ESA study H2O DIAL value is included for comparison. So is ADM-AEOLUS, assuming a
150mJ/100Hz transmitter in bursts of 10s (including warm-up time) with 28s repeat period, with a
1.5m diameter telescope.
Tables 2.2-3 – 2.2-5 give an overview on the instrument requirements with respect to pointing and
laser performance as well as the required auxiliary data.
63
Table 2.2-2: Required PA for the different classes of active optical instruments investigated in this
Study; taken from [TN 240]
DIAL
IPDA
Power
* Ap.
RequireWavePower
*
Aperture
Product (Wm2)
2
Product
(Wm
)
ment
Species
length
(µm)
Heterodyne
Type
Direct Hetero- Surface Direct Detection
Detect. dyne
albedo
CW
Pulsed
CW
Pulsed
CO2
1.6
Target
Threshold
CO2
2.1
Target
CH4
1.6
too
large
235
too large
20
Target
120
90
2.3
Threshold
3.9
Target
16-30
km
Strat. 16-45
O3
km
13-33
km
Trop. + Strat.
O3
too large
36
Target
N2O
too
large
Threshold
Threshold
CH4
270
too
large
too
large
too
large
too
large
0.7
0.31
0.08
0.31
7.0
1.6
speckle limited
0.08
4.4
0.16
0.13
0.08
0.09
0.02
0.02
0.2
14
12
0.2
2
0.2
0.95
0.85
0.04
0.29
0.03
1.8
1.7
0.07
1.0
0.1
1.0
0.03
0.05
14
1.4
0.26
0.02
0.6
0.06
10
0.08
0.6
23
0.02
100
0.02
0.2
too
large
0.02
too
large
0.02
0.02
2.4
Threshold
5
0.3
7.2
Threshold
inadequ
. for
UV
0.5
too
large
too
large
0.35
speckle speckle
limited limited
integr.
ozone inadequ inadequ
. for
. for
profiles
UV
UV
not
useful
125
0.76
Breakthrough
global
NWP
n.a.
n.a.
Temperature
0-6 km
0.77
Target
400
n.a.
H2O WALES
0.94
Thresh. for
oper. NWP
17
AEOLUS
0.355
Surface
Pressure
0.08
impractical because of background light / receiver noise
Threshold
0.03
24
0.3
2.4
n.a.
n.a.
integrated temperature profiles not useful
5.5
64
Table 2.2.3: Instrument pointing requirements; taken form TN 220
65
Table 2.2-4: Required auxiliary data; taken from [TN 220]
66
Table 2.2-5: Required laser parameter; taken from [TN 220]
67
2.3
Performance Summary of Active and Passive Sensors
2.3.1
Temperature
Differential Absorption Lidar (DIAL) applied in the oxygen A-band around 0.760 nm is not suited for
temperature profiling form space-borne platform. Despite the low orbit considered in the analysis, the
required measurement precision can only be fulfilled with an unrealistic large instrument.
In contrast there are several new passive sensor combinations or even single sensors
(AIRS/AMSU/HSB, IASI) are planned which are expected to satisfy the requirements for global NWP.
2.3.2
Pressure
For the pressure variable the situation is more favourable than for temperature measurements. Pulsed
direct detection IPDA performed in the oxygen A-band is expected to satisfy the threshold
observational requirements for global NWP (1 hPa, 100 km integration length) with an instrument of
moderate size. To fulfil the target requirements (0.5 hPa, 15 km) would require a significant bigger
instrument.
Passive sensors cannot measure surface pressure precisely enough to meet the needs of future NWP.
2.3.3
Ozone
DIAL performed in the UV can be regarded as a potential candidate instrument for the measurement
of stratospheric ozone from space-borne platform. An instrument of moderate size (5 Wm²) would
fulfil the threshold observational requirements over a height range from 16 km up to 45 km with a four
wavelength instrument. Measurements in the UTLS and troposphere in all climates, however, would
require a relative large instrument which is not available from current technology. A big system is also
required for fulfilling the target requirements in the stratosphere.
Passive sensors do not meet the stringent requirements posed in this study because of their lack of
vertical resolution.
2.3.4
Carbon Dioxide CO2
Active remote sensing at 1.6 µm or 2.05 µm is regarded an appropriate means for CO2 monitoring
from space-borne platform. Favourable candidate instruments are based on IPDA approaches
incorporating pulsed or coded cw lasers. Both sensor types are expected to meet the stringent target
observational requirements even over the ocean at low target reflectivity. The analysis of the different
sources of systematic errors indicates that an overall accuracy of ~ 0.3 ppmv could be attainable which
would mark a major breakthrough in space-borne remote sensing of CO2. However, this accuracy can
only be achieved if the target altitude can be determined with an accuracy of about ± 3 m and if
scattering from aerosol and clouds can be sufficiently suppressed. Furthermore, both sensor types
require ancillary information on the surface pressure (±0.5 hPa) and the temperature profile (1 °K).
Conventional DIAL in the boundary layer is still possible but would require a significantly bigger
68
system. Range-resolved DIAL would require heterodyne detection. But with this instrument only the
threshold observational requirements can be fulfilled.
Currently no passive satellite systems exist for which it has been demonstrated that the observational
requirements drawn in this study can be met. There are, however, satellite systems which are expected
to meet at least the threshold requirement for column measurements in the near future (e.g.,
SCIAMACHY over land (requires more studies), the TIR nadir instruments AIRS, TES, and IASI, and
the NIR nadir instrument OCO (launch in 2007)). For the latter theoretical studies indicate that an
accuracy in the order of 1 ppmv is attainable with the three band concept.
2.3.5
Methane (CH4)
Similar to CO2, active remote sensing is regarded an appropriate means for CH4 monitoring from
space-borne platform. Favourable candidate instruments are IPDA with cw lasers and IPDA with
pulsed lasers. Both sensors operating at 1.6 µm are expected to meet the target observational
requirements. For the CW sensor auxiliary measurements of the target altitude (±10 m) would be
mandatory. Both sensors require additional information on the surface pressure (±1 hPa) and the
temperature profile (±1 °K). Conventional DIAL in the boundary layer is still possible with a
significantly bigger system. Range-resolved DIAL would require heterodyne detection. But with this
instrument only the threshold observational requirements could be fulfilled.
For CH4 currently no passive satellite system exists for which it has been demonstrated that the
requirements can be met. There are, however, satellite systems which have the potential to meet even
the target requirement for the column measurements in the near future (e.g., SCIAMACHY over land
(requires more studies), AIRS, TES, and IASI (launch 2005)).
2.3.6
Nitrous Oxide (N2O)
Active remote sensing of N2O from space can be performed with cw or pulsed IPDA instruments, but
the (technological) risk level increases for longer wavelengths in the IR. Both sensors are expected to
meet the threshold observational requirements. The target requirements, however, cannot be met with
a realistic system. For the cw sensor auxiliary measurements of the target altitude (± 10 m) would be
mandatory. Both sensors require additional information on the surface pressure (±1 hPa) and the
temperature profile (±1 °K). Conventional DIAL for boundary layer measurements would require an
unrealistic big DIAL instrument.
No passive satellite systems exist for which it has been demonstrated that the requirements for N2O
can be met. Concerning future systems such as IASI more studies are needed to reliably assess if the
requirements can be met.
2.3.7
Potential Synergy Between Passive and Active Sensors
Instruments for temperature soundings
Temperature DIAL operating in the oxygen A-Band cannot be regarded a potential candidate
instrument for space applications. We are not aware whether any other active sensor could provide a
better performance than the passive ones above. Synergistic aspects are not of practical relevance.
69
Instruments for pressure soundings
The temperature profile received from advanced passive sensors above would help to improve the
accuracy of surface pressure soundings with an active instrument. On the other hand, an active optical
sensor which uses ns pulses would provide additional information on aerosols and clouds also needed
to improve TIR retrievals of passive sensors. Particular benefits for NWP would be expected from colocated measurements using a high spectral resolution TIR sensor such as AIRS or IASI for the
temperature profile and an active sensor for the surface pressure.
Instruments for ozone soundings
Passive instruments cannot provide the spatial resolution demanded by the observational requirements
even in the stratosphere. Here a small size UV-DIAL could complement limb and nadir sounding
passive instruments in sensitive regions were strong ozone gradients occur by providing ozone data
with better accuracy and spatial resolution. In particular the simulation show that the critical area
where the chemically induced ozone depletion in the polar stratosphere occurs could be very well
captured by an ozone DIAL that performs best in the height range 13 km - 33 km. The third
harmonics of the Nd:YAG laser can serve as off-line for ozone DIAL in the stratosphere. This would
allow to strongly benefit from Aeolus and EarthCARE instrument development for a future ozone
mission. The off-line channel can be used to measure polar stratospheric clouds as well as aerosols
similar to EarthCARE, hence there is a strong link to former lidar missions. With the UV-Lidar
additional trace gases such as NO2 and SO2 could be measured. With the UV-Lidar additional trace
gases such as NO2 and SO2 could be measured.
Instruments for greenhouse gas soundings
An active IPDA instrument operating at 2.05 µm in the far wing of an absorption line would be
sensitive to sources and sinks in the low troposphere. Such a sensor could complement thermal
infrared sensors which have their maximum sensitivity in the upper troposphere in order to obtain
profile information on CO2. On the other hand IPDA would benefit from water vapour and
temperature information provided by passive TIR sensors such as AIRS and IASI. As the viewing
geometry of scanning IR sensors (45° cross track) and the footprint (25 x 25 km for IASI) would
significantly differ to nadir viewing Lidar sensors, the aerosol and cloud information obtained from
the latter instruments are expected to be not really relevant for IR sensors.
In the solar backscatter region synergisms between passive and active sensors are also not so obvious.
Both sensors would apply differential absorption methods at 1.6 µm and 2,05 µm. Even the sampling
characteristics are quite similar. OCO could benefit from complementary IPDA measurements at night
time and at high latitudes or at mid-latitudes in winter time where OCO cannot measure. This would
certainly help to constrain CO2 transport on a global basis.
As the light path of the OCO mission is significant different to an active system, the oxygen channel
of the OCO mission cannot be used to derive surface pressure information for improvement of IPDA
lidar measurements. Despite some better cross track coverage offered by the OCO sensor which
samples over a strip of about 10 km length the benefit of OCO for an active sensor is only marginal.
IPDA instruments for greenhouse gas observations could also complement activies sensors in the
optical and microwave spectral region for the measurement canopy heights.
70
2.4
Parameter Selection
2.4.1
Selection Criteria
In view of the selection of one or two parameter for further activities the performance of the active
instruments have been analysed in detail and compared to the performance of passive instruments. In
total, seven evaluation criteria have been identified worthwhile for the selection process. The first
criterion deals with the expected measurement performance of the active instruments for each
parameter as emerging from simulations. The second criterion highlights the relevance of such a
sensor for advanced sciences studies, examination of international conventions, advanced NWP as
well as the degree of innovation and contribution to the advancement of European Earth Observation
capabilities. The third selection criterion deals with synergistic aspects. Here common observation
scenarios are assessed where passive and active sensors are regarded to complement each other. The
fourth evaluation criterion focuses on programmatic aspects. Here heritage, risk areas, feasibility and
level of maturity have been assessed.
2.4.2
Potential Advantage of an Active Sensor for CO2
As a result of the assessment in this study it appears that CO2 is regarded the most promising
parameter for an active optical instrument worthwhile to be studied in more detail. Monitoring of CO2
is of high scientific relevance because it is the most important anthropogenic greenhouse gas also
banned by international conventions. Active remote sensing performed at 1.6 µm or 2.05µm using
Integrated Path Differential Absorption (IPDA) Lidar is expected to meet the stringent target
observational requirements over land and the ocean in all climates. Following potential advantages are
associated to a space-borne IPDA lidar for CO2:
•
IPDA lidar measurements do not suffer from spectral variability of aerosol scattering and
surface reflectance. For soundings in the 1.6 µm and 2.0 µm spectral regions, the spectral
separation between the transmitted on- and off-line wavelengths in the vicinity of a CO2
absorption line can be minimised such that the residual error caused by aerosol scattering can
be neglected. A typical wavelength separation between on- and off-line is only 0.0005 µm.
Opposed to this OCO uses a three channel approach at 1.6 µm, 2.05µm and 0.76 µm with a
wavelength separation of up to 1.3 µm, and hence suffers strongly from spectral variability of
aerosol scattering.
•
Both the light path and the length of the total column are precisely determined by using laser
pulses with a pulse width of a few ns. This allows the determination of the path length with an
accuracy of a few meters even under cloudy conditions. In case of a cw laser sophisticated
random modulation approaches have to be applied for range-resolved measurements. It is
expected that the accuracy of the path length determination will only slightly degrade
compared to a pulsed instrument. For the cw laser the spot size is about 1 m only. The small
spot size on ground will enable to collect useful data even in broken cloud fields. The cloud
fields and aerosol layers can easily be discriminated by active instruments with ranging
capability. As for aerosols, passive sensors such as OCO on the other hand are hindered by
cirrus scenes because scattering from clouds alters the effective path length and hence the total
column amount if not precisely corrected for. This correction procedure is primarily based on
accurate knowledge on the scattering function for cirrus clouds which is always not complete
and therefore erroneous. Errors in the correction of the CO2 column due to thin cirrus clouds
71
and aerosols can introduce an unacceptably large bias in the measurement because cloud and
aerosol occurrence may be seasonally and regionally dependent.
•
For a nadir pointing lidar mission the sampling characteristics is similar to the OCO mission.
Lidar, however, is not limited by the sun angle; hence the effective sampling of the
atmosphere is expected to be significantly larger than for the OCO mission. Moreover a
significant improvement over OCO can be expected for monitoring of surface fluxes at high
latitudes or mid latitudes during winter time. Also a lidar can sample during day and night.
•
OCO will frequently change its observation geometry between nadir, glint and calibration
mode. The different observation modes are latitude dependent such that a bias may be
introduced, which is not the case for the Lidar instrument that always sounds in nadir
direction.
•
As optically thin clouds can partly be penetrated by the lidar signals this would enable to
extract profile information on the CO2 columns measured above, in between and below cloud
layers. Moreover, the unique DIAL technique can be applied to the IPDA lidar measurements
which do not need any sensor calibration and thus would be a major improvement to passive
sensors. Under clear sky conditions profile information can be obtained by sounding at
different wavelengths within the line profile of a CO2 absorption line. Measurements in the far
wing of a line would weight the boundary layer in the low troposphere where the sources and
sinks of CO2 reside. Measurements near the line centre would weight the middle troposphere
whereas soundings in the line centre put the weight on the upper troposphere and lower
stratosphere.
•
IPDA instrument offering a small footprint of 50-200 m are advantageous over OCO
measurement, particularly in the convective boundary layer or under partly cloudy conditions.
Here an upper bound for the footprint is set by the size of the clear air spots. In the convective
boundary layer the diameter of the cloud holes is expected to be smaller than the boundary
layer height which is typically 1-2 km. This means that the footprint of OCO is just on the
edge to fulfil this requirement.
•
IPDA lidar measurements use narrow-band tuneable radiation sources. This allows for
selection of suitable wavelengths which can be optimised with respect to maximum
measurement sensitivity for the individual height range according to the vertical weighting
function, negligible water vapour and other trace gas interference, and low temperature
dependency.
2.4.3
Need for Auxiliary Data
Extraction of the CO2 dry air mixing ratio column from IPDA lidar measurements in the 1.6 µm and
2.05 µm spectral ranges requires additional information on meteorological parameters such as the
surface pressure as well as the water vapour and temperature profiles of the sounding atmosphere.
The requirement on the temperature profile is not critical. This data can be adopted from operational
NWP centres. An assumed bias of 1 °K on a climatologic basis which is a rather large value would
cause a systematic CO2 error of only 0.05 ppmv.
Stronger requirements exist for the water vapour profile particular in the boundary layer in the tropics
where most of atmospheric water vapour resides. Here a humidity bias of 5% would cause a 0.08
72
ppmv systematic error in the CO2 measurement. This data is also expected to be provided by the
operational NWP network.
Accurate knowledge of the surface pressure is a prerequisite for extracting the CO2 mixing ratio from
differential absorption measurements. The required level of accuracy for the surface pressure is of
comparable size as the needs for CO2 soundings. A 0.5 hPa surface pressure uncertainty results in a
measurement bias of 0.2 ppmv in the CO2 mixing ratio column. ECMWF will provide surface
pressure data in the near future with an accuracy of 0.7 hPa on a global base, which is close to the
required one [TN 110]. Over land, however, some gaps may remain, particularly in date sparse regions
and at locations where the elevation strongly changes. The latter is due to the strong link of surface
pressure to terrain height. A pressure variability of 0.7 hPa requires knowledge on the elevation of
about 6 meters.
Further requirements exist for the knowledge of the line parameters of the selected CO2-line. If
soundings are performed in the wing of corresponding lines, where the cross talk from bias in the
temperature profiles on the absorption cross section calculation is small this would not induce a
significant regional bias even when the line parameter are known at an accuracy level of only 1-2%.
Although a 1% deviation of the line strength parameter, for example, would give rise to an error of 3.8
ppmv this is regarded a constant bias which does not account for flux budget estimation.
3 Potential Instrument Concepts for CO2
Two instrument lines are found potentially suited for CO2 measurements form space-borne platform
using active instruments. Both sensor approaches are based on IPDA lidar measurements using hard
target reflection. The first candidate will incorporate modulated CW laser sources and makes use of
heterodyne detection. This sensor is preferable operated at 2.05µm where the weighting function is
more favourable for the measurement of sources and sinks of CO2 in the low troposphere. The second
candidate uses the principle of direct detection and nanosecond pulses to achieve the required
measurements performance. This sensor is preferably operated at 1.6 µm as detector degradation limits
its performance at longer wavelengths. For both sensors it is assumed that the auxiliary data on surface
pressure, water vapour and temperature are obtained from NWP centres with sufficient accuracy.
Furthermore it is assumed that the sensors will fly on a sun-synchronous polar orbiting satellite. An
orbit altitude of about 400 - 450 km has found to be adequate.
3.1
CW Heterodyne IPDA with Ranging Capability
The first alternative employs the heterodyne detection principle, where the interference signal of the
received echo with a local oscillator is detected. The down-converted RF frequency signal represents
the optical signal in amplitude and phase, and hence the information on the gas column density. Sunsynchronous as well as non sun-synchronous orbits can be selected. The latter require a somewhat
increased effort and reduce slightly the measurement performance. The demands on the satellite bus
are moderate except for the altitude control system. As current baseline a 430 km sun-synchronous
orbit is assumed.
73
3.1.1
Detailed Instrument Requirements
Apart from the issue of instrument size required to achieve the desired measurement accuracy the
following issues need to be considered for a heterodyne CW IPDA lidar:
1. Pointing stability
2. Diffraction limited beam
3. Multiplexing of on-line and off-line signals
4. Narrow band laser stabilised on an absolute frequency
5. High commonality of on-line and off-line receive chains
6. Compensation of motion Doppler
7. Determination of target altitude and suppression of atmospheric echos
Pointing stability
The heterodyne detection principle requires coherent imaging. With 0.8 m aperture and 430 km
altitude the laser footprint is about 2.1 m diameter (1/e2). This small spot size requires a consideration
of satellite rotation around orbit. During the round trip time of about 2.89 ms the footprint is shifted by
about 1.9 m. This effect has to be considered in the instrument optics by a slight pointing angle
between transmit and receive beams. In addition the small footprint requires a pointing stability to a
fraction of the footprint (<10%) during the round trip time. The resulting stability requirement on
pointing is 0.052°/s in pitch and roll.
Diffraction limited beam
The diffraction limited imaging on transmit and receive paths as needed to achieve an acceptably high
heterodyne efficiency implies an identical aperture size for transmitter and receiver and a monostatic
telescope design.
Multiplexing of on-line / off-line signals
The monostatic telescope design requires a beam separation between receive and transmit for the online and off-line channels. It is proposed to apply polarization beam splitting for separation of transmit
and receive beams and in-field separation for separation of on-line and the off-line channels.
As a consequence of the heterodyne reception principle even a perfect overlay of on-line and off-line
footprint would only in a statistical sense yield the same reflection properties because the two
wavelengths are sufficiently different to yield a different speckle pattern.
Narrow band laser stabilised on an absolute frequency
For a heteorodyne measurement principle the satellite velocity and the aperture size determine the
maximum detection bandwidth. For the proposed system this is in the order of 19.1 kHz. A higher
processing bandwidth increases the noise but does not provide additional information about the
radiated atmospheric volume. As a consequence the frequency stability of the transmitter within the
ground return time needs to be better than a fraction of the 19.1 kHz/3ms.
74
As the measurements are directly linked to a dedicated spectral line the transmit frequency has to be
absolutely stabilised over lifetime. This can be achieved by locking the transmit frequencies to the
spectrum of a gas absorption cell.
High commonality of on-line and off-line receive chains
The stability requirements for a diffraction limited system and constraints resulting form in-field
separation require a high commonality of the on-line and off-line receive chains. It should be aimed to
use a common detector chip for both channels. If this is feasible depends on the techniques for beam
splitting and the superposition of the local oscillator signal.
Compensation of motion Doppler
Doppler shifts resulting from pointing variations of the reflecting surface or vertical motion need to be
compensated. Pointing variations are sufficiently controlled by the attitude and orbit control system of
the satellite (see above). Vertical target motions are expected only over ocean and will be controlled
by a dedicated tracking loop.
Determination of target altitude and suppression of atmospheric echoes
A principle deficiency of cw IPDA systems is that there is no direct determination of the altitude
respectively the target range possible. While for ground reflection this information could be obtained
from a digital terrain model (DTM) reflections from cloud tops can not be evaluated.
In principle the missing data can be generated either by an additional laser altimeter or by applying
modulation techniques for ranging measurements. A simple laser altimeter however will provide a
much wider beam width than the IPDA lidar. This will cause uncertainties concerning atmospheric
water vapor impact and target stability. For a heterodyne system therefore a modulation technique is
favorably.
While background light is of little concern to heterodyne measurements owing to its narrow
bandwidth, echo signals from altitudes above the target interfere with the measurement objective,
because they are subjected to attenuation by a different column height. A basic cw-IPDA does not
provide any suppression of echoes from altitudes above the target. A modulated system however can
provide suppression according to its pulse response function.
The motivation for applying ranging techniques based on modulation schemes results from the
capability for using the measurement beam itself and for suppressing the impact of atmospheric
reflections.
3.1.2
Critical Instrument Sub-Systems
Wavelength control
For wavelength control it is assumed that an absolute frequency reference will be generated by locking
the cw laser source to the edge of the selected CO2 absorption line in a gas cell aboard the satellite.
The on- and off-line wavelengths to be transmitted into the atmosphere are directly produced via EOM
which shifts the frequency to the desired wavelength position.
75
Signal Modulation
A Bi-phase modulation technique was found appropriate for determination of the target altitude and
suppression of echo signals from the atmosphere above the hard target. This can be accomplished by
phase modulating the transmitted cw signal via EOM with a pseudo random sequence of sufficient
length to avoid ambiguities in the atmosphere. In principle this setup can be compared with the
transmission of the useful signal (the echo signal characterised by the speckle bandwidth) through a
spread spectrum (transmitted signal characterized by the inverse chip rate). The code locking loop
provides the altitude information (with a resolution determined by the chip rate) while the ratio of the
echo signal bandwidth to the spread bandwidth determines the attenuation of the unwanted
atmospheric echoes.
Since the transmitter nevertheless operates in continuous mode (or quasi continuous mode if also pulse
widths in the ms range are considered) the suppression capability depends on the autocorrelation
function of the (in tendency infinitely long) modulation signal (or more generally the ambiguity
function if range and Doppler ambiguity have to be considered simultaneously). A quasi continuous
operation e.g. very long (up to echo roundtrip time in the order of 2.6ms) pulses with pauses (50%
duty cycle) or truly continuous operation with a periodic modulation and simultaneous transmit and
receive operation can be considered.
In principle linear frequency modulation (LFM) or phase modulation can be considered alternatively.
However, using LFM would cause range-Doppler coupling which is a driver for pointing errors even
at moderate range-resolution.
Potential Laser Source
CW lasers which are based on Tm,Ho:YLF or Ho:YLF materials are promising candidate instruments
for a cw heterodyne IPDA sensor operating at 2.05 µm. The advantage of using a laser over an OPO
is the better control of the beam quality in the laser resonator. Furthermore, compact, reliable and
efficient cw pump sources are available without the need to develop an additional single-frequency cw
Nd:YAG pump laser. As an example a unidirectional Tm,Ho:YLF or Ho:YLF ring laser, which is
resonantly pumped with Tm-doped silica fibre lasers can be used. High-power fiber lasers are already
commercially available in Europe and similar fibre lasers have been space qualified. An overall wallplug efficiency of the cw IPDA transmitter of > 25% is expected by such a design [TN320].
Telescope
The telescope (with its primary) is certainly one of the most critical single components in the receiver
chain of a space-based cw- IPDA. The difficulty is to manufacture a telescope mirror in the 1-m class
providing diffraction limited performance which required an rms wavefront error of < 150 nm. At the
same time this mirror should have a low areal density to reduce costs. However, the technical bases are
already available. Astronomical telescopes such as the SIRTF/Spitzer (0.85m, 67nm rms) have already
been launched. Even larger space telescopes are planned in the near to mid-term future so that there is
no concern that an adequate telescope will not be available in due time.
Detector
Extended wavelength InGaAs PIN diodes are fully sufficient as detectors for a space-based cw- IPDA.
These detectors that are widely used in optical telecommunication are commercially available. They
76
have also successfully shown their capabilities in state-of-the-art coherent lidar systems at 2µm
wavelength.
3.1.3
Expected Measurement Performance of a Coded CW IPDA
Table 3.1-1 Expected sources of systematic error from CW IPDA; taken from [TN 230]
Expected Bias
[ppm]
Error Source
Assumption/Uncertainty
Atmosphere
Temperature
T-dependence of line parameter
0.035
Path length
0.080
0.5 °K
2 % line strength accuracy @0.5K
T-bias
1 % pressure shift [email protected]
T-bias
2% pressure broadening accuracy@
0.5K T-bias
2 % T scaling exponent accuracy
@0.5K
3 m (1/3 of raw range resolution)
Surface pressure
0.200
0.5 hPa
H2O mixing ratio
0.080
5% in the tropics
H2O line interference
0.003
20% in the tropics
0.004
ESA RMA
0.002
Suppression factor 29 dB
0.015
Suppression factor of 29 dB
0.100
< 0,3 MHz
0.210
relative accuracy of power monitoring
10-3 TBC
0.210
relative accuracy of heterodyne
efficiency calibration 10-3 TBC
0.401
RMS Sum of the above
0.05
0.05
0.025
Wavelength dependence
scattering
echoes from aerosol
echoes from cirrus clouds
of
0.085
aerosol
Transmitter/Receiver
Frequency drift
Power monitoring error
(on-,off-line)
Heterodyne efficiency
calibration (on-,off-line)
Error budget
Due to the narrow processing bandwidth (19 kHz) the following error sources are insignificant to bias
error in a PNCW IPDA:
•
•
Spectral purity of transmitter
Doppler Error (the Doppler-knowledge is necessarily better than the receiver bandwidth)
77
It is clearly seen from above error analysis that the driver of the systematic error is the uncertainty in
the surface pressure. The second critical parameter is to the frequency stability of the radiation source.
Table 3.1-2: Performance summary of coded CW IPDA; taken from [TN230]
Parameter
Threshold
Target
Horizontal resolution
50 km
50 km
Required precision including 50 %
cloud coverage
5 ppmv
1.5 ppmv
Required power aperture area product
over land to meet the precision level
0.14 Wm2
0.85 Wm2
over sea to meet the precision level
0.16 Wm2
0.95 Wm2
1 ppmv
0.1 ppmv
Random error
Bias
Maximum allowed bias
Expected bias
3.2
0.4 ppmv
Pulsed Incoherent IPDA Instrument
As a second alternative we propose a pulsed IPDA instrument. This instrument aboard the satellite
transmits two laser pulses at the wavelengths each with the on-line and off-line in nadir direction and
measures the intensity of the return signals backscattered by the earth surface and clouds. As in the
coherent case it is assumed that the satellite will fly on a polar orbit with an altitude of about 400 - 450
km. Sun-synchronous as well as non sun-synchronous orbits can be selected. The requirements on the
attitude control system are less demanding but nevertheless stringent.
The instrument employs the direct detection principle where the received optical signal is directly
converted to the photon current which results from the illumination of a photo sensitive detector area.
The measured number of photoelectrons from target reflection contains all information on the total
number of CO2 molecules within the atmospheric column.
3.2.1
Instrument Requirements
Random error characteristics
Different to the visible and ultraviolet spectral region where solar background radiation is the
dominant noise source, noise from detector limits the measurement sensitivity of a direct detection
IPDA sensor for wavelengths > 1.5 µm. In this case the random error scales inversely proportional to
the power aperture area product which is different to the case of a background noise limited
backscatter lidar
78
Frequency calibration and band width control
To avoid systematic errors an accurate control of the transmitted wavelengths is required. The
requirement to sound in the wing of the line rather then in the centre strongly has an impact on the
required frequency stability. For a low bias level the transmitted wavelength must be locked to a
reference with very high accuracy (< 0.3 MHz). Different to the strong frequency calibration issue the
requirement for the laser bandwidth (~ 30 MHz, FWHM) is more relaxed.
Spectral purity
In contrast to heterodyne detection a direct detection instrument strongly suffers from spectral
impurity of the emitted signal energy. If not corrected for which accurate knowledge of the spectral
shape of the transmitted signal energy would be required, a spectral purity of as high as 99.98% is a
prerequisite for a low bias level. By using a narrowband spectral filter (~ 1 GHz FWHM; resolving
power ~ 200.000) in the receiver channel would lead to a relaxation of this value to about 99.9%.
Another possibility to get rid of a large amount of spectrally impure radiation relates to filtering of the
outgoing radiation by means of dispersive optical elements such as prisms or gratings. This frequency
discrimination is similar to a spectrum analyser where the size of the slit in front of the detector is
replaced by the size of the footprint on the ground.
Pointing
Pointing off nadir would need compensation of the Doppler shift if the misalignment exceeds a value
of 0.067 mrad along track and 1 mrad across track. Strong requirements have to be considered for the
relative pointing between the transmitted on- and off-line pulses, particular in regions where strong
albedo jumps occur. It must be ensured that on- and off-line pulses overlap as much as possible on
ground.
Temporal separation
To avoid errors which may arise from different detector sensitivity it is proposed to use the same
detector for both on- and off-line measurements. This requires a minimum temporal separation of
~133 µs between on- and off-line pulses in order to avoid signal ambiguity within a 2* 20 km
atmosphere.
Raw data altitude resolution
The requirement for the path length determination which is about 3 meters facilitates the need of a
high sampling frequency of the reflected signal. The resulting spatial spread of the ground or cloud
echo is a function of the height variability of the distributed targets within the laser ground spot, the
laser pulse duration, and the inverse sampling frequency. If ns pulses are incorporated the maximum
sampling frequency typically determines the minimum spread of ground echos from flat terrain.
However, if buildings, ocean waves, trees or steep slopes in the mountains are hit by the laser pulses
on the ground the measured spread of the signal can easily exceed tens of meters. To avoid biasing in
the path length determination, particularly in regions with inhomogeneous target reflection, some
degree of over sampling is recommended.
79
Relative calibration of transmitted energy and receiver channel
In contrast to DIAL, an IPDA lidar using hard target reflection needs calibration of the relative
transmitted pulse energies of on- and off-line and the relative gains behind this measurement within
the telescope and the receiver channels. For the required measurement accuracy this task is very
demanding. Especially drifts in the telescope area are difficult to be monitored. Potential solutions are
a dedicated high resolution pointing monitor directly attached to the telescope or a cross calibration by
a cross coupling of the on-line transmitter with the off-line optical path and vice versa. Difficulties
result from the high pulse energies and the high accuracy for a stable long life switching device.
For a regular monitoring of the signal characteristics at the output of the laser sources it would be very
helpful to directly couple a small part of the laser beams into the receive chain and to measure the online and off-line power values with the standard detector. A difficulty of the scheme is the coupling
method of the two laser links to a single polarisation of the receive channel.
This method detects gain variations and amplitude variations caused by pointing drifts and other
effects. Slight spectral drifts of the laser sources caused by temperature variations or by aging can not
be detected. An adequate monitoring scheme for spectral purity has not yet been identified.
Compared to the heterodyne system the calibration tasks of the direct detection system are more
demanding. In the heterodyne system gain variations are directly monitored by the LO signals,
frequency variations are directly monitored by the receiver and spectral spectral purity is not critical.
3.2.2
Critical Instrument Sub-Systems
Laser Transmitter at 1.6µm for pulsed IPDA
For the laser transmitter in the 1.6 µm spectral region three solutions are possible. The first one is a
fiber pumped injection-seeded Er:YAG laser. The benefit of this approach is the compactness, the
reliability, and the efficiency of the pump laser. However, no high-average power, high-single-shot
pulse energy operation has been demonstrated yet by means of pumping with fibre lasers. From fibre
pumped injected – seeded Er: YAG laser a wall-plug efficiency of 6% is expected.
The second and the third approach needs an additional pulsed Nd:YAG pump laser. The second
approach is a gain-switched Cr4+:YSAG laser. The advantage of this system is that low requirements
are set to the beam quality and frequency stability of the pump laser. However, operation with high
single-shot pulse energy operation of Cr4+:YSAG has still to be demonstrated. From Nd:YAG laser
pumped gain-switched Cr4+:YSAG laser a wall-plug efficiency of 2% are expected.
The third approach is an efficient singly-resonant OPO (SRO) in a ring cavity. In contrast to the
second approach above, this approach requires a single-longitudinal frequency, pulsed Nd:YAG pump
laser with high beam quality. Furthermore, efforts are essential for maintaining a high beam quality.
All these approaches are in principle master-slave configurations so that an ML or injection-seeder is
needed. The availability of this low-power ML is not critical, as, for instance, DFB lasers are
commercially available in the wavelength region of interest from telecommunication research. A
remaining important issue is the operation of this seeder with high frequency stability as well as
frequency control of the SL cavity. From Nd:YAG laser pumped SRO a wall-plug efficiency of 3 %
are expected.
80
Frequency control
A gas absorption cell in combination with a frequency locking scheme can be used to stabilize a
reference signal on the line centre of the CO2 absorption line. To enable frequency locking of the
transmitted on-, and off-line wavelengths, this signals can be monitored by a Fabry-Perot
interferometer which measures their relative frequency shift with respect to the reference signal. Stable
frequency operation can be achieved by locking on-, and off-line to the difference frequency of the
Fabry-Perot interferometer. This design offers highest flexibility of wavelength selection and it allows
precise absolute frequency control for each of the selected wavelengths. Different frequency
stabilization techniques should be compared with respect to their performance and suitability for space
operation. Particularly, the dither lock-in, the side-of-fringe, and the Drever-Hall technique as well as
Doppler-free spectroscopy should be investigated.
Receiving telescope
The requirements on the telescope for a direct detection 1.6 µm IPDA are not too stringent as in
contrast to heterodyne detection no diffraction limited performance is required. Adequate telescopes of
the 1-m class have already been launched into space.
Spectral filter
Narrowband filter for suppression of spectrally broad lidar echos are required. Filter technologies
based on interference filters, Fabry-Pérot, or birefringent filters show a degree of maturity that
suggests their successful application to compensate for detrimental spectral properties of the laser
transmitter. Filters combinations of the above mentioned filter concepts have shown to achieve
resolving powers of the order of > 100.000 and to simultaneously provide decent acceptance angles.
These filters gain their heritage from solar astronomy where similar requirements are demanded.
Nevertheless, the prospect to relax the stringent requirements put on the spectral purity of the lidar
transmitter depends on both, the passband of the filter(s) and the detailed spectral behaviour of the
laser. Therefore, this issue needs further analysis based on appropriate models of the spectral shape of
the emitted radiation for the different transmitter concepts.
Detector
As detecting elements that combine highest maturity and adequate performance avalanche photo
diodes (APD) are considered. The devices have widely been used successfully for lidar applications.
Using state-of-the-art APDs that are either commercially available or have demonstrated their
specifications in prototype devices will allow to reach the target requirements of 1.6µm direct
detection IPDA for CO2 column measurements [TN240]. However, on the field of detector
technology there still seems to be room left for beneficial developments of near infrared detectors with
high responsitivity and low noise. New materials or concepts that particularly lead to a better noise
performance of NIR detectors may prove to be very beneficial for lidar applications. For example, the
pulse energy requirements for the laser transmitter may be relaxed. In this case, a low-energy (highrepetition rate) laser source would present an alternative that may prove to be superior to the base-line
concept in terms of cost, weight, and spectral or spatial properties. While range resolved sensing using
the DIAL technique currently suffers from the comparatively poor performance of near-infrared
detectors this may become feasible in the future when cutting edge technologies lead to improved NIR
detectors.
81
3.2.3
Expected Measurement Performance of Pulsed IPDA at 1.6 µm
Table 3.2-1: Expected systematic errors of a pulsed IPDA at 1.6µm; taken from [TN240]
Error source
Expected Bias
[ppm]
Assumption/Uncertainty
Atmosphere
Temperature
0.035
0.02
0.04
0.01
0.04
Path length
0.08
Surface pressure
0.2
0.5 K NWP model T-bias
2 % line strength accuracy @0.5K
T-bias
1 % pressure shift [email protected]
T-bias
2% pressure broadening accuracy@
0.5K T-bias
2 % T scaling exponent accuracy
@0.5K
2m
0.08
0.003
0.0035
0.5 hPa (NWP model bias TBD, see
TN 110)
5 % in the tropics (TBD)
20 % in the tropics
ESA RMA
0.008
0.1
0.08
0.09
15 MHz (FWHM)
0,3 MHz at 1.6 µm
99.9% with 1 GHz filter
5x10-4 accuracy (TBD)
0.09
5x10-4 accuracy (TBD)
Pointing/Timing
Doppler shift along track
Doppler shift cross track
Relative on-/off-line pointing
Temporal inter pulse separation
(on/off-line)
0.1
0.01
0.018
0.018
0.067 mrad pointing
1 mrad pointing
ln (βon / βoff) < 10-4
ln (βon / βoff) < 10-4
Error budget
0.32
geometically added
H2O mixing ratio
H2O line interference
Aerosol scattering
Transmitter/Receiver
Bandwidth
Frequency drift
Spectral purity
Rel. pulse energy calibration
(on-,off-line)
Rel. detection channel
calibration (on-,off-line)
In case of the direct detection IPDA instrument the total systematic error is estimated to be 0.3 ppmv
for CO2 measurements. The driver of the error budget is the uncertainty in the surface pressure.
Further critical parameter are connected to the frequency stability and the calibration of the relative
pulse energies and detection channels for the on- and off-line measurements. Pointing needs also
particular attention. In this error analysis we assumed that the spectral impurity can be sufficiently
suppressed by filtering either the outgoing or incoming radiation
82
Table 3.2-2: Performance summary of pulsed IPDA at 1.6 µm; taken from [TN340]
Parameter
Threshold
Target
Random Error
Horizontal resolution
Required precision including 50 %
cloud coverage
over land to meet the precision level
over sea to meet the precision level
Bias
Maximum allowed bias
Expected bias
50 km
3 ppmv
50 km
0.7 ppmv
0.16 Wm2
1.6 Wm2
0.7 Wm2
7 Wm2
< 1 ppmv (tbd)
0.1 ppmv
0.32 ppmv
83
3.3
Performance Synthesis
Table 3.3.1 gives an overview on the assumed risk level for the two instruments. While both sensors
are expected to meet the target observational requirements with a moderate size instrument it is still
challenging to fulfil the stringent systematic error budget which should be considerably less than the
random error budget. Especially, the stringent requirement for the laser spectral performance of the
pulsed direct detection sensor needs particular attention.
Tab 3.3.1: Performance synthesis and risk level; taken from [TN240]
Parameter analysed
Random error
Risk level for
Pulsed direct
detection IPDA
Lidar
Risk level for
PN CW IPDA
lidar
Comments
Low-moderate
Low-moderate
Risk level for achieving the target
requirements
Systematic error budget Moderate-high
Moderate-high
This assumes that the expected error
budget of 0.27 or 0.4 ppmv lies in
between
target
and
threshold
requirements. The error is driven be
the ground pressure uncertainty
smallb
Driven by the target requirement over
water
Power aperture
product
area mediuma
Laser spectral
performance
high
Moderate
Risk level of required bandwidth,
frequency stability and spectral purity
Path length
determination
low
Moderate
Risk level of applied technology for
achieving
Aerosol/cloud
interference
low
Moderate
Pointing requirement
Moderate
Moderate
a
medium correspondes to values with 2 < PA < 10 Wm2
b
small correspondes to values with pa < 2 Wm2
4 Technology Status, Risks and Limitations
4.1
Transmitter Sub-System
Table 4.1-1 summarises potential candidates and their limitations of transmitter sources at 1.6 and 2.05
µm spectral region. In reference to the maturity level for the various radiation sources shown in the
84
table below, some preferences are given to the 2.0 µm spectral region where the benefits from past and
current instruments are expected to be largest.
Tab. 4.1-1 : Risk level of transmitter sources for pulsed and cw IPDA taken from [TN320]
4.2
Receiver Sub-Systems
4.2.1
Telescope
The telescope itself presents no challenges: telescope primaries of the same diameter class as needed
as well as the optical structure and accommodating the volume and mass of this structure will be done
by use of the SiC technology which gives lower mass and higher stiffness than conventional
technologies. This technology was already used for existing space telescopes or demonstrators (for
example the FIRST demonstrator (φ=1.35m)) for future mission.
4.2.2
Blocking Filter
The required filters present no technological risk. The use of optical filters will be restricted to the 1.6
µm direct detection IPDA concept in order to overcome possible detrimental spectral properties of the
lidar transmitter. While the filters are anticipated to rely on conventional designs the exact layout must
remain open and should undergo further analyses as this strongly depends on the properties of the laser
radiation.
85
5
Summary of Baseline Recommendation
Table 5.1 summarises the overall instrument comparison in terms of risk level estimates for the
various trade-off criteria. The table indicates that both instruments show comparable risk level
estimates. Therefore, we recommend considering both instrument lines pulsed IPDA direct detection
at 1.6 µm and CW IPDA at 2 µm for follow-on activities on CO2. But we would like to point out, if
better near IR detectors at 2.0 µm become available the direct detection IPDA should also be
investigated at 2.0 µm on instrument level.
Table 5-1: Summary of baseline recommendations; taken from [TN340]
Trade-off Criteria
Performance analysis
Technical Aspects
Transmitter
Rx Optics & Detector
Wavelength control
Opto-mech&thermal
Calibration
On-board Signal Proc.
Resource Demands
Risk Level
Random modulated IPDA
at 2 µm
Compliant with target
requirements
Pulsed direct detection
IPDA at 1.6 µm
Compliant with target
requirements
medium-high
low
medium
medium-high
medium
low-medium
medium
medium-high
high
medium
low-medium
medium-high
low
high
86
6 Study Conclusion and Recommendations
6.1
Summary
In this study the background for the definition of a future space-borne lidar system capable to monitor
greenhouse gases CO2, CH4 and N2O as well as ozone (O3) and also pressure (p) and temperature (T)
are investigated in detail. This includes the definition of the observational requirements for the
individual parameters, an estimation of the measurement performance of active optical instruments,
and a comparison to the performance of existing and planned passive instruments.
After trading-off pros against cons, the greenhouse gas CO2 was finally selected as potential candidate
worthwhile to be further studied on the instrument and mission levels. The method of Integrated Path
Differential Absorption (IPDA) lidar using hard target reflection in the near IR has been found
potentially suited for fulfilling the target observational requirements for the random error.
The observational requirements for CO2 are harmonised with the results from the parallel IPSL study.
Two sets of requirements are established for measurements at 1.6 µm and 2.0µm. The availability of
required surface pressure data with an accuracy <1 hPa was confirmed by ECMWF. More realistic
geophysical data on surface reflections over land and the ocean were implemented for an improved
simulation of the surface pressure induced CO2 measurement errors.
Two baseline instrument concepts are proposed in the second part of the study. The first one uses a
modulated cw laser as transmitter and a heterodyne detection principle in combination with spread
spectrum techniques to achieve the range-information directly from IPDA measurements. The second
system uses a pulsed laser source which emits nanosecond pulses into the atmosphere and a direct
detection receiver. For each sensor the availability and status of the critical subsystems we reviewed.
Finally, the various sources of measurement errors in connection to the specific sensor concept were
critically reviewed. A systematic error of 0.3 ppmv for the direct detection and 0.4 ppmv for the RM
cw system was found. Although the direct detection instrument is regarded more demanding, both
instrument lines are found worthwhile to be further studied.
6.2
Recommendations on Follow-on Activities
Despite the achievements obtained in this study several questions remain which first have to be
answered prior to an optimisation of single hardware elements.
6.2.1
Validation of Statistical Albedo Variations
The low surface reflection over water is the design driver for sizing the instrument. If sources and
sinks over land are the main focus of the mission, then the required power aperture product could be
relaxed by a factor of ten. However, any further optimisation of a future CO2 mission with respect to
maximum scientific merits requires also dedicated observations over the ocean since 2/3 of the Earth’s
surface is covered by water.
The definition of a CO2 measurement system depends to a large extend on the values of the target
albedo and their statistical variations during overflight. The statistics and even the magnitude of the
surface reflectivity over land (albedo jumps) and water (specular reflections) are not clear for a nadir
87
viewing instrument operating in the 1.6 or 2 µm wavelength. As such data depend on target
characteristics as well as on spot size and wavelength it would be very useful to arrange validation
measurements by flying an unmodulated laser together with a roughly representative telescope and to
monitor the return signal with a heterodyne receiver.
A significant error source of IPDA measurements is also associated to difficulties in path length
determination (spread of the ground return, etc) over 50 km sampling interval, especially over
inhomogeneous terrain. It should be investigated if ranging measurements can be realised with
adequate effort together with the albedo measurements.
6.2.2
Error Analysis of Supporting Data
A target bias level of 0.1 ppmv is hard to achieve even with an active system. Here a consolidated
threshold value for the upper bound of the systematic error is required. A small bias strongly impacts
the requirements for the spectral performance and frequency stability of the used laser source,
especially for the direct detection system.
From differential absorption measurement the column number density of CO2 is obtained. For
calculation of the mixing ratio, surface pressure data is required from an external data source. This is
an additional source of error and we recommend an analysis of the surface pressure error in ECMWF
data in more detail. Moreover, it should be investigated whether the observational requirements for the
dry air mixing ratio can be reformulated in the total column of the number density of CO2.
In addition the validation scenario using in-situ data and co-located measurements from other mission
should be analysed. This would include a requirements definition for the cross validation with other
sensors.
6.2.3
Breadboarding Activities
For the cw IPDA instrument @2.068 µm the stability of the heterodyne efficiency should be validated.
As most error contributions depend on details of the design the performance only can be validated by
hardware testing. For this purpose the core elements of the instrument except beam expander should
be breadboarded in a space representative manner and should be operated with a reflector plate a few
meters apart.
The most critical issue of the heterodyne concept is the realisation of the 3-niveau laser with an M2
close to 1 and a sufficiently low spectral short term stability. None of the other available laser sources
(e.g. Meteor from CTI, LISA laser, etc.) directly meets the requirements on wavelength, output power,
frequency stability, beam quality and space compatibility.
Especially the beam superposition and the spectral stability strongly depend on design details. At least
the short term stability (Allan variance), the beam jitter and the beam quality parameters should be
validated by testing. In addition all major parameters affecting the beam superposition should be
varied. Mainly concerned are thermal effects and micro-vibrations.
The detailed optimisation of H/W components should be realised after successful realisation of 6.2.1 6.2.3.
88
Direct Detection System
The most critical technical parameter for the direct detection instrument is the spectral purity of the
measurement signal. For the radiation source any value > 99.9% at the wavelengths 1572, 1580, and
1610 nm would be highly desirable. We expect that further improvement of the spectral purity by
about one order of magnitude might be possible by spectral filtering either the outgoing laser pulses or
the received signals.
The spectral characteristics of the envisaged radiation source and the transmit chain filter should be
estimated by performance simulation. The effects of error contributions on the measurement should be
analysed. Different filtering techniques should be investigated and the most promising one should be
selected for a breadboard phase. Within this phase the theoretical values should be confirmed by longpass absorption measurements. At the end of the study a spectral purity value of ~ 99,98% should be
demonstrated. Nevertheless, the prospect to relax the stringent requirements put on the spectral purity
of the lidar transmitter depends on both, the passband of the filter(s) and the detailed spectral
behaviour of the laser. Therefore, this issue needs further analysis based on appropriate models of the
spectral shape of the emitted radiation for the different transmitter concepts.
Heterodyne System
Besides the laser source there are a few other elements needing a more detailed investigation: The
phase modulator probably will be realised with an EOM and for the system a complete carrier
suppression is desired. As the modulation depth depends on an absolute value of high voltage it is
doubtful that the modulation depth can be kept stable over lifetime. It is therefore recommended to
establish a control loop by measuring the remaining carrier by mixing the signals in front and after the
EOM and to control the modulation voltage. For risk reduction this device should be breadboarded.
Another topic of the modulator is the switching time for phase modulation. No critical effects are
expected but the switching times, i.e. the output spectrum of a fairly representative modulator should
be validated.
89
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water vapor measured by an airborne DIAL, J. Geophys. Res. 104, 31,351-31,359
7. ESA ITT AO/1-3654/00/NL/DC, “Evaluation of Spaceborne Differential Absorption Lidar
for Water Vapour, Ozone and Carbon Dioxide”: Technical Note 2a, “Review of Instrument
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97
8 Documents and Technical Notes Related to the Study
Statement of work
[SOW-2003] ESA/ESTEC, Concept study for advanced lidar instrument, Statement of work,
EOPP-FP/2003-09-828, Issue 1, 17.10.2003
Technical proposal written in response to ESA RFQ/3-10880/03/NL/FF:
[PROP-DLR-2003] DLR and partners, Requirements Definition for Future DIAL
Instruments, Technical Proposal, Reference: 3 472 749, 12. November 2003.
Observational requirements documents
[TN 110] Analysis and Definition of Observation Requirements for Pressure and
Temperature Measurements to be used In Numerical Weather Prediction (NWP),
by Hans Volkert and Gerhard Ehret, DLR
[TN120] Analysis and Establishment of Threshold and Target Observation Requirements for
Space-Based Measurements of CO2, CH4, and N2O, by Martin Heimann, MPJ, Sander
Houweling, SRON
[TN 130] Ozone Requirements, by Jos Lelieveld, MPM
Instrument Concepts and Performance Analyses documents
[TN 210] Techniques, instrument requirements and performance of space-based passive
systems for measuring T, p, and GHG, by Michael Buchwitz, Rüdiger De Beek, and Heinrich
Bovensmann, IUP, Bremen
[TN 220] Review of Lidar instrument concepts and the physics of measurement of active
techniques for T, p, O3, and GHG monitoring, by Martin Wirth, DLR and Andreas Behrendt,
UHOH
[TN 230] Sensible instrument and mission parameter, by Uwe Kummer and Susanne Nikolov,
ASG
[TN 240] Performance models of active techniques and results from preliminary performance
simulations, by Christoph Kiemle and Gerhard Ehret, DLR
[TN 250] Synthesis, techniques comparison and requirements for lidar systems measuring p,
T and GHG profiles/columns, by Gerhard Ehret, DLR
98
Instrument analysis and definition, study evaluation
[TN 310] DIAL instrument analysis and definition, by Uwe Kummer and Hans-Reiner
Schulte, ASG
[TN 320] Laser Transmitter concepts and technology trades, by Volker Wulfmeyer, UHOH
[TN 330] Receiver concepts, by Andreas Fix, DLR
[TN 340] Study Evaluation, by Gerhard Ehret, DLR
99

Requirements Definition for Future DIAL Instruments - Emits

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