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Acquisition of Bidirectional Reflectance Data
Using the Swiss Field-Goniometer System (FIGOS)
Stefan Sandmeier, Willy Sandmeier*, Klaus I. Itten, Michael E. Schaepman, and Tobias W. Kellenberger
Remote Sensing Laboratories, Department of Geography, Univ. of Zurich,
Winterthurerstr. 190, CH-8057 Zurich - Switzerland
FAX: +41 1 362 52 27, TEL: +41 1 257 52 46, EMAIL: [email protected]
* Max Lehner & Co AG, Maschinen- u. Apparatefabrik, Suhrerstr. 1, CH-5722 Gränichen - Switzerland
FAX: +41 64 31 21 80, TEL: +41 64 31 15 12
ABSTRACT: Most natural objects expose a non-Lambertian behaviour, i.e. the reflectance characteristics
vary with changing illumination and viewing geometry. Numerous models have been developed to describe
bidirectional reflectance effects and to involve them in the preprocessing of remote sensing data. However,
only a few ground reference data is available to validate these models, and much of it is derived from laboratory experiments. In order to obtain bidirectional reflectance factor (BRF) data of naturally illuminated targets
a transportable field-goniometer system (FIGOS) has been developed. It is operated together with a GER3700 spectroradiometer. The goniometer consists of an azimuth full-circle and a zenith semi-arc of 2 m radius
each. It enables to observe a target in the centre of the hemisphere from any desired viewing direction. In a
field-campaign the bidirectional reflectance of a plane meadow is measured over the hemisphere within 15
minutes in a resolution of 15° and 30° in zenith and azimuth direction, respectively. A Spectralon panel
measured periodically during the BRF-data acquisition allows for normalisation of the changes in atmosphere
and solar irradiance. The resulting 66 BRF-data are used to model the bidirectional reflectance distribution
function (BRDF) of the target. Special emphasis is given to the solar principal plane where the BRDF-effects
are most pronounced. The obtained results clearly show the non-Lambertian reflectance characteristics of the
target.
INTRODUCTION
Applying radiometric corrections to satellite data,
in particular over rugged terrain, the variations in the
sun-sensor-target geometry have to be accounted for
(Sandmeier et al., 1994) (Sandmeier, 1995). A major
drawback in such radiometric correction models is
often the Lambertian assumption. Although many
authors have shown that most of the natural surfaces
expose anisotropic reflectance distributions (Kriebel,
1978), (Deering and Eck, 1990) the impact of the
sun-sensor-target geometry on the bidirectional
reflectance distribution function (BRDF) is often
neglected. Radiative transfer codes like the 6S
(Vermote et al., 1994) have been updated in order to
couple bidirectional effects in modelling the satellite
signal. Among the BRDF-models used are those of
Minnaert (1941), Hapke (1981) and Verstraete
(1990). In order to validate such BRDF-models the
bidirectional reflectance characteristics of the targets
under considerations have to be known first. Also
the development of sensors with off-nadir tilting
capabilities like ASAS (Irons et al., 1991) and MISR
(Diner et al., 1991) emphasises the need of BRDF
ground reference data in a hyper-spectral resolution.
Rather few BRF-data has been acquired in recent
years, however, and most of it is out of laboratory
experiments neglecting atmospheric and environmental effects.
In order to obtain bidirectional reflectance factor
(BRF) data under natural atmospheric and illumination conditions a transportable field-goniometer has
been constructed. FIGOS (field-goniometer system)
allows for measuring the target reflectance over the
hemisphere by user-defined viewing angles. It is
operated together with a PC-controlled GER-3700
spectroradiometer which covers the spectrum
between 400 and 2500 nm in 704 bands with a resolution of 1.5 nm (400-1050 nm), and 8.4 nm (10502500), respectively.
TECHNICAL FEATURES
The field-goniometer has been built by Willy
Sandmeier under support of Fa. Lehner & Co AG,
and in co-operation with the Remote Sensing Laboratories at the University of Zurich (Fig. 1). The
planning and construction were performed in 1994
and required about 700 working hours.
The goniometer consists of three major parts: a
zenith arc, an azimuth arc, and a motor driven sled
on which the GER-3700 spectroradiometer is
mounted (Fig. 2). All parts are of coated aluminium.
The zenith arc of 2 m radius is constructed out of
three 40x40x3 mm profiles following the technique
used for cranes. In spite of its small weight of 48 kg
it is very stable due to the construction technique.
For reasons of transportation the zenith arc can be
separated into two parts (Fig. 3) which are assembled before mounting on the azimuth rail (Fig. 4).
The azimuth arc consists of twelve sockets on
which a rail of 2 m radius for the zenith arc is
mounted (Fig. 3). It weights about 150 kg. For
transportation the arc can be split into the twelve
basement parts, but in order to save time a separation into four quarters is more convenient. Four
interconnected wagons on the azimuth rail allow to
rotate the zenith arc by 360° (Fig. 1). Two of them
serve as a base for the zenith arc, and two prevent it
from blocking. The ball-bearing of the wagons on
the azimuth rail are arranged in a way that the zenith
Fig. 1
arc cannot be removed without dismantling the
azimuth circle. A prop linking the centre of the
zenith arc with the azimuth rail serves as a further
stabilisation and helps to guide the cables. As a
consequence the field-goniometer can be used in
sloped terrain as well.
The zenith arc is mounted eccentrically on the
azimuth rail in order to prevent from shadowing the
target when measuring in the solar principal plane.
The minimum distance between the centre of the
target and the shadow of the zenith arc aligned in the
solar principal plane is 14 cm. As the field of view
of the GER-3700 is approx. 3°, measurements
within the solar principal plane are free from
shadow of the zenith arc.
FIGOS always points to the same spot in the
centre of the hemisphere, i.e. all hemispherical data
corresponds to the same target in the centre of the
azimuth arc (Fig. 5). Naturally, the footprint of the
sensor's field of view (FOV), which is a circle of
10.5 cm diameter at nadir position, is distorted to an
ellipse at view zenith angles larger than 0°. Fig. 6
depicts the distortion of the FOV. a and b are the
backward and forward major semi-axis of the
elliptical FOV following equations (1) and (2):
a = r ⋅ sin(θ v ) − h ⋅ tan  θ v −

FOV 
2 
The field-goniometer system (FIGOS) during the field-campaign of 25 July
1995 on the campus of the University of Zurich
(1)
b = h ⋅ tan  θ v +

Light source
(Sun)
Sled with
radiometer
Zenithal arc
2m
Target
Azimuthal arc
4m
Fig. 2
Concept of the field-goniometer
Fig. 3
Assembling of the azimuth arc; in the
background the separated zenith arc can be
seen
Fig. 4
Mounting of the zenith arc
FOV 
− r ⋅ sin(θ v )
2 
(2)
where r is the radius of the hemisphere, h the height
of the radiometer above the target and θv is the view
zenith angle.
According to Fig. 6 the footprint of the FOV never
touches the azimuth rail, even for extreme view
zenith angles of ±75°.
Freely placable labels on the zenith arc allow for
an automated positioning of the spectroradiometer.
It is also possible to drive the sled-motor manually
from a remote control unit to any desired position on
the zenith arc. By default the labels are set every 15°
resulting in 11 measurements with zenith angles
-75°, -60°, -45°, -30°, -15°, 0°, 15°, 30°, 45°, 60°,
and 75°. The positioning precision on the zenith arc
is within ±0.2°. The geometric precision of the
zenith arc is depicted in Fig. 7 with the help of a
laser moving over the zenith arc on plane ground.
The deviation of the laser spot, representing the
FOV's centre, shows values within ±3.5 cm. It is
introduced by mechanical problems in bending the
aluminium profiles.
The roundness of the zenith arc is nearly perfect
showing deviations of the laser spot from the centre
within ±1 cm between -60° and + 60°.
The azimuth view angle is given by a scale
engraved in the azimuth basement. At the current
status the zenith arc is positioned manually with the
help of a pointer, and a brake fixing the position of
the zenith arc (Fig. 8). As it is reasonable to position
the zenith arc within a tolerance of ±0.7 cm, a
position accuracy of ±0.2° can be assumed for the
azimuth arc. The azimuth arc is almost perfectly
round. A laser spot pointing vertically from the
centre of the zenith arc on the ground moves less
than ±1 cm when the zenith arc is rotated.
By default an increment of 30° is set on the
azimuth arc resulting in 6 measurement profiles,
each containing 11 measurements on the zenith arc.
Thus to cover the full hemisphere 66 measurements
are needed.
The sled with the spectroradiometer mounted
weights 20 kg. It is driven by a 24 V DC braking
motor. A precision chain serves as a guideway for
the 3/8'' cogwheel.
Fig. 5
Goniometric measurements with FIGOS and
a GER-3700 spectroradiometer
length of FOV [cm]
Fig. 8
FOV semi-axis a
60
3
The motor velocity is set to 5 m/min, i.e. it takes
about 6 seconds to move the sled by 15° to the next
position on the zenith arc. The scan time of the
GER-3700 is about 50 ms and therefore negligible
compared with the motor's velocity. A full hemisphere is covered in approx. 15 minutes, including
time for repositioning of the zenith arc, and for
actually taking the measurements.
The total weight of the field-goniometer amounts
to about 230 kg. The maximum weight of a single
part is 61 kg which is the zenith arc with the sled to
be mounted on the azimuth rail. Therefore it is
possible for two people to transport and assemble
the goniometer. Less than 2 hours are needed for the
set-up of the goniometer.
2
FIELD EXPERIMENT
FOV semi-axis b
a+b
40
20
0
0°
15°
30°
45°
60°
75°
90°
view zenith angle [°]
Fig. 6
Brake at the azimuth arc
80
Major semi-axis of the elliptical FOV a
and b according to equations (1) and (2)
cm
-75°
•
-60° •
-45°
•
1 • -30°
•-15°
0°
-3
-1 +15°
-2
+30° ••
+45° •
+60°
•
1
-1
2
3
orientation
of zenith arc
-2
•
+75° •
Fig. 7
-3
Trace of a laser moving over the zenith
arc
Experimental BRF measurements were taken from
a plane meadow on flat terrain on the Zurich University campus (47°24' N / 8°33' E ) at 25 July 1995.
The sky was cloudless but rather hazy. The target
consists of mainly grass and clover.
A Spectralon panel of 10x10 sq.in. was used for
measuring the solar irradiance at each nadirposition. The resulting 6 panel measurements of a
full hemispherical data set allow to monitor the solar
irradiance during data collection. In the near future
changes in atmospheric conditions and solar
irradiance will be monitored with a Reagan sunphotometer (Ehsani and Reagan, 1992).
The hemispherical data depicted in Fig. 9 is
acquired between 13:19 and 13:34 UT. The sun's
zenith angle changed from 34.9° to 36.8°. The 6
BRF-measurements at nadir-position are constant
within ±5% in the 551 nm band. The integration and
plotting of the BRDF data are performed using the
interactive data language IDL. The polar coordinates
of the view zenith angles are first transformed onto a
x-y plane as a vector of unit length. Then the BRF
data are interpolated using a Delaunay triangulation
in order to obtain a regular grid which can be
displayed as a BRDF. The positive zenith angles
represent measurements with the sun at the back of
the radiometer.
The shape of the BRDF is rather varying over the
spectral range obtained, particularly between the
visible/near-infrared (a) and (b) and the farther nearinfrared range (c) and (d). Therefore the influence of
the wavelength on the shape of the object's BRDF
cannot be neglected. Surprisingly, the meadow does
not expose a strong forward and backward scatter
component as could be expected from the BRDFdata of a similar target (Sandmeier, et al., 1995).
Typically, however, are the high reflectance values
at large zenith angles.
Details of the object's BRDF in the solar principal
plane (SPP) and the temporal changes are shown in
Fig. 10 for the vis and nir spectral range. The SPP
data in Fig. 10 taken under a sun's zenith angle of
35° corresponds to the BRDF-data in Fig. 9. It
shows the strongest deviation from a Lambertian
reflector. The SPP data of the measurements taken
under higher sun positions are less pronounced. The
low reflectance values at view angles +30° are
influenced by the shadow of the radiometer. The
break at -30°, however, is a true bidirectional effect.
The BRF-data taken under a sun's zenith angle of
35° vary in the SPP at 551.0 nm between 3.3% at
-30° and 11.5% at +75°. At 850.1 nm the variation
lies between 24.2% at -30° and 53.5% at -75°. The
reflectance characteristic of the meadow is therefore
highly dependent on the view zenith angle and confirms the non-Lambertian assumption.
Fig. 9
BRDF of a meadow canopy at (a) 551 nm,
(b) 850 nm, (c) 1250 nm and (d) 1648 nm,
for sun' zenith angle 36°
Fig. 10 BRF-data acquired in the solar principal plane at sun's zenith angles 28°, 29°, 30° and 35° (from top
left to bottom right)
CONCLUSION
A unique field-goniometer system (FIGOS) has
been constructed to perform field measurements of
the bidirectional reflectance over the hemisphere
under natural conditions. FIGOS is operated with a
GER-3700 spectroradiometer and provides hyperspectral BRF-data within the reflective range
between 400 and 2500 nm in 704 bands. The total
weight of the FIGOS-instrument amounts to 230 kg,
the radius of the hemisphere is 2 m. The set-up time
is approx. 2 hours. A complete hemispherical measurement with a resolution of 15° and 30° in zenith
and azimuth (66 measurements) is done in about 15
minutes. In contrary to other goniometers, FIGOS
points at the same target in the centre of the hemisphere for all viewing angle positions. It provides
essential information on the BRDF of objects in
order to overcome the Lambertian assumption.
Extensive field experiments combined with
controlled laboratory measurements will form the
base for a validation of the BRDF-models used in
radiometric preprocessing steps. A system for
storing and analysing the bidirectional data is in
progress (Schaepman et al., 1994) and the BR(D)Fdata will be integrated in the physically-based
radiation model described in Sandmeier (1995).
ACKNOWLEDGEMENTS
This study is supported by "Stiftung für wissenschaftliche Forschung an der Universität Zürich".
The support of the members of RSL in particular of
C. Müller in performing the field campaign is
greatly appreciated.
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