Global Positioning System: How it Works

Global Positioning System:
How it Works
Featuring contributions from:
Peter H. Dana, via The Geographer's Craft Project
Jeff Hemphill
Kevin Knight
Chris Rizos, U. New South Wales
What is GPS?
• GPS is a Satellite-based Navigation
System
• Generic term: Location Determination
Technology
• Funded by and controlled by the U. S.
Department of Defense (DOD). Originally
NAVSTAR
• Designed for military tasks: Dual Use
• Civil uses now far exceed military
One of three such systems
• GLONASS
• Galileo
• GPS Blocks I and II
– Block III and beyond
– Many new LDTs, some using GPS
The GPS System has the
following components
• Space Segment
• Ground/Control Segment
• User segment
•Space Segment
• Space vehicles (SVs) send radio signals
from space.
• The nominal GPS Operational Constellation
consists of 24 satellites that orbit the earth
in 12 hours 20,200km out in space.
• Often more than 24 operational satellites as
new ones are launched to replace older
satellites.
What the Satellite Does
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Radio transmitter
Pseudo-random code
Time
Position & ephemeris
The constellation
24 satellites
6 orbital tracks
4 satellites/track
12 hour orbits
Orbits and Ground Tracks
00:00:00 9/29/98 to 00:00:00 9/30/98
Ascending and descending ground tracks (2
orbits) for 27 GPS satellites plotted over a 15
degree graticule.
Complete global coverage, including the poles.
The Space Segment II
• The orbit altitude is such that the satellites repeat
the same track and configuration over any point
approximately each 24 hours (4 minutes earlier
each day).
• There are six orbital planes (with nominally four
SVs in each), equally spaced (60 degrees apart),
and inclined at about fifty-five degrees with
respect to the equatorial plane.
• This constellation provides the user with between
five and eight SVs visible from any point on the
earth at any time
Control Segment
• The Control Segment consists of a system of tracking stations
located around the world.
• GPS Master Control and Monitor Network located at Schriever Air
Force Base in Colorado
• These monitor stations measure signals from the
SVs which are incorporated into orbital models for each satellite
• The models compute precise orbital data (ephemeris) and SV
clock corrections for each satellite
• The Master Control station uploads ephemeris and clock data to
the SVs as they pass over
• The SVs then send subsets of the orbital ephemeris data to GPS
receivers over radio signals
How it works: Trilateration
• Four satellites are required to compute the four
dimensions of X, Y, Z (position) and Time.
• GPS receivers are used for navigation, positioning, time
dissemination, and other research.
• Navigation in three dimensions is the primary function of
GPS.
• Navigation receivers are made for aircraft, ships, ground
vehicles, and for hand carrying by individuals.
GPS Navigation
• Precise positioning is possible using GPS
receivers at reference locations providing
corrections and relative positioning data
for remote receivers.
• Surveying, geodetic control, and plate
tectonic studies are examples.
Positional solutions: d = c * t
GPS Positioning Services
• Specified In The Federal Radionavigation
Plan
• PPS: Precise Positioning Service
• SPS: Standard Positioning Service
Precise Positioning Service
(PPS)
• Authorized users with cryptographic
equipment and keys and specially equipped
receivers use the Precise Positioning
System.
• U. S. and Allied military, certain U. S.
Government agencies, and selected civil
users specifically approved by the U. S.
Government, can use the PPS.
PPS Predictable Accuracy
• 22 meter Horizontal accuracy
• 27.7 meter vertical accuracy
• 200 nanosecond time (UTC) accuracy
Standard Positioning Service
(SPS)
• Civil users worldwide use the SPS without
charge or restrictions.
• Most receivers are capable of receiving and
using the SPS signal.
• The SPS accuracy can be intentionally
degraded by the DOD by the use of Selective
Availability.
• SA turned off (permanently?) May 1st, 2000
Nominal SPS Predictable
Accuracy with SA
• 100 meter horizontal accuracy
• 156 meter vertical accuracy
• 340 nanoseconds time accuracy
Receiver manufacturers use
various accuracy measures.
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Root-mean-square (RMS) error is the value of one standard deviation
(68%) of the error in one, two or three dimensions.
Circular Error Probable (CEP) is the value
of the radius of a circle, centered at the actual position that contains
50% of the position estimates.
Spherical Error Probable (SEP) is the spherical equivalent of CEP, that
is the radius of a sphere, centered at the actual position, that contains
50% of the three dimension position estimates.
As opposed to 2drms, drms, or RMS figures, CEP and SEP are not
affected by large blunder errors making them an overly optimistic
accuracy measure
Some receiver specification sheets list horizontal accuracy in RMS or
CEP and without Selective Availability, making those receivers appear
more accurate than those specified by more responsible vendors using
more conservative error measures.
•GPS Satellite Signals
• The SVs transmit two microwave carrier
signals.
• The L1 frequency (1575.42 MHz) carries
the navigation message and the SPS code
signals.
• The L2 frequency (1227.60 MHz) is used to
measure the ionospheric delay by PPS
equipped receivers.
•Digital signal processing
• Three binary codes shift the L1 and/or L2
carrier phase.
• The C/A code
• The P-code
• The Y-code
• Plus the navigation message
Influence of the Ionosphere
• Each complete SV data set includes an
ionospheric model that is used in the receiver
to approximates the phase delay through the
ionosphere at any location and time.
• Each SV sends the amount to which GPS
Time is offset from Universal Coordinated
Time.
• This correction can be used by the receiver to
set UTC to within 100 ns.
• Other system parameters and flags are sent
that characterize details of the system.
Decoding the signal
• The GPS receiver produces replicas of the C/A
and/or the P (Y)-Code.
• The receiver produces the C/A code sequence
for a specific SV with some form of a C/A code
generator.
• Modern receivers usually store a complete set of
precomputed C/A code chips in memory, but a
hardware, shift register, implementation can also
be used.
• The C/A code generator produces a different
1023 chip sequence for each phase tap setting.
Pseudo-Random Code?
• Very weak signal
• Each satellite makes its own unique code
(not really random)
• Can be easily distinguished from
background noise
Pseudo-Random?
Received
Signal
In GPS
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GPS knows each sat’s code
Duplicate code running in GPS
Synchronizes it’s code to received signal
Time difference is the time signal took to get
there
ECEF XYZ to Geodetic
Coordinate Conversion
• Latitude and longitude are usually provided in the
geodetic datum on which GPS is based (WGS-84).
• Receivers can often be set to convert to other userrequired datums.
• Position offsets of hundreds of meters can result from
using the wrong datum.
• Velocity is computed from change in position over time,
the SV Doppler frequencies, or both.
• Repeat values at stationary receivers can be averaged
Carrier Phase Tracking
(Surveying)
• Carrier-phase tracking of GPS signals has resulted in a
revolution in land surveying.
• Line of sight along the ground is no longer necessary for
precise positioning.
• Positions can be measured up to 30 km from reference
point without intermediate points.
• This use of GPS requires specially equipped carrier
tracking receivers.
• The L1 and/or L2 carrier signals are used.
• L1 carrier cycles have a wavelength of 19 cm. If tracked
and measured these carrier signals can provide ranging
measurements with relative accuracies of millimeters .
GPS errors : A combination of
noise, bias, blunders.
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Noise errors are the combined effect of PRN code noise (around 1
meter) and noise within the receiver noise (around 1 meter).
Bias errors result from Selective Availability and other factors SA is the
intentional degradation of the SPS signals by a time varying bias.
SA is controlled by the DOD to limit accuracy for non-U. S. military and
government users. The potential accuracy of the C/A code of around 30
meters is reduced to 100 meters (two standard deviations).
The SA bias on each satellite signal is different, and so the resulting
position solution is a function of the combined SA bias from each SV
used in the navigation solution.
Because SA is a changing bias with low frequency terms in excess of a
few hours, position solutions or individual SV pseudo-ranges cannot be
effectively averaged over periods shorter than a few hours.
Differential corrections must be updated at a rate less than the
correlation time of SA (and other bias errors).
•Other Bias Error sources
• SV clock errors uncorrected by Control Segment: 1
meter
• Ephemeris data errors: 1 meter
• Tropospheric delays: 1 meter. Complex models of
tropospheric delay require estimates or
measurements of these parameters.
• Unmodeled ionosphere delays: 10 meters. The
ionosphere is the layer of the atmosphere from 50 to
500 km that consists of ionized air. The transmitted
model can only remove about half of the possible 70
ns of delay leaving a ten meter un-modeled residual.
Other Sources of GPS Error
• Multipath: 0.5 meters. Multipath is caused by reflected signals
from surfaces near the receiver that can either interfere with or
be mistaken for the signal that follows the straight line path from
the satellite.
• Multipath is difficult to detect and hard to avoid.
• Blunders can result in errors of 100s of km.
• Control segment mistakes due to computer or human error can
cause errors from 1 meter to 100s of km.
• User mistakes, e.g. incorrect geodetic datum selection, causes
errors from 1 to hundreds of meters.
• Receiver errors from software or hardware failures can cause
blunder errors of any size.
• Noise and bias errors combine, resulting in typical ranging
errors of around fifteen meters for each satellite used in the
position solution.
Multi-path Error
?
Earth as Ellipsoid
Earth Models and Datums
The Datum
• An ellipsoid gives the base elevation
for mapping, called a datum.
• Examples are NAD27 and NAD83.
• The geoid is a figure that adjusts the
best ellipsoid and the variation of
gravity locally.
• It is the most accurate, and is used
more in geodesy than GIS and
cartography.
Geometric Dilution of Precision
(GDOP) and Visibility
• GPS ranging errors are magnified by the range vector
differences between the receiver and the SVs.
• The volume of the shape described by the unit-vectors from
the receiver to the SVs used in a position fix is inversely
proportional to GDOP.
• Poor GDOP, a large value representing a small unit vectorvolume, results when angles from receiver to the set of SVs
used are similar.
• Good GDOP, a small value representing a large unit-vectorvolume, results when angles from receiver to SVs are
different.
GDOP
• computed from the geometric relationships between
the receiver position and the positions of the
satellites the receiver is using for navigation.
• For planning purposes GDOP is often computed
from Almanacs and an estimated position.
• Estimated GDOP does not take into account
obstacles that block the line-of-sight from the
position to the satellites.
• Estimated GDOP may not be realizable in the field.
•GDOP Components
• PDOP = Position Dilution of Precision (3D), sometimes the Spherical DOP.
• HDOP = Horizontal Dilution of Precision
(Latitude, Longitude).
• VDOP = Vertical Dilution of Precision
(Height).
• TDOP = Time Dilution of Precision (Time).
WAAS network
• ~25 precisely
measured WRS (Wide
Area Reference
Stations)
• Receive signal from
GPS satellites, check
for error, then
broadcast the
correction
Differential GPS (DGPS)
Techniques
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The idea behind all differential positioning is to correct bias errors at
one location with measured bias errors at a known position.
A reference receiver, or base station, computes corrections for each
satellite signal.
Because individual pseudo-ranges must be corrected prior to the
formation of a navigation solution, DGPS implementations require
software in the reference receiver that can track all SVs in view and
form individual pseudo-range corrections for each SV.
These corrections are passed to the remote, or rover, receiver which
must be capable of applying these individual pseudo-range
corrections to each SV used in the navigation solution.
Differential corrections may be used in real-time or later, with postprocessing techniques.
Real-time corrections can be transmitted by radio link as WAAS
Common DGPS Methods
• The U. S. Coast Guard maintains a network of
differential monitors and transmits DGPS corrections
over radio-beacons covering much of the U. S. coastline.
• DGPS corrections are often transmitted in a standard
format specified by the Radio Technical Commission
Marine (RTCM).
• Corrections can be recorded for post processing. Many
public and private agencies record DGPS corrections for
distribution by electronic means.
• Private DGPS services use leased FM sub-carrier
broadcasts, satellite links, or private radio-beacons for
real-time applications.
DGPS Characteristics
• DGPS removes common-mode errors, those
errors common to both the reference and remote
receivers (not multipath or receiver noise).
• Errors are more often common when receivers
are close together (less than 100 km).
• Differential position accuracies of 1-10 meters
are possible with DGPS based on C/A code SPS
signals.
• Special software is required to process carrierphase differential measurements.
Pts w/ No Diff Correction
Pts w/ Diff Correction
Some Corrected More – Why?
Experiment
Yellow = dGPS much better
Red = dGPS better
Blue = Neutral or GPS better
Precision Agriculture
GPS enabled innovation
GPS: Magellen vs. Garmin … Garmin wins
Trimble is for surveying, and commercial
applications
Accuracy
Usually about, 6-8m
With correction (WAAS), 1-3m
$150-300 gets you a new GPS
Garmin Quest $599
WAAS
Garmin eTrex Vista
$60-150
Consider used 1st to try it
Casio Protrek GPS Watch
Garmin Forerunner 201
$100-150
Magellan Meridian
Gold GPS $250
WAAS