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INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION
Introduction To GlobalPositioning System (GPS)
Chris Hecker
Department of Earth Systems Analysis
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Acknowledgements
GPS Overview by Peter H. Dana, Department of Geography,University of Colorado, USA.
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
GPS Tutorial by Trimble Navigation Ltd.
http://www.trimble.com/GPS
Contributions by Bart Krol, ITC
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.htmlhttp://www.trimble.com/GPShttp://www.trimble.com/GPShttp://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html8/14/2019 GIS Feb05 - GPS_principles_2004
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Why GPS ?
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Why GPS ?
Finding out where you are andwhere youre going
Positioning is crucial in handlinggeographical data
A world wide system for positioning
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What is GPS ?
A world wide radio-navigation system
Uses satellites as reference points
to calculate positions
Three components: Space segment Control segment
User segment
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GPS components
Space
segment
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GPS components
Control segment User segment
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How GPS works
1. The basis for GPS is ranging from satellites
2. For ranging a GPS receiver calculates distance to satellitesusing travel time of radio signals
3. To measure travel time, GPS needs exact timing
4. Along with distance you need to know the satellite position
5. For accurate positioning you must correct for errors
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1. Resection from satellites
3 satellites for a position fix
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2. Measuring distance to satellites
Distance is about 22,000 kmWe cannot see satellitesWe cannot measure exact distance
Distance = velocity * travel time
Using radio signal to calculate distanceThis signal travels with speed of lightSpeed of light = 299,174 km/sec
Calculating distance to satelliteUsing travel time of radio signalTravel time = ?
approx. 0.07 sec !
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3. Exact timing
1 msec
Satellites signal
GPS receivers signal
How to measure travel time satellite and receiver generate radio signal
at the same time travel time = phase difference between
signals
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3. Exact timing
Very precise clocks for exact timing satellites : highly accurate atomic clocks
(about USD 100,000 each )
receivers : moderately accurate quartz clocks
Difference in clock accuracy use a 4 th satellite to correct for clock error
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3. Exact timing
Correcting for clock errors in 2D:
At least:3 satellites for 2D fix4 satellites for 3D fix
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4. The satellite position in space
Using satellites as reference points for positioning
also requires that you know the exact position in spaceof each satellite, at any place and at any time.
The GPS control segment monitors the satellite positionin space.
All details of satellite orbits is available in an almanac
This satellite status information can be downloadedto the GPS receiver
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5. Correcting for errors
Main GPS error sources Clock errors Signal errors (noise) Interference in ionosphere and troposphere Multipath error Satellite position ( Ephemeris ) error Geometrical error ( Geometric Dilution of Precision) Intentional errors ( Selective Availability ) Human errors Receiver errors (hardware, software, antenna)
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5. Correcting for errors
Geometric Dilution Of Precision (GDOP)
POOR GDOP (2-6)GOOD GDOP ( 2)
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5. Correcting for errors
GDOP, continuation
Were somewherein this box At close anglesthe box gets bigger
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5. Correcting for errors
GDOP, continuation
GOOD COMPUTED GDOP AND BAD
VISIBILITYRESULTS IN POOR GDOP
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5. Correcting for errors
Selective Availability (SA)
The US military can introduce intentional errors to limitaccuracy for civil GPS users
SA introduces an artificial clock error into the radio signaland writes an error in the satellite status information
If SA is on a potential horizontal accuracy of 30 meterswill be reduced to 100 meters .
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5. Correcting for errors
Selective Availability (SA)
SA off (after May 2000)SA on (before May 2000)
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5. Correcting for errors
Some typical errors
Satellite clock error 2 meters Receiver noise 0.5 meter Interference in
ionosphere and troposphere 5 meters Multipath error 1.4 meter Satellite position (ephemeris) error 2 meters
poor GDOP up to 200 meters Human error up to hundreds of meters
Receiver errors(hardware, software, antenna) any size possible
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Using a handheld GPS receiver
Typical accuracy: 10 m Horizontal(civil use, good GDOP)
Results for a Month (Garmin 12XL)
Horizontal Accuracy (50%) 3.9 metersVertical Accuracy (50%) 9.6 meters
Horizontal Accuracy (95%) 9.3 metersVertical Accuracy (95%) 21.9 meters
Source: GPS ACCURACY MONITOR by Dennis Milbert(http://mywebpages.comcast.net/dmilbert/handacc/accur.htm )
http://mywebpages.comcast.net/dmilbert/handacc/accur.htmhttp://mywebpages.comcast.net/dmilbert/handacc/accur.htm8/14/2019 GIS Feb05 - GPS_principles_2004
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Other "GPS Systems
Russian GLONASSSystem(Global Navigation Satellite System)Managed by: Russian Space Forces1st Satellite: 1982Accuracy: 50-70m horizontal (99.7%)
Info: http://www.glonass-center.ru/
European GALILEOSystemDeveloped by: European Space Agency1st Satellite: 2006Fully operational: 2008Accuracy: around 1m
Info: http://www.esa.int/export/esaSA/GGGMX650NDC_navigation_0.html
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How GPS works
In conclusion
3. Exact timing1. Ranging from satellites 2. Distance to satellites
5. errors4. Position in space
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