GIS Feb05 - GPS_principles_2004

8/14/2019 GIS Feb05 - GPS_principles_2004

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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.html


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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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Geometric Dilution Of Precision (GDOP)

POOR GDOP (2-6)GOOD GDOP ( 2)


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GDOP, continuation

Were somewherein this box At close anglesthe box gets bigger


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GDOP, continuation

GOOD COMPUTED GDOP AND BAD

VISIBILITYRESULTS IN POOR GDOP


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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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Selective Availability (SA)

SA off (after May 2000)SA on (before May 2000)


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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.htm


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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

GIS Feb05 - GPS_principles_2004

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