1. The Global Positioning System
Bart Krol / Jeroen Verplanke
INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

2. The Global Positioning System

3. The Global Positioning System
Acknowledgements
 GPS Overview by Peter H. Dana, Department of Geography,
University of Colorado, USA.
 GPS Tutorial by Trimble Navigation Ltd.

4.  Finding out where you are and where you’re going
Why GPS ?
 Finding out where you are and
where you’re going
 Positioning is crucial in handling
geographical data
 A world wide system for positioning

5.  A world wide radio-navigation system
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

6. GPS components
Space segment

7. GPS components
Control segment
User segment

8. distance to satellites using travel time of radio signals
How GPS works
1. The basis for GPS is resection from satellites
2. For resection a GPS receiver calculates
distance to satellites
using 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

9. 1. Resection from satellites
3 satellites for a position fix

10. 2. Measuring distance to satellites
= velocity * travel time
Calculating distance to satellite
Using travel time of radio signal
Travel time = ?
Distance is about 22,000 km
 We cannot see satellites
 We cannot measure exact distance
approx sec !
Using radio signal to calculate distance
This signal travels with speed of light
Speed of light = 299,174 km/sec

11. 3. Exact timing How to measure travel time
 satellite and receiver generate radio signal
at the same time
 travel time = phase difference between signals
1 msec
Satellite’s signal
GPS receiver’s signal

12. 3. Exact timing  Very precise clocks for exact timing
 satellites : highly accurate ‘atomic’ clocks
(about USD 100,000 each)
 receivers : moderately accurate quartz clocks
 Clock error due to difference in clock accuracy
 use a 4th satellite to correct for clock error

13. 3. Exact timing Correcting for clock errors in 2D: At least:
3 satellites for 2D fix
4 satellites for 3D fix

14. 4. The satellite position in space
 Using satellites as reference points for positioning
also requires that you know the exact position in space
of each satellite, at any place and at any time.
 The GPS control segment monitors the satellite position
in space.
 All details of satellite orbits is available in an ‘almanac’
 This satellite status information can be downloaded
to the GPS receiver

15. Main GPS error sources  Clock errors 5. Correcting for errors
 Signal errors (noise)
 Interference in ionosphere and troposphere
 Multipath error
 Satellite position (“ephemeris”) error
 Geometrical error (Geometric Dilution of
Precision - GDOP)
 Intentional errors (Selective Availability - SA)
 Human errors
 Receiver errors (hardware, software, antenna)

16. Geometric Dilution Of Precision (GDOP)
5. Correcting for errors
Geometric Dilution Of Precision (GDOP)
GOOD GDOP ( 2)
POOR GDOP (2-6)

17. GDOP, continued 5. Correcting for errors We’re somewhere in this box
At close angles
the box gets bigger

18. GDOP, continued 5. Correcting for errors
GOOD COMPUTED GDOP AND BAD VISIBILITY
RESULTS IN POOR GDOP

19. Selective Availability (SA)
5. Correcting for errors
Selective Availability (SA)
 The US military can introduce intentional errors to limit
accuracy for civil GPS users
 SA introduces an artificial clock error into the radio signal
and writes an error in the satellite status information
 If SA is ‘on’ a potential horizontal accuracy of ± 30 meters
will be reduced to ± 100 meters.

20. Some typical errors 5. Correcting for errors
 Satellite clock error ± 2 meter
 Receiver noise ± 0.5 meters
 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 errors up to hundreds of meters
 Receiver errors
(hardware, software, antenna) any size possible

21. Using a handheld GPS receiver
Typical accuracy: ±10 m Horizontal
( civilian use, good GDOP)
Results of measurements over one month (Garmin 12XL)
Horizontal Accuracy (50%) ± 3.9 meters
Vertical Accuracy (50%) ± 9.6 meters
Horizontal Accuracy (95%) ± 9.3 meters
Vertical Accuracy (95%) ± 21.9 meters
Source: GPS ACCURACY MONITOR by Dennis Milbert (

22. To sumarize How GPS works 1. Resection from satellites
2. Distance to satellites
3. Exact timing
4. Position in space
5. errors

23. How to operate the Garmin
Basic buttons:
Next page
change up/down
Enter
Power

24. How to operate the Garmin
Navigation screen:
Mode
Accuracy
Units
Expected sat position
Horizon
Battery level
Zenith (90º)
Skyline at 45º
Known sat position
Signal strength
Satellite number

25. Set up the GPS and go outside
In the classroom
Familiarize yourself with the GPS
Set-up the GPS
Enter Waypoints
Outside the main entrance
Check position
Try using the compass in the GOTO function
Go and find the waypoints