1. Global Positioning Systems (GPS)
Jeroen Verplanke
Department of Urban and Regional Planning and Geo-Information Management

3. 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

4. GPS components
Space segment

5. GPS components
Control segment
User segment

6. How GPS works
1. The basis for GPS is resection from satellites (triangulation)
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

7. What GPS accuracy do we need?
Soil map (1:250,000) :
Road map (1:100,000) :
Topographical map (1:50,000) :
Ecological map (1:25,000) :
Cadastral map (1:10,000) :
Architectural plan (1:1,000) :
Sketch map (1: ???) :

8. Cálculo de la posición usando la tecnología GPS? X X
Superficie de la Tierra

9. 1. Resection from satellites
Resection from satellites: determination of an unknown location using 1, 2 and 3 distances to known locations of satellite(s). X Da A Da
1 satellite A X

10. 1. Resection from satellitesX A Da Db B Da Db X
2 satellites
A + B

11. 1. Resection from satellitesDc X Da Db A B
3 satellites
A + B + C

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

13. 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

14. 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

15. 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

16. 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 GDOP)
 Intentional errors (Selective Availability - SA)
 Human errors
 Receiver errors (hardware, software, antenna)

17. 5. GPS Signal – optimal

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

19. 5. GPS Signal – good GDOP, poor visibility

21. 5. GPS Signal – beware of yourself

22. 5. GPS Signal - solution

23. 5. Correcting for errors Selective Availability (SA)
SA on (before May 2000)
SA off (after May 2000)

24. 5. Correcting for errors Some typical errors
 Satellite clock error meter
 Receiver noise meters
 Interference in
ionosphere and troposphere 5 meters
 Multipath error 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

25. 5. GPS settings
units’ settings: position format: hddd.dddddo, map datum: WGS 84, and units: metric
‘interface’ settings: i/o format: NMEA out, and baud rate: 4800.

26. About GPS accuracy Accuracy can be from 100m to 1mm
Accuracy depends on purpose
Soil map (1:250,000) :
Road map (1:100,000) :
Topographical map (1:50,000) :
Ecological map (1:25,000) :
Cadastral map (1:10,000) :
Architectural plan (1:1,000) :
Sketch map (1: ???) :

27. 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 meters
Vertical Accuracy (50%) meters
Horizontal Accuracy (95%) meters
Vertical Accuracy (95%) meters
Source: GPS ACCURACY MONITOR by Dennis Milbert (

28. In conclusion 1. Resection from satellites 2. Distance to satellites
3. Exact timing
4. Position in space
5. errors