1. The Global Positioning System
GPS
Part : 2 By
Mohammed Chessab Mahdi
Al-Furat Al-Awsat Technical University

2. How GPS works 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
Mohammed Chessab Mahdi

3. 1. Resection from satellites
3 satellites for a position fix
Mohammed Chessab Mahdi

4. Mohammed Chessab Mahdi

5. Simple mathematical model of GPS
If receiver was in
The four satellites were in
They transmit at
Where:
C Velocity of light
Receiving moment
Mohammed Chessab Mahdi

6. 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
Mohammed Chessab Mahdi

7. 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
Mohammed Chessab Mahdi

8. 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
Mohammed Chessab Mahdi

9. 3. Exact timing Correcting for clock errors in 2D: At least:
3 satellites for 2D fix
4 satellites for 3D fix
Mohammed Chessab Mahdi

10. Atomic Clock
An atomic clock is a clock that uses the resonance frequencies of atoms as its resonator. The resonator is "regulated by the frequency of the microwave electromagnetic radiation emitted or absorbed by the quantum transition (energy change) of an atom or molecule
Mohammed Chessab Mahdi

11. 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
Mohammed Chessab Mahdi

12. 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)
Mohammed Chessab Mahdi

13. Geometric Dilution Of Precision (GDOP)
5. Correcting for errors
Geometric Dilution Of Precision (GDOP)
GOOD GDOP ( 2)
POOR GDOP (2-6)
Mohammed Chessab Mahdi

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

15. 5. Correcting for errors GDOP, continued
GOOD COMPUTED GDOP AND BAD VISIBILITY
RESULTS IN POOR GDOP
Mohammed Chessab Mahdi

16. 5. Correcting for errors Selective Availability (SA)
The US military can introduce intentional errors to limit accuracy for civil
GPS users
S/A was designed to prevent America’s enemies from using GPS against
us and our allies.
In May 2000 the Pentagon reduced S/A to zero meters error.
S/A could be reactivated at any time by the Pentagon.
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.
Mohammed Chessab Mahdi

17. 5. Correcting for errors Some typical 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
Mohammed Chessab Mahdi

18. Thank you
Mohammed Chessab Mahdi