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

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

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

Mohammed Chessab Mahdi

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

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. 0.07 sec ! Using radio signal to calculate distance This signal travels with speed of light Speed of light = 299,174 km/sec Mohammed Chessab Mahdi

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

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

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

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

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

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

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

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

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

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

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

Thank you Mohammed Chessab Mahdi