Presentation by Tony Murfin and Marcia Bates of Novatel
Tony is one of our own, being a Chartered Electrical Engineer.
The GPS satellite system currently consists of 27 satellites located in
6 orbital planes at 5 degrees to the equatorial plane. They orbit at an
altitude of 20,000 km. with an orbital period of 12 hours. The system provides
world wide and continuous coverage with line of sight and all weather operations.
The basic GPS can resolve a position to within 25 p; 100 metres.
A differential GPS can resolve positions to within 3 p; 5 metres.
Accuracies down to 2 cm. can be attained in real time utilising a portable
base station.
Post processing allows that accuracy to be improved to within 5mm.
The GPS belongs to the U.S. military and as such security is of major importance.
Selective Availability (SA) limits GPS accuracy to non military users to
ensure foreign interests do not have the capability to operate in the precise
positioning mode.
SA is achieved by incorporating satellite clock dithering and broadcast
orbit accuracy degradation.
Errors and inaccuracies can arise from many sources
including:-
Multipath signals p; signals reflecting off nearby surfaces and
being received as a true signal.
Poor signal quality.
Satellite and Receiver clock errors.
Ionospheric and Tropospheric delays.
Cycle slip and carrier phase ambiguity.
The industry is currently focusing on the reduction or elimination of these
errors.
