akafrank07
7th Aug 2012, 20:27
GPS system errors:
Clock Bias
Satellite ephemeris and
Ionosphere propagation
Can anyone explain these 3 system errors simply of the GPS, before someone takes the head off me, i have researched but of no avail
keith williams
7th Aug 2012, 21:02
Ephemeris Error.
These are errors in the satellite’s calculation of its position. The gravitational effects of the sun, moon and other planets in the solar system cause them. The ground monitoring stations check satellite position every 12 hours, so the maximum error is 2.5 metres.
Satellite Clock errors.
Caused by the inherent inaccuracy of the atomic clocks used in the satellites. They are checked by the ground monitoring stations every 12 hours, so the maximum position error caused by clock errors is 1.5 metres.
Ionospheric Propagation errors.
The effect that the ionosphere has on radio waves is proportional to the frequency of the waves. The satellite broadcasts contains a model of the ionosphere, which is used to make corrections to the time taken fro signals to reach the receivers. But the condition of the ionosphere is constantly changing, so the corrections made are imperfect. The model of the ionosphere is corrected by the ground stations every 12 hours, so the maximum position error is 5 metres.
Other GPS errors include the following
Tropospheric Propagation Errors.
Variations in pressure, temperature and air density cause variations in the speed at which radio waves move through the troposphere. This affects the time taken for satellite signals to reach the receivers. The maximum position error caused by troposheric propagation errors is between 0.3 and 0.5 metres.
Receiver Noise Errors.
Internal noise produced within the circuits of the receivers causes position errors of up to 0.3 metres.
Multi-Path Reception Errors.
Reflections from the ground and parts of the aircraft can cause the receivers to receive signals from more than one path. The maximum position error caused by these effects is 0.6 metres.
Geometric Dilution Of Precision Errors (GDOP).
These occur when the range arcs produced by the satellites cross each other at poor angles. They are greatest when the satellites are relatively close to each other.
Aircraft Manoeuvre Errors.
These are caused when manoeuvring of the aircraft temporarily blocks the reception of signals from one or more of the satellites that are being used.