In a paper submitted by Witte and Wilson, December 2004, they established that the accuracy of GPS systems was within 0.2 metres per second for 45% of their test results with a further 19% being within 0.4 m/s.
They concluded that consumer GPS systems were acceptable for the measurement of speed and distance calculations even under conditions where satellite reception becomes moderately degraded.
This was for non-differential GPS - more modern systems use EGNOS or WAAS error correction networks and even consumer receivers are able to give an accurate position within 2-3 metres over 95% of the US and accuracies of less than a metre over 99% of Europe.
Originally, the Satellite signals were degraded by the US Military by inserting timing errors which reduced the accuracy to around 100 metres ("Selective Availability") . This was turned off in 2000 at the instruction of Bill Clinton, to assist the military in the Gulf War, some of whom were using domestic type Garmin receivers.
Military GPS units are slightly more accurate because they use more sophisticated error checking and validation techniques, using multiple signal channels to make them more suitable for munitions guidance. There is no longer any degradation of the service - its just that theirs is even better.
Interestingly, the time signals from the satellites are given a rate offset to allow for the effects of relativity, as Einstein predicted that a clock operating at a typical orbital distance from the Earth would run approximately 38 microseconds per day more slowly.
Some vehicle sat-navs may not be enabled to this level of accuracy, but I have verified my TomTom unit against an EGNOS enabled Garmin receiver and I can confirm that they are in agreement. Where there may be some discrepancy is in hard acceleration where the receiver's sampling time may not keep up with the change in velocity.
"From the NAVSTAR GPS User Equipment Introduction document Section 3.7:
GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.
Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on. "
In other words - you can trust your sat-nav speed readings
Last edited by ukmike2008; 05-11-09 at 14:10.