Thanks, Tim. I can genuinely say that having read the article, I'm not much better off (though I did understand the word "sphere", so that's something). I'm happy to take your assurance on this one!
Dave
According to the operators of the GPS system, the accuracy offered by the satellite data is theoretically quite high, but it also depends on the implementation in the receiver:
How accurate is GPS for speed measurement?
As with positioning, the speed accuracy of GPS depends on many factors.
The government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec over any 3-second interval, with 95% probability.
This measure must be combined with other factors outside the government's control, including satellite geometry, signal blockage, atmospheric conditions, and receiver design features/quality, to calculate a particular receiver's speed accuracy.
