How is distance measured so accurately given GPS is only accurate to a few m?
GPS position has limitations in accuracy. If we were to use GPS position to measure distance, then we could only achieve distance accuracies of a few metres rather than centimetres. For this reason we use the velocity (derived from Doppler measurements) to measure distance instead. This eliminates the majority of the normal errors associated with GPS accuracy, and allows us to measure distance to within a few centimetres during a brake stop.
For more information, see also: