VP959
AFAIK all but the cheapest GPS receivers (which, I assume, we are not using for flight tests...) use doppler information on the GPS signals (change of frequency due to movements of transmitter or receiver) to compute velocity, or speed, whatever. This doppler shift data is
very precise, and usually much more reliable than position data.
ref: I think I read that in Hofmann/Wellenhoff's GPS book, easily to be found on the internet.