I don't want to cause thread drift here, but how do GPS speedometers give a true ground speed reading when the vehicle is going uphill or downhill?
Dave
Multiple satellite reception from a range of angles would ameliorate what would in any case be extremely low impact on horizontal speed errors.
I guess a Eurofighter in vertical climb would present a problem although even then GPS altitude changes could give a speed reading.
ETA: the fourth satellite requirement for minimum 3D location is needed for almanac data received from satellite. Most GPS receivers have 12 channel or more capability.
