Like Eddie said GPS means less than nothing in a model. Since at that speed you cover more than 100 metres a second (360kph) a +- 10m resolution (IF you have suffiecient satelites visible to your ever moving antenna) gives a 20% error at BEST.
On our high speed UAV there are mayor discrepancies in airspeed between GPS and pitot airspeed. If it was not for the INS capability of the autopilot you would have no idea of your position second to second in the small space models fly. GPS is just not good enough. DGPS is a lot better but systems suitable for aviation is very expensive and not as compact.
Andre Baird