I believe that GPS is in fact listed as an approved device once you reach 10k feet altitude. I carry mine all the time, but never had a chance to use one. Most car GPSes (Tomtom, Garmin etc.) will have a hard time locking on to the signals when trying to cold/warm start at such high speed. If one was locked on to the signal on the ground it may continue to be locked - but it would be a violation of FAA regulations to leave them on during takeoff.