I am a private pilot. Flying my unpressurized Cessna I never had to fly over 10000 feet but my gps always gave me a fix up to at least 7000 feet. iPad GPS is not a real gps. Real gps use satellite signal to triangulate their position. iPad GPS used cell towers to triangulate their positions. To the maximum height is more a maximum distance from ground cell towers which at 10000 feet is roughly 3 kilometers.