Originally Posted by
Loren Pechtel
GPS on an airplane is going to be very marginal at best because the amount of sky you can see from your seat is quite limited and GPS requires something approximating line of sight to 4 satellites or it just thumbs it's nose at you.
Actually three for 2D coordinates (triangulation), 4 to add 3D (which yields altitude). Nowadays more handheld GPSs and Smartphone support many of the other positioning systems too (GLONASS, Galileo, Beidou, etc.) With well over 100 positioning satellites now, it's virtually impossible to not be able to see at least a half dozen or so of these at any time at say FL350 (from the non-electrified window anyway).