On earth you can (in theory) get a position if you have a lock on three GPS satellites as you know that you are on the surface (intersection of 3 spheres). If y

On earth you can (in theory) get a position if you have a lock on three GPS satellites as you know that you are on the surface (intersection of 3 spheres). If you're higher up, you be a fourth sat. I can't conceive how they could lock it down to low speed objects, as it is a one way communication link, but I'd be interested to know if it is possible. Edit: looks like this is a limit built into GPS receiver devices, not the protocol. If you build your own receiver, then you can get around those restrictions. Not sure if there are licencing issues to worry about, but I can't imagine an adversary to care about that kind of thing, or a company that is launching 'rogue' sats.