> encrypted GPS

Encrypting what?

Except if you want people not to be able to use the GPS signal, which sure make sense in military context (and even then, it's pretty meaningless, except if you change the key pretty regularly and you have a safe way to propagate it that won't be possible for enemies to get their hands on).

You know right that GPS signal is a one way signal? The receiver doesn't transmit anything, he just receive and use the timing and its data with some fancy math to be able find your position.