Come to think of it, spoofing the GPS signal shouldn't bee too hard, either. You know where the satellites will be at any given time, and you can calculate what the true signal will look like. Blocking the "true" signal with your spoofed version is a question of getting the right antenna and transmitter close enough to the area of interest.

I don't know any details (and anything interesting would probably be classified anyway), but I do know this is one area where the military has active research going on. I would also hazard a guess that systems less vulnerable to spoofing would be pretty high on the military Christmas wish list.