This is where encryption comes into play, and verification of signals. If the GPS maker has any sense about them, they have a control mechanism in place to verify what signals they are receiving. One thing is to measure power levels. If you are receiving a signal that is as strong as if it were right next to you, something is fishy there. Perhaps the GPS satellites need to be updated with a CRC or MD5 hash of sorts verifying the satellite they are coming from so they cannot be spoofed. Just a thought.