Receivers and transmitters are synchronized in time, before they are used. So if the timestamp of the signal deviates of GPS's internal clock, GPS receiver kno

Receivers and transmitters are synchronized in time, before they are used. So if the timestamp of the signal deviates of GPS's internal clock, GPS receiver knows it. Now for commercial receivers, they assume their internal clock is wrong, and set their own time to new GSP time. Not sure what military GPS receivers do, but I imagine, they kind of thought about this, and just ignore signals that have timestamps that differ too much. This probably means that they need pretty accurate internal clock. Another way to encrypt a signal is to just have an encrypted sequence going on. If you get a sequence you have already seen something is wrong. Think of it as hash+salt or something similar.