The GPS shuts down at 1200mph or at an altitude of 60,000 feet to ensure it won’t be used in a guided missile!
The GPS has built in limits based on speed and altitude. This is because the US Defense department originally developed the technology. During the early years of the Cold War, CoCOM or Coordinating Committee for Multilateral Exports Control was established. It put arms embargos on Eastern Bloc countries.
When the GPS was developed, CoCOM limits were put in place so the Russians couldn’t use it in guided missiles. The fact that the GPS still has these limits is somewhat funny, since the Cold War is long over. Although, it could still be used by other hostile forces.