Gizmodo provides the factual reasons why GPS doesn't work at Mach 2 or at an altitude above 60,000 feet. Apparently, you can thank the communists. Gizmodo says, citing
Wikipedia:
The CoCOM, or Coordinating Committee for Multilateral Export Controls, was established in the early years of the Cold War to put an arms embargo on Eastern Bloc countries. After GPS was developed by the Defense Department, COCOM limits were put in place so that the government could be sure the technology wouldn't end up in commie-guided missiles.
Of course, the Cold War is over. That your GPS stops functioning when you're moving that fast or that high (which is to say not often) is because it's a relic of a bygone age.
