I've been building new GPS Simulators for a customer in Texas. The simulator uses a PICAXE-08M to send sentences that look like the GPGGA sentences of a balloon in flight. Eventually I'll move this to a larger micro and have it simulator all the sentences of a GPS receiver on a balloon flight. Using the simulator lets you test a flight computer's program on the ground.
A ground test like this lets you find errors without leaving the ground (where it's a lot cheaper to correct).