The restrictions on GPS prevent ballistic missiles, not MANPADs. Typical limits are 515 m/s and 18,000 meters (try using your phone's GPS on a commercial flight, it works fine near a window). Update rate is probably the biggest issue with GPS and MANPADs.
Are these chips so much better at calculating GPS position than general purpose CPUs or consumer FPGAs? Feels like a silly restrictions for anyone capable of building a ballistic missile. On the other hand it seems relatively computationally expensive to do a speed check every time for low energy devices.
