They shut themselves down if they detect you are travelling around Mach 2 or above 18000 metres altitude
The intention was for a GPS device to disable itself if it was traveling faster than 1k knots AND above 60k feet, not traveling faster than 1k knots OR above 60k feet. Unfortunately, many GPS devices improperly implement this as an OR condition, rather than an AND condition. This becomes problematic when using a GPS in a high-altitude weather balloon to determine altitude and location, which a few friends and I did a couple years ago. We had to be very careful when selecting the GPS we used to avoid this pitfall.