It's worth noting that the artificial limit of commercial GPS 1,900 km/h AND over 18,000m.
Of course, you can get round those limits either by constructing your own receiver, or asking a supplier very nicely, like the Copenhaagen Suborbital guys did:
http://www.wired.com/wiredscience/2013/09/bypassin-us-gps-limits-for-active-guided-rockets/
It would be interesting to know if GLONASS also shares those limits.