GPS could be turned off or made very unreliable if the military needs it that way (as it was in the olde days when the best you could hope for was maybe 10m precision using satellites alone). So there are options now (not mutually exclusive):

1) The Russians have a similar system. You can use it (for a price, probably). Of course it's subject to the same military usage issues as GPS, only it's somebody else's military.
2) The Europeans are in the process of launching their own system. Once it's working, you can use that (for a price, probably). This one sounds like it's commercial, like SPOT image satellites.
3) RRs have lots of radio towers around for voice and other data communication. Those sites could be used for differential GPS, and a few probably will be anyway were satellite access is spotty.

Bottom line: GPS has its potential problems, but for most purposes it's fine. The real expense in the whole PTC thing is developing and implementing databases and data communication systems that allow locos to be used pretty much anywhere as they are now, not using GPS as a backup location method within those databases.