Gps
Until a satisfactory civil owned and operated system of satellites is in place GPS will rely on US and Russian military systems that can be shut down at any time. For this reason the FAA will not allow airlines to use GPS as a primary navigational aid. A few years ago there was a satellite jammer sytem on open sale at a Russian trade show. Imagine this in the hands of a terrorist organisation!
GPS can only be an aid, not a substitute for navigation.