People have to realize that this is already happening. see
GPS Jammers Threaten Air Traffic Navigation: Video - Bloomberg
http://www.gps.gov/multimedia/presen...TS/merrill.pdf
GPS jammers and spoofers threaten infrastructure, say researchers | Ars Technica
These jammers have a range of 5 - 10 miles dependent on manufacture and power - that is a hemisphere of 5 - 10 miles so above the jammer at FL410 an aircraft ADS-B may be reporting a totally incorrect position based on jammed GPS. (See figure 8 of this paper looking at the effect on ships and the difference between eLORAN position and GPS position when there is jamming
http://www.navnin.nl/NIN/Downloads/G...Navigation.pdf )
Given jamming like that the FMC (and crew) thinking it is well off track will try to navigate back to 'where it should be' and in reality fly the aircraft well off track all the while broadcasting its
incorrect position on ADS. Imagine the effect in a high traffic area with all the aircraft giving false positions.
It was the managed version of this jamming that was demonstrated by some Universities in 'taking control' of an unmanned aircraft and possibly the way the Iranians brought down a reconnaissance UA.
This is an area that the proponents of moving everything to GNSS based systems need to analyze, and then they will need to mitigate all the potential risks before closing the radars down and removing all the NDB/VORs.
The only backup system with sufficient coverage and more security is eLORAN, and there is a battle over whether to fund it. .