Steve is correct. If you spoof GPS in an area, it will affect all GPS in range. I don't believe it will be purposely done, at least in secret, and certainly not by the government, for the following reason:
During the last 25 years, the FAA has been trying to get out of the earth-station based air navigation business. Reason? Annual maintenance cost for VOR land based for point to point air navigation, and ILS radio transmitter (Instrument Landing System) maintenance is costing over 1$billion a year. GPS, using space based satellites do the job far better and at lower cost.
For years now, nearly all aircraft have been using GPS for both en-route and terminal approach to landing procedures. They have been so successful, entirely new navigation techniques have been developed which depend on GPS. When flying enroute, for example, they now fly 10 aircraft in the same physical space as earth based systems only allowed 2. Systems such as TCAS (Collision Avoidance) use GPS positions and altitude as a basis of allowing aircraft to approach MUCH closer than most people realize as they fly along having their super expensive diet drinks. Many of the other new systems, ADS-B for example, depend on accurate GPS position and altitude to function correctly.
If a decision was made to turn off (or "spoof") GPS, all of the aircraft in the area would be effected and to possibly devastating results. The Pilots would know that the system had gone totally off, but would NOT know if it was simply sending the wrong data. They have multiple GPS units, and all units would be showing the same. Some aircraft, with multiple redundant navaids would know pretty quickly - others would not.
In GPS positioning, a 100 foot error at 40,000 feet isn't a big deal. When terminating an approach into DC during a snow storm, 100 feet and you miss the runway.
In EMP parlance, think of the airborne GPS as the power infrastructure of landlines. Having it go down would be a really bad thing.
WW
shoot straight - stay safe
