NavList:

But isn't it true that airport WAAS employs differential GPS? You place a GPS receiver at a fixed location with a well-surveyed latitude and longitude at the airport. Then you broadcast the live difference between the instantaneous GPS fix at that known location and its actual location to nearby aircraft. They adjust their GPS fixes by that offset vector, and you instantly get an order of magnitude improvement in fix accuracy. Differential GPS like this also over-rules SA and spoofing and even ionospheric noise. They're completely cancelled out. The only exception that I can think of is when the spoofer is using short-range equipment located aboard the aircraft and is possibly a passenger on the aircraft. That's an important loophole. 

Frank Reed

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WAAS is the acronymn for "WIDE AREA augmentation system" so it can't deal with local spoofers or jammers.

gl