NavList:

Gary L, you wrote:
"But what about WAAS as used by aircraft making precision instrument approaches down to 200 feet above the ground, in the clouds, to a runway only 200 feet wide? SA reduced the accuracy to 100 meters, 95% of the time with larger errors the remaining 5% of the time. And spot jamming or spoofing with low power equipment placed need the airport by hostile power or terrorists?"

They could fly an airplane right into the ground. Set the clock back a bit, and it's the setup for Die Hard 2. To which I say, "Yippee Ki Yay, Mister Falcon!!" Mister Who?

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