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."

No. WAAS doesn't work that way. WAAS and the other SBASs are state-space systems, which use a wide-area network of ground stations to determine satellite orbit and clock corrections and a grid of ionospheric delay values transmitted to all users by geostationary satellites. Perhaps you are thinking of GBAS -- ground-based augmentation system, where receivers at airports provide differential corrections and integrity information via a VHF data link. GBAS has not been widely deployed yet but there are a number of stations deployed in North America and Europe. Various coast guard LF/MF DGPS stations operate on a similar principle. The U.S. Coast Guard stations are to be shut down between now and 2020 as it believes WAAS is sufficient for the needs of mariners.

-- Richard Langley