NavList:

I think this is worthy of a pause. Frank Reed was wrong?!  Not that he would not (and obviously did) recognize it. It is just that in the 10 years or so of being on this forum (not calling it “the list”, Frank) I have NEVER seen any evidence of Frank being wrong. He is mortal. And the world is right again. 😇

Richard Langley, you wrote:
"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."

Aha. Yes, apparently I have had that wrong for many years, and I think you're right: I was assuming that the ground-based augmentation was an addition to WAAS.

Frank Reed