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    Re: Is Celestial Navigation really a backup to GPS Navigation?
    From: Frank Reed
    Date: 2018 Sep 20, 09:28 -0700

    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

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