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    Re: GPS and the Chandler Wobble
    From: Paul Hirose
    Date: 2018 Nov 6, 19:12 -0800

    The official specification for the GPS signal is an Interface Control
    Document. I think the one of most interest is the first on the page,
    Interface Specification IS-GPS-200.
    
    https://www.gps.gov/technical/icwg/
    
    See p. 103 in that document for the format of the ephemerides broadcast
    by the satellites. The orbit parameters have the appearance of the
    Keplerian orbital elements long used in astronomy, but they actually
    yield the WGS84 coordinates of the satellites. In other words, the
    rotation of the Earth and wandering of the pole have been absorbed into
    the ephemeris.
    
    But what about the encrypted military signal ("Y-code")? Does it include
    additional data for improved accuracy? Apparently not: "The NAV data,
    D(t), includes SV [Space Vehicle] ephemerides, system time, SV clock
    behavior data, status messages and C/A to P (or Y) code handover
    information, etc. The 50 bps data is modulo-2 added to the P(Y)- and
    C/A- codes; the resultant bit-trains are used to modulate the L1 and L2
    carriers. For a given SV, the data train D(t), if present, is common to
    the P(Y)- and C/A- codes on both the L1 and L2 channels."
    
    Upgrades have made new GPS signals available to civil users, such as the
    "CNAV" data described in Appendix III of the document. They include
    Earth orientation parameters: x and y of the pole, UT1-UTC, and the time
    derivatives of those values. See p. 176. I don't know how many
    satellites broadcast the new data, or to what extent consumer grade
    devices apply the corrections.
    

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