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    Re: GPS time for celestial
    From: Robert Goley
    Date: 1997 May 23, 10:59 EDT

    I checked the 1995 edition of NIMA's Pub 9, aka Bowditch.  Its
    references to time, in Chapter 18, are essentially the same as
    those in the 1984 edition, Vol. 1.
    The 1996 edition of Pub 117, Radio Navigational Aids, has similar
    information.  Neither publication has much to say about "GPS
    time" in their chapters on time.
    However, they both have chapters on satellite navigation, Chapter
    11 in Bowditch and Chapter 6 in Pub 117.  Bowditch uses terms
    such as "GPS system time", "clock data", "clock dither", and
    "receiver clock bias".  Quite frankly I have only a rough
    understanding of what these terms mean.  I have not and will not
    have time to read the chapter in depth.  But the following
    sentence caught my eye and seems germane:  "The navigation
    message ... contains coefficients for ionospheric delay models
    used by C/A receiver and coefficients used to calculate Universal
    Coordinated Time (UTC)."  From this I'm assuming (!!!!) that "GPS
    time" is UTC.
    Lt. Bob Goley, AP, Rockville (Md.) PS, D5
    Nautical Cartographer
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