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    Re: GPS as a time authority
    From: UNK
    Date: 2009 Sep 15, 14:31 -0400

    On Monday, September 14, 2009 12:05 PM, John Parsons 
    > While assisting the Race Committee at a regatta this weekend, I heard
    > the timekeepers say that they were using "GPS time."  I noted that my
    > $79 wristwatch, which I check monthly against the US Bureau of
    > Standards atomic clock (ph: 303-499-7111) and which doesn't lose a
    > second per month, was 8 seconds ahead of the Committee's time.
    Eight seconds is a surprising error. As George Huxtable noted [in
    NavList 9744], the current difference between GPS time and UTC is 15
    seconds, that being the number of leap seconds since 6-Jan-1980 00:00:00
    UTC when the GPS count began. A possibility that occurred to me was that
    the GPS unit in question was so old (or stupid) as to be confused by the
    GPS week number roll-over* in 1999, but there have only been two leap
    seconds since then.
    * GPS time (the "Z-count") is the number of weeks since 6-Jan-1980
    concatenated with the number of seconds since 00:00:00 of the current
    week. Since the week count is a 10-bit field, it rolls over every 1024
    weeks (about 19.6 years). The number of roll-overs is included in the
    GPS message stream, but some early receivers didn't use it, since it was
    zero until 22-Aug-1999.
    The UTC correction is repeated at about 12-minute intervals. If a
    receiver hasn't cached the correction, it will be (today) 15 seconds off
    when it starts up until the next time it sees the correction.
     -- Peter Smith
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