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On Monday, September 14, 2009 12:05 PM, John Parsons 
said:
> 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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