Daniel R. Tobias wrote:
>It's a few seconds off from TAI, isn't it?  It was synchronized to
>UTC in 1980 (I think),

Yes.  The epoch for GPS time is 1980-01-06T00:00:00 UTC, which is
1980-01-06T00:00:00 GPS time.  Not having leap seconds, it effectively
tracks TAI, with the equation

        TAI(GPS) = GPS + 19 s

TAI(GPS) generally stays within about 15 ns of TAI.

The GPS time signal uses a ten-bit count of weeks, which wraps every
twenty years, so the raw signal is ambiguous between, for example,
1987-05-20 and 2007-01-03.  They are, nevertheless, distinct points on
the GPS time scale.  I recall there being some plan for a new field in
the navigation message that would disambiguate them.

>                                      They probably should just have
>used TAI if they wanted a time scale without leap seconds, rather
>than ending up creating a different one.

Would have been nice.  Actually, since the only real significance of
GPS time is that it's part of the signal format, they could just as
well have picked an unconventional but space-efficient encoding (say,
32-bit count of seconds, wrapping every 4 Gis).  I think GPS time is
best viewed as an encoding of TAI(GPS).


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