[LEAPSECS] epoch of TAI, and TAI vis a vis GPS

Steve Allen sla at ucolick.org
Tue Mar 3 21:23:37 EST 2015

On Wed 2015-03-04T00:04:10 +0000, Tony Finch hath writ:
> They have different epochs:
> TAI: 1958-01-01 T 00:00:00 Z
> PTP: 1970-01-01 T 00:00:00 Z
> GPS: 1980-01-06 T 00:00:00 Z

Getting meaninglessly pedantic, in Survey Review v19 #143 p7 (1967)
A.R. Robins had been talking with Sadler and Smith and with that
information in hand he wrote that atomic time was identical to UT2 at
1958-01-01 T 20:00:00 Z

This, of course, disagrees with Guinot's memoir, but the various
realizations of UT2 then differed by centiseconds and the different
versions of atomic time were subsequently realigned by milliseconds.
And that date of 1958-01-01 was decided ex post facto at the 1959
August meetings where the US and UK decided to try coordinating their
broadcast time signals using cesium.  So there really isn't an epoch
for TAI.

On Tue 2015-03-03T14:31:13 -0800, Hal Murray hath writ:
> Since GPS time is a fixed offset from TAI, it's easy to convert.

I believe that BIPM would disagree because of the different kinds of
steering at the nanosecond level.  The stance of the BIPM was expressed in
where TAI "should not be considered as an alternative time reference."

Without the assent of the BIPM it is hard for there to be an agreed
upon name for real-time versions of time scales that are trying to
track the value of TAI (which will not actually be available until the
next issue of Circular T).

Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
UCO/Lick Observatory--ISB   Natural Sciences II, Room 165    Lat  +36.99855
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