Re: [LEAPSECS] what should a time standard encompass?

From: Steve Allen <>
Date: Mon, 27 Jan 2003 14:09:45 -0800

On Mon 2003-01-27T21:48:54 +0000, Ed Davies hath writ:
> Steve Allen replied:
> # GPS did choose TAI.
> # For all practical purposes GPS = TAI - 19 s.
> (GPS = TAI - 19) & (x != x - 19) => (GPS != TAI)

GPS could not choose TAI, for TAI is not known _now_.

The TAI of _now_ is known next month after all the clocks that
participate in TAI have been intercompared and averaged.
If your clock is one of those clocks, then next month you can
figure out how wrong your clock was at time _now_, and then
you know what the TAI of _now_ was.

GPS chose what it could choose, which is the time of _now_ according
to the clocks that set GPS time. If GPS had chosen TAI it would
almost always be wrong, currently by up to about 50 ns, and previously
by notably more. The constant difference of 19 s is there to prevent
automatically thinking that GPS time is TAI in the hopes that users
will take the effort to educate themselves on exactly these points
about practical vs. precision time keeping

The galling aspect of the original posting was this:
> If UTC drifts away from UT1 then astronomers can reasonably be assumed
> to have the understanding and motivation required to deal with the
> change without significant problems.

Understanding and motivation do not equal hardware budget and manpower
for systems whose designed lifetime was supposed to be 30 years.

Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064      Voice: +1 831 459 3046
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93
Received on Mon Jan 27 2003 - 14:09:59 PST

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