Re: [LEAPSECS] A lurker surfaces

From: Steve Allen <sla_at_UCOLICK.ORG>
Date: Tue, 2 Jan 2007 21:35:24 -0800

On Tue 2007-01-02T22:16:19 -0700, M. Warner Losh hath writ:
> changed in later revisions to be the same as GPS time. There's an
> extreme reluctance in the time community to call something without
> leap seconds "TAI" or "TAI + fixed offset". TAI means something very
> specific. That's the other problem with just using TAI, btw, but
> explaining that point is very hard...

It would almost seem to consistent with established notation
to define

TAI(GPS) = GPS + 19 + W1K * n

> Producing a number that corresponded to TAI time was OK, and
> likely the least confusing thing to do (we give a second number and
> UTC time in 'YYYY-MM-DD HH:MM:SS Z' as well as the channel and the
> measurement for that time in out output), but actually calling it TAI
> would 'confuse' the really smart time geeks out in the world. I asked
> him for a reference where I could read up on this, and he shrugged and
> said he just knew it and didn't know of any good write up.

This is my "tail wags the dog" point.

The national metrology agencies are tasked by their national laws
and funding agencies to produce the legal time scale for each country.
Depending on the state of legislation that is either GMT or UTC.
In the US the time agencies have chosen to interpret GMT as UTC
by taking advantage of the imprecision of the federal law.

The national metrology agencies are not *directly* tasked to keep TAI,
but by being parties to the Metre Convention their own version of UTC
plus leap seconds contributes to TAI.

So each national contributing source to TAI is really based on that
country's version of UTC. Despite the appearances of the equations
the versions of UTC are the primary entities and TAI is secondary.

And, yes, explaining all this is very hard. It's not obvious to the
geek that the political and funding realities are more real than the
underlying physics, but that's the way the world works.

Steve Allen                 <>                WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99858
University of California    Voice: +1 831 459 3046           Lng -122.06014
Santa Cruz, CA 95064     Hgt +250 m
Received on Tue Jan 02 2007 - 21:35:43 PST

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