# Re: [LEAPSECS] A lurker surfaces

From: Magnus Danielson <magnus_at_rubidium.dyndns.org>
Date: Wed, 03 Jan 2007 07:21:07 +0100 (CET)

From: Steve Allen <sla_at_UCOLICK.ORG>
Subject: Re: [LEAPSECS] A lurker surfaces
Date: Tue, 2 Jan 2007 21:35:24 -0800
Message-ID: <20070103053524.GA21506_at_ucolick.org>

> 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

Actually, in BIPM Circular T they use another notation to avoid confusing TAI
with that of a particular local TAI realization, TA(k). So the above formula
would be

TA(GPS) = GPS + 19 + C0

where C0 is being reported in Clause 5 of Circular T. Actually, they write it
as:

[UTC-GPS time] = -14 s + C0, [TAI-GPS time] = 19 s + C0, global uncertainty is of order 10 ns.

(direct quote from Circular T No 227).

For UTC they gladly refer to UTC and UTC(OP) or whatever laboratory they choose
to discuss.

> > 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.

Notice that some laboratories actually maintain their own TAI in addition to
maintaining their own UTC. You can even see that they produce different data
for UTC and TAI.

> 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.

Indeed. I investigated the different translations of the European Commission
summer time directive and it turns out that most translations referred to GMT
where as only a few actually said UTC. It gives the filmtitle "Lost in
translation" yeat another meaning. Sigh. Interestingly was the Danish
translation indicating the transition times in UTC where as Denmark yeat has to
legally accept UTC (where as it in reality thanks to a certain friend of ours
here for all practicall reasons actually is UTC). Sigh.

Cheers,
Magnus
Received on Tue Jan 02 2007 - 22:21:35 PST

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