[LEAPSECS] Terminology question

Michael Deckers michael.deckers at yahoo.com
Wed Mar 10 20:08:21 EST 2010

On 2010-03-10 22:42, Steve Allen wrote:

> In the lingo of the atomic horologists I would say

> "the relationship between UTC(TAI) and TAI is simple."


> Here UTC(TAI) means "the version of UTC constructed

> in arrears by using the contents of Circular T".

But IERS Bulletin C would suffice for the
relationship between UTC and TAI, and that is
available a bit earlier.

> The relationship between any other realization of UTC

> and TAI is not simple.

Yes, modulo IERS Bulletin C it is as complicated as the
relationship of the realization of UTC to UTC proper.


> > If I understand you correctly: my computer gives me UTC(my_computer)

> > and I can convert that easily to TAI(my_computer)


> In the lingo of the atomic horologists there can be only one TAI.

> There is no published entity such as TAI(anything else), and the

> transcripts of discussions indicate that people get chafed when anyone

> uses such a lingo.

Oops, you are of course right. I should have written TA(my_computer).


> > I do not understand how the formal definition of UTC limits its

> > precision to 1 ms. UTC can be determined with the same

> > uncertainty as TAI.


> Yes, but only as of next month, when the next Circular T is published.

> That is unsuitable for an operational time scale which is needed now.

I see. Nevertheless it may be useful to apply the conversion from
UTC to TAI also to approximate UTC values: the resulting approximate
TAI values are closer to a proper time scale, and this may be desired
when computing the length of time intervals.


> I believe it is this distinction that prompted the creation of GPS

> time and BeiDou system time as opposed to calling those system times

> by any sort of name related to TAI.


> Also notice carefully that the ICD which defines GPS time indicates

> that it is based on UTC(USNO), not on TAI. I suspect that in the

> statutory language of its mission there is no requirement for the USNO

> to deal with TAI, only UTC. It is only the compliance with the

> treaties which brings them to contribute to TAI.

Yes, GPS time is steered to UTC(USNO) (modulo some leap seconds).
The construction of GPS time out of the participating clocks sounds
even more complex than the construction of TAI because GPS time is
also used as the time coordinate of an ephemeris. And the
relationship to UTC(USNO) is available in real time with at
most 90 ns error!

Thanks for the comments!

Michael Deckers.

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