[LEAPSECS] Terminology question

Steve Allen sla at ucolick.org
Wed Mar 10 17:42:07 EST 2010

On Wed 2010-03-10T22:01:50 +0000, Michael Deckers hath writ:

> Yes, the relationship between UTC and TAI is simple.

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".
The relationship between any other realization of UTC
and TAI is not simple.

> 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.

> 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 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.

Steve Allen <sla at ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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