Timesys note review

Steve Allen sla at ucolick.org
Thu Nov 29 05:12:06 CET 2018

On Tue 2018-11-27T13:23:50+0100 Markus Demleitner hath writ:
> I've slightly updated the Timesys note, incorporating some feedback; in
> particular, this fixes a broken example and updates descriptions of the
> time scales (volute revs. 5235 and 5236).  Before we publish version 1.1:
> does anyone have other changes?  Or corrections to the changes?
> A pre-built PDF is available on
> http://docs.g-vo.org/timesys-draft.pdf, the source is still available
> from
> https://volute.g-vo.org/svn/trunk/projects/time-domain/timesysnote.

Table 1

I strongly object to the notion that the term UTC shall be used for
time stamps that were originally labelled as GMT.  The term used for
that should be just plain UT.

UTC has always referred to an atomically-regulated time scale.  There
were no atomically-regulated time scales available outside of
laboratories before 1960.  All available timestamps before 1960 were
based on UT.  UT is the name for the scale which has continuity with
UTC and which does not have the ambiguity of 12 hours that GMT has at
1925.  The term UTC is not found in any publication before 1965.

The distinction between UT, UT0, UT1, and UT2 was worked out after the
1955 IAU GA and starting 1956-01-01 all radio broadcasts were directed
to supply timestamps of UT2.  But time signal broadcasts from the UK
had started to broadcast not GMT but "PUT" (Provisional Uniform Time)
to correct for what would be called UT1 by 1950, and they were
correcting for what would be called UT2 starting in 1952.  Radio
broadcasts by USNO and US NBS were also attempting corrections of this
sort before and after 1956 with USNO broadcasts calling their time
scale names like "N3c" and striving more toward variable frequency
with fewer, smaller steps of time and US NBS broadcasts striving more
toward uniform frequency with more, bigger steps of time.

Even after 1961 when all radio broadcast time signals were supposed to
follow the frequency/rate and big steps as given by the BIH the
individual stations performed their own smaller time steps and
frequency changes ad hoc.  Some of those peculiar changes are recorded
in BIH Bulletin Horaire, and some are not.  Meanwhile the Soviets and
Chinese were broadcasting using their own scheme of coordination.

So without digging through the tomes of BIH Bulletin Horaire and the
circulars published by the radio broadcasters to find out just exactly
what their time stamps meant the best that can be said for a time
stamp before 1960 (and even after up until 1972-01-01) is that it was
simply "UT".

For TAI note that although TAI can be extrapolated back to mid 1955
there was a step of frequency of 1e-12 on 1977-01-01.  Although some
time labs were constructing their own atomic time scales before 1960,
The CCDS did not recommend any attempt to compare those with each
other until their meeting of 1961-04-11/12.  The first published
attempt at something that would become TAI appeared by 1967-07 and was
called Temps Atomique Integré.  The CCDS first published a definition
for TAI during their meeting of 1970-06-18/19, and the 14th CGPM
authorized that on 1971-10-04.

The IAU directed that the catalog used for time determination changed
from FK3 to FK4 on 1962-01-01, and that caused a shift of 1.5 ms in
the values of UT that were being provided by various observatories.
The extrapolated estimates of the value of TAI at that date and before
have uncertainties and differences of similar size.

Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
1156 High Street               Voice: +1 831 459 3046         Lng -122.06015
Santa Cruz, CA 95064           http://www.ucolick.org/~sla/   Hgt +250 m

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