[tz] What's "right"?

Steve Allen sla at ucolick.org
Tue Nov 17 16:21:22 UTC 2020

On Tue 2020-11-17T13:23:36+0000 Michael H Deckers via tz hath writ:
> UTC was defined by the BIH since 1961 as a function of TAI.

Alas the process was not that simple, and the issues of BIH
Bulletin Horaire have the tabulations that show how messy it was.

One of the UK radio time signals had begun to be based on the
Greenwich atomic time scale in the late 1950s.  The US NBS had a
similar scheme for WWV based on the NBS atomic chronometers, and the
USNO time signals used their own scheme which was more directly
connected to astronomical observations of recent years.  US NBS
gave a smoother time scale and USNO gave a time scale closer to
astronomical time needed for navigation.

Folks running those schemes got the 9th CCIR plenary assembly in 1959
April to recommend that all time signals should be based on cesium
chronometers.  In August H.M.  Smith of UK had folks from the US for
afternoon tea at his house and they agreed to use the same frequency
offsets and steps.  This was "coordination".

Early in 1960 USNO and one of the UK time signal stations were using
the same scheme.  In April both of the main UK time signal stations
started using that same scheme.  In May the UK and US met to make a
formal agreement about the coordination scheme.  "Full coordination"
between the US and UK time signals was achieved 1961-01-01.

In 1961 April the CCDS saw that various sites with cesium chronometers
had begun to construct atomic time scales.  They recommended that BIH
should use its equipment and skill to compute an atomic time scale
as BIH had always been doing with astronomical time.  By October
Anna Stoyko had created tabulations of atomic time scales beginning
on 1961-01-01.

In 1961 August the 11th IAU General Assembly learned that radio time
signals agreed at the unprecedented level of 1 ms, and with their
oversight of the BIH they recommended that BIH should compute the
frequency offsets and steps starting for 1962.

The Stoykos retired from BIH in 1965, Guinot became head of service.
For 1964 Guinot had begun to change the BIH tabulations that had
been in use since the 1920s.  The original tabulations had regarded
the time at observatories as the reference.  The new tabulations
regarded the time of the broadcast time signals as the reference.

Guinot began tabulating "UTC" for the year 1964.  Those issues of
Bulletin Horaire were not published until 1965, but they are the
first appearance of the term "UTC" (actually TUC) anywhere.
For 1964 "UTC" was as all previous BIH tabulations; it was determined
by combining/averaging the received radio broadcast signals of all
stations claiming to be emitting coordinated time.  It was not
based on the BIH time scale.

For 1965 Guinot determined the frequency offset and steps for
coordinated time, and starting with 1965 UTC(BIH) was computed as a
function of the BIH atomic time scale.  Starting 1965 old UTC begins
to be based on what would become TAI.

It was not until 1968 that the US NBS and the USNO decided to bring
their broadcasts of coordinated time into sync at the microsecond

Up until 1972-01-01 the Soviet radio broadcast time signals were using
their own values of frequency offset and steps which were not the same
as BIH values for coordinated time.

During most of this interval the US NBS was broadcasting old UTC
(never using that name) and stepped atomic time on two different
stations.  Old UTC had been deemed illegal in Germany by a law passed
in 1969 July, but the German DCF77 continued broadcasting old UTC at
some hours of the day until 1970 April.

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           https://www.ucolick.org/~sla/  Hgt +250 m

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