[LEAPSECS] leapseconds, converting between GPS time (week, second) and UTC

[Steve Allen]

On Wed 2019-01-16T22:24:46-0800 Paul Hirose hath writ:
> On 2019-01-16 1:35, Steve Allen wrote:
> >
> > Does it know that rubber seconds do not apply to timestamps in central
> > Europe made using DCF77 from 1970-01-01 to 1972-01-01?
>
> All my DLL knows is what's in the UTC file. If the file indicates no rate
> offset in the years 1970-71 and some fractional second step adjustments,
> that's what you get. An existing file could be edited to match the DCF77
> flavor of UTC. Then construct a UtcTable object from the file:

There lies the problem with timestamps, then and now.

The folks running DCF77 had convinced themselves that German law did
not permit them to broadcast seconds which were not SI seconds, so
they stopped broadcasting the rubber second version of UTC which was
then the CCIR standard.  Instead they convinced themselves that
broadcasting "mostly legal" SI seconds was more acceptable under
German law.

The issues of Bulletin Horaire over the decades show that the
existence of any difference between the time signal broadcasts of any
service was not acceptable and repeatedly resulted in internatioal
meetings with recommendations putting pressure on every transmitter to
conform.

This was the reason for the creation of UT2.  In the 1950s
the issues of Bulletin Horaire show that the UK had started
correcting their broadcasts, first for what would become called UT1,
and then for what would be called UT2.  The US Navy broadcasts
did likewise, but using a different expression for what would become
called UT2.  The US NBS WWV broadcasts chose to prefer more steps and
fewer frequency offsets than US Navy because the NBS was more
interested in standard frequency than in astronomical time.

Bulletin Horaire says that by 1955 there were at least 4 different
schemes being used to produce broadcasts of time scales akin to what
would become called UT2.  Bulletin Horaire explains that was the
motivation behind the 1955 IAU decision to create UT0, UT1, and UT2.

Then folks became uncomfortable with the way that different stations
attacked the problem of broadcasting something like UT2.  The US Navy
method of approximating UT2 using fewer time steps and more changes of
frequency offset was good for navigators who would not see their
position suddenly shift.  The US Navy method was also an irritant, in
part to folks who had been driving CCIR recommendations toward making
better use of spectrum by reducing spacing between stations using
adjacent frequencies, and in part to engineers who had to retune the
transmitting gear.

The existence of cesium frequency standards made it easy for everyone
to see the changes in offsets that had previously been hard to
measure, but it also allowed for the agreement on the original form of
rubber second UTC where everyone agreed to use the same frequency
offsets and time steps.  The first version of that agreement was
forged over tea in the living room of H.M. Smith, one of the
Greenwich engineers associated with UK radio broadcast time signals
over the span of his career.  After the US and UK had codified their
rubber second agreement it was taken to URSI, CCIR, and IAU to get
official international status as recommendations.

After the CGPM approved the cesium SI second, the German decision that
in 1970 they would no longer conform to rubber second UTC led to
urgent need to create yet another agreement on a different way of
broadcasting time that could be legally tolerated in all countries.
There was also dissatisfaction with the changes in frequency and time
steps from the LORAN engineers who had to re-tune and re-phase entire
chains of transmitters, and the LORAN users who could not navigate
during the interval when the chain was retuning and rephasing.

So for your software the German decision means that the only way to
decode timestamps in central Europe from 1970/1972 is to find the
publications where they announced when DCF77 inserted their markers
which were spaced by 1.2 SI seconds.  The USNO circulars which
announced changes in their broadcasts were printed on high-acid paper
which has self-destructed while sitting in the few libraries which
bothered to store them.  I do not even know the name of the German
publications which would have announced their time signal steps.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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