[LEAPSECS] DCF77 and the inception of leap seconds

[Steve Allen]

There is more history leading to the inception of leap seconds evident
in close inspection of the issues of Bulletin Horaire.

During the early 1960s the German federal time broadcast station DCF77
did not broadcast time signals 24 hours a day.  Historically this was
also true of many other time broadcasts which used equipment that was
shared between many purposes.  As the 20th century progressed many
time signals were generally broadcast during those hours when the
ionosphere was quiescent between transmitter and the expected set of
navigational users.  Ships near those transmitters knew to tune in and
check their clocks at the right time.

Furthermore, the signals from different places did not agree with each
other, but they did agree with longitudes on local navigational
charts.  Different regions of the earth had assumed different local
origins of longitude when making their charts, and those were not
globally consistent with Greenwich at the fine level.  Time was
longitude, and longitude was time, and they had to agree.

The local broadcasts used times offset from each other, but consistent
with the charts in use.  These broadcast time offsets persisted until
new charts were issued and decreed to come into general use.  I
suspect that close inspection of regional navigational charts in the
1960s will show changes that are consistent with those of the time
signals.

In the early 1960s the DCF77 broadcasts were run by the German
Hydrographic Institute for the sake of navigation.  The time scale was
not "coordinated".  This continued several years after the various
scientific bodies and CCIR Rec 374 said that everyone should be
broadcasting the "coordinated" original form of rubber second UTC.

About 1965 DCF77 changed its broadcasts such that there were two
different kinds of broadcasts at different times of day.  (Bulletin
Horaire is not entirely clear on when; it had always been the case
that BIH learned about a lot of changes after the fact.)  The German
Hydrographic Institute continued to control broadcasts at some times
of day, and those were the CCIR-approved form of rubber second UTC.
At other times of day the broadcasts were controlled from the German
Metrology Institute (PTB).  The PTB-controlled broadcasts were pure SI
seconds thus making those broadcasts a form of Stepped Atomic Time
which was approved as experimental by CCIR Rec 374-1 in 1966.

Sometime after 1967 October when the CGPM approved the SI second based
on cesium the German broadcasters convinced themselves that the rubber
second form of UTC was no longer legal to broadcast in Germany.  This
departure from the agreed form of "coordination" led to the urgent
series of CCIR working group meetings during 1968 and 1969 which
resulted in the proposal for leap seconds presented to the CCIR
assembly in 1970 February.

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