leap history and air traffic

From: Steve Allen <sla_at_ucolick.org>
Date: Sat, 23 Aug 2003 01:55:17 -0700

The report of commission 31 (Time/l'Heure) to the IAU General Assembly
of 1970 contains a terse and dense summary of the activities of
various national and international agencies during the preceding
triennium. Some of the agencies had disregarded requests from others,
and there was notable dissatisfaction with the situation. On page 345
there is a section on "Coordinated Time" which mentions reports from
the CIPM and the CCIR followed by an amazing trio of sentences (I have
attempted to add bits of context using brackets):

    According to the authoritative intervention of D.H. Sadler [who
    strongly opposed the new scheme of leap seconds for UTC] and G.M.
    Winkler [vice president of IAU Comm. 31], who examined thoroughly
    the serious dissensions between those concerned with standard
    frequencies and those who request a time system connected with the
    Earth's rotation (astronomers, geodesists, navigators etc.) and
    in particular the world-wide collision avoidance system for
    aircraft (CAS), which cannot admit stepping time adjustments
    [i.e., leap seconds], serious objections can be made as regards
    the proposals of the two documents; also because said Commissions
    ignore the needs of a great minority who cannot prescind from the
    definition of above mentioned U.T.C. [the original 1960s scheme
    that tracked UT2 using "rubber seconds"] and in particular cannot
    accept differences [between broadcast time signals and UT] greater
    than +-0.1s. It is stated as a necessity that activities
    concerning time service should be completely independent of the
    activities of frequency standards laboratories.

    Discarding the suggestion (for practical reasons and to avoid
    confusions) of two time scales, one approaching U.T. (the present
    [meaning original] U.T.C.) and the other without offsets and
    adjustments, only three alternatives remain: (a) step adjustment
    of 0.1s or 0.2s to maintain U.T.C. sufficiently near to U.T.2 to
    permit to ignore the difference in most of the applications; (b)
    complete disuse of [the original] U.T.C. system replacing it with
    a coordinated uniform time scale without offsets and steps and
    therefore not approaching U.T.; (c) step adjustment of 1s exactly;
    anyway only further discussions will solve the problem.

Between the time of the report and the end of the General Assembly it
had become clear that option (c) was going to happen and that the IAU
had no alternative other than recommending some guidelines regarding
the implementation of leap seconds in the new UTC. In 1973 the IAU GA
resolved that the new UTC "provides mean solar time directly" and
could continue to use the name coined for the previous UT2-based

The tone of the discussions almost makes it seem as if the time and
frequency community has been holding some sort of grudge against
astronomers ever since the ephemeris second was adopted during the
1950s -- if only those !_at_#$% astronomers had picked a second with a
length that matched the mean solar second in the year 2020 instead of
1820 we might have had another century before we had to face the
problems of leap seconds.

In any case I find it most curious to see the mention of air traffic
collision avoidance. This issue was raised when leap seconds were
first adopted in 1970. It has also been raised during the current
round of discussions about broadcast time signals.

Has there really been no improvement in the air traffic control
situation during the past 33 years? If not, why not?

That makes it seem as if somebody has not been trying, or that the
leap second scheme for UTC as implemented is so broken that there can
be no progress.

Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla_at_ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93
Received on Sat Aug 23 2003 - 01:55:35 PDT

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