[time-nuts] NIST UT1 NTP server results

Steve Allen sla at ucolick.org
Sat Jul 23 18:21:25 EDT 2016

On Sat 2016-07-23T12:36:21 -0700, Tom Van Baak hath writ:
> I've always been curious about the conflict between accuracy and
> stability with these various time scales.
> If the purpose of a UTx clock is long-term timekeeping, then I can
> see that smoothing is helpful.  OTOH, if the purpose of a UTx clock is
> to pinpoint a telescope, then smoothing is exactly what you don't
> want.  That is, what you want is to know where to point the telescope
> right now, not the average of where you would have pointed the
> telescope each day over the past year.  The former is a precise
> actionable measurement; the latter is a filtered historical statistic.

I wrote this up for the 2011 Future of UTC colloquium in the context
of what we would have to do if leap seconds were abandoned
Of those telescopes the ones operated by humans do not care because
the operators understand the quirks and will be able to adapt to
whatever kind of time is provided.  For the telescopes pointed by
computer using software that we do not own we are fortunate to be able
to tell their software lies about the longitude.  (Basically the older
telescopes are built like battleships, by the same foundries, and they
point like battleships.  The newer telescopes have software components
which expect UTC with leap seconds as input.)

> Can you shed light on this?  Who / how / why was UT0 vs.  UT1 vs.
> UT2 used in practice?

UT0 was the name coined around the end of 1955 for what had been
called just plain UT before there was any consistent scheme for
dealing with the fine details of nonuniform earth rotation.  Except as
a stepping stone in the calculations to produce a globally consistent
UT1 my impression from reading the contemporary accounts is that
references to UT0 bordered on disdain that anyone would use it.  As is
the case for lots of terminology when the original meaning evolves,
literature continued to have examples of just plain UT found both in
cases where the user did not know the distinctions and in cases where
the user knew that the distinctions were not relevant.

For all practical purposes UT1 has the same meaning now as it always
did.  It is the angle of earth rotation calculated using the
conventional expressions and measured in the reference frame that are
currently trendy.  That remains the notion by which calendar days are

My impression of UT2 was that its invention was motivated by a desire
not to be tweaking the frequencies used in radio broadcasts during a
year by using conventional expressions for the mostly-predictable
annual variations in earth rotation.  Tweaking frequencies more often
than that was not considered given that during the 1960s the BIH
publications of "Heure Definitive" were coming out a year after the
measurements which went into calculating that "time which should have
been broadcast".  Thus UT2 was the best that a radio engineer could
try for frequency stability while keeping time broadcasts close to
the earth.

The agencies contributing to BIH improved their methods and the BIH
got better computing capability.  It became clear to astronomers and
the BIH that the unpredictable variation of earth rotation, even
within one year, could be bigger than the annual corrections.  At the
IAU general assembly in 1970 the usage of (just plain UT) and UT2 was
still in active discussion.  By the IAU general assembly in 1973 a
specialist in atomic clocks and radio transmission opined that BIH
should change its publications to tabulate discrepancies of radio
broadcasts from UTC rather than UT2.  I think this is basically saying
that the introduction of leap seconds had already satisfied the
underlying needs of folks who wanted frequency stability, so that
purpose of UT2 was no longer relevant.  BIH did change its tabulations
soon after, and that was effectively the end of UT2 for any purpose
other than re-interpreting historical observations.

As a result of the radio-centric control over "what time is it?"  the
original draft of the CCIR leap second document which became Rec.  460
specified that the leap seconds should try to keep time close to UT
(just plain UT), and in the context of earlier CCIR recommendations on
broadcast time signals they meant UT2.  At the IAU General Assembly in
1970 it was pointed out that the almanac expressions used for
navigation were based on UT1, not UT2, so the radio broadcasts with
leap seconds should track UT1.  That suggestion and others from the
1973 IAU GA went into the first revision of CCIR Rec.  460-1.

So what is the goal of a NTP server giving UT1 instead of UTC?

If the goal is the best frequency smoothness over an interval of a
year then UT2 makes sense now just as it did to the radio engineers in
the 1960s.  That would be close enough to earth rotation time and
calendar days for almost any civil purpose, but I do not know if any
existing system still needs that frequency smoothing goal.

If the goal is the closest accuracy to instantaneous earth rotation
then UT1 makes sense.  That would be desirable if the intended use is
as direct input to navigation or pointing, but I can't point to any
existing system that can use such an input.

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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