Hi Steve,

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.

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

Thanks,
/tvb

----- Original Message -----
From: "Steve Allen" sla@ucolick.org
To: "Discussion of precise time and frequency measurement" time-nuts@febo.com
Sent: Saturday, July 23, 2016 8:52 AM
Subject: Re: [time-nuts] NIST UT1 NTP server results

Interesting plots. A couple of points.

1. These look like the data points are taken at 0h and without intermediary measurements as the data points are connected by straight line segments. If we don’t know what the intermediary data points are, the plots, to my mind, should be presented « with steps ».
2. It is not beyond the abilities of IERS to determine these intermediary data points, and I expect that they already do so, or for them to disseminate them in real time.. If that were the case then any self respecting UT1 server would be able to reproduce the phase differences exactly. That is what I personally would like, but would perfectly happy in the meantime with interpolated data points. Steps aren’t good enough for reasons I previously outlined.

I suppose that the answer to that depends on the objective of having a readily available accurate UT1 timescale realization. It has an intrinsic value unrelated to leap seconds so we should have it. It’s dissemination over GPS/radio would also allow the possibility of having a civil timescale without leap seconds where there would not be ‘rare moments’ of 86400 second days, but every day could have exactly 86400 seconds.

I don’t think that proponents of having UT for a civil time scale have the characteristics that you attribute to them. Get out of bed the wrong side?

"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw

On Sat 2016-07-23T12:36:21 -0700, Tom Van Baak hath writ:

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
http://futureofutc.com/2011/program/presentations/AAS_11-678_Allen.ppt.pdf
http://futureofutc.com/2011/preprints/38_AAS_11-678_Allen.pdf
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.)

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
defined.

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@ucolick.org              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
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Hi Mike,

Here's the two IERS sites I frequent. Maybe you can locate data with greater precision or shorter tau:

https://hpiers.obspm.fr/eop-pc/

http://maia.usno.navy.mil/

I like the plots a lot. These are phase measurements of a continuous oscillator (earth). It's ok to connect time/phase points. You can argue about the rate/frequency data: For this, Stable32 uses steps; TimeLab uses cubic spine. I would suggest you work with the raw data (which includes error bars as well). It's the same data I used for:

http://leapsecond.com/museum/earth/

Are you asking for fraction of a day resolution at tens of microseconds accuracy? I don't think VLBI gives that. This is not unlike any of us trying to measure a quartz oscillator -- there's a trade-off between resolution and accuracy; between short-term noise and long-term precision. We all face this, whether using nixie counter or a TimePod. Every oscillator (including earth) and every measurement system (including VLBI) has an ADEV curve.

I believe this is what Judah was talking about when he mentioned interpolation; that is, the sub-day precision of UTx measurements. So see what you can find, look into different interpolation schemes, see what practical effect it would have on the average computer running NTP. There might be a best case; there might be a simplest case. It's probably worth looking into.

Take a step back and consider time & frequency measurement 101. The earth is an oscillator. You cannot measure it continuously or with perfect resolution. I know you don't like steps. Yes, oscillators are analog. But when you make periodic, digital measurements you get steps, in both time and value. Even a GPSDO has steps (the DAC).

Earth is a very noisy, wandering, drifting, incredibly-expensive-to-measure, low-precision (though high-Q) clock. I think it's cool to try to discipline the quartz oscillator in a computer against earth. But I'm still wondering if you or someone else could outline the set of advantages this provides.

Thanks,
/tvb