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Geoffrey Kolbe wrote:
> Gary was comparing his clocks against WWV, which
> is UTC or "broadcast time". But UTC is itself
> being constantly compared to UT1 and periodically
> a second of time is inserted (or taken away) from
> UTC so that UTC continues to agree with UT1 to
> within +/- 0.9 seconds. So, what Gary is actually
> comparing his clocks against is UT1, not UTC.

I disagree, Geoffrey. Like Gary, I periodically compare clocks to UTC
(WWV) and log the results. It is not the same as comparing to UT1. Logs
of clock performance with respect to UT1 and UTC would would have
noticeable differences. Clock error could be up to 0.8 second different.

Daily rate would be the same except after a leap second, when the UTC
log would show a momentary deviation from normal. By adjusting for the
leap second as I explained in a previous message, the rate discontinuity
can be eliminated. However, the discrepancy in clock error remains.

To compare a clock to UT1, Gary can first note its error relative to
UTC. Then, by listening to the double ticks in the WWV audio, determine
DUT1 (= UT1-UTC rounded to 0.1 second). Algebraically subtract DUT1 from
clock error (with respect to UTC) to obtain error with respect to UT1.
For example, if clock error (UTC) is +0.1 s, and DUT1 = +0.1 s, clock
error (UT1) = 0.

That can be done retroactively if records of clock error relative to UTC
exist. Daily precise values of UT1-UTC are in the IERS Bulletin B archive.

No special procedure is required for leap seconds, because there is a
step adjustment to UT1-UTC at the same time. See Bulletin B at the end
of June 2012:
ftp://hpiers.obspm.fr/iers/bul/bulb_new/bulletinb.294

Nevertheless, I prefer to compare clocks to UTC. The main reason is that
there's no UT1 time standard. UT1 must be derived from UTC every time
the clock is checked. It's less trouble to use UTC directly and apply an
adjustment for the occasional leap second.

In effect, my log shows clock error relative to UTC, and rate relative
to atomic time.

Back in the 1970s when I was a shortwave listening hobbyist there was a
time station (BPM in China?) which broadcast UT1. It was on or close to
the WWV frequency. You could hear them simultaneously, and the offset in
the ticks was obvious. With such a station it would be easy to compare a
clock to UT1.

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