[LEAPSECS] Metrologia on time
michael.cook at sfr.fr
Thu Aug 4 05:25:37 EDT 2011
Le 04/08/2011 07:27, Tom Van Baak a écrit :
> The question is -- does anyone use this any more? I'd guess
> that any person or any system that needs DUT1 has long since
> switched over to telephone, fax, or the internet to obtain this
> information. The last LF or SW radio that decoded DUT1 was
> made decades ago.
What must be clear now is that the currently transmitted resolution of
DUT1 is not much use for anything except for a long range warning that
a leap may be coming. I have seen references that a couple of Russian
transmitters are or were transmitting DUT1 to 0,1s and a dUT1 to 0,02s
resolution, so when they were installed I guess someone was interested
in a better transmitted value. I agree that it is not clear that anyone
is using it now.
> The internet is faster, more reliable, and far more global than
> LF or short-wave timecodes ever were. Further, you now get
> 4 or 5 digits of precision instead of just 1, as well as history
> and predictions tens or hundreds of days in advance. All with
> one line of code and a URL.
Hmm. I agree with the principle, but the historical data is not in a
standard format and is not scaled uniformly so it needs more than one
line of code to make it usable.
> After that issue is settled, then permitting DUT1 to temporarily
> spill over 1.0 seconds is the next hurdle.
Though I have no particular love for leap seconds, but there is logic
in ITU-R TF.460-6 at least for me, as it provides for the minimum
requirements for any time transmission.
a) Ticks of SI seconds, used by all. Even though UTC is not uniform, the
actual tick is on a TAI sec boundary +/- a few nano seconds.
b) Current value for DUT1. Possibly superfluous as c) is available on air.
c) A civil time scale, UTC, used world wide as a legal time scale,
directly descending from and now synonymous with GMT and approximating
to UT1 which is still the legal definition in many countries laws such
that "day" has its traditional meaning.
d) All that available in one place if everyone plays ball.
The American (just happens to be them) proposition to modify 460-6 only
takes account of a) , not coherently addressing any of the others and in
fact positively discriminating against them, not even deigning to
recommend the transmission of the difference between new UTC and UT1.
New DUT1 will only be available from IERS. Not only that, but it is just
assumed that that data will be "made available" in a uniform format and
machine readable manner. That means that anyone wanting to keep time
with respect to generally understood notion of day has to get on the
internet download the tables maintained in an ad hoc and voluntary
basis and make the change, hoping of course that the scaling used hasn't
changed by magic between one access and the next. I hope their paper is
flushed down an ITU-R toilet with due ceremony. (end of rant)
I do like agree with most here that the transmitted time scale should be
uniform and quite like Steve Allen's proposition, though there are as he
freely admits issues to be taken care of. But I think that really ITU-R
WP7A should be packed off to a large padded cell and not let out until
they have a formal proposition covering points a-d above.
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