On Mon 2003-06-02T17:58:36 +0100, Markus Kuhn hath writ:
> backgrounds, one of the first presentations that gave a case for
> abandoning leap seconds was given by William Klepczynski. He took the
> ADS-B (Automatic Dependent Surveilance Broadcast) system as an example,
> in which GPS receivers installed in civilian aircraft broadcast their
> own position and velocity vectors, such that any receiver can implement
> a "virtual radar" from this data. He gave ADS-B as an application
> example, where a high-precision time scale has (or is about to) become
> an element of a real-world safety-critical air-traffic control system,
> and where the parallel use of several time scales such as UTC and "GPS
> time" could lead to rare and difficult to test malfunctions related to
> UTC leap seconds. UTC would be used to display data to operators
> (pilots, controllers) in real time, but a uniform timescale will also be
> needed to solve motion equations.
This begs the question:
If the system presumes that it has access to GPS, why would its
system design bother to do any operations or data exchange in UTC?
Within the system the designers should choose a single timescale that
makes sense, and for such systems UTC is obviously not it. I don't
expect that there is an answer for this question, else we would have
read about ADS-B long before it was used as a bombshell for the Torino
colloquium.
The Galileo GNSS is going to use TAI, not "GPS time", so I don't see
how crippling UTC resolves the problem of having multiple time scales,
and adding "TI" into the mix in 20 years only makes that problem
worse.
> One claim made was that in many
> real-world systems that run on UTC, the insertation of the leap second
> is today actually delayed until the current operation has finished. This
> leads temporarily to a loss of synchronization by one second for some
> time after each leap second.
I hope that they bothered to name these systems, mostly because it
would set a precedent in the history of this mailing list. So far
nobody has named a system here which cannot tolerate leap seconds.
> John also handed out a paper that he wrote after discussion with list
> members on the role of UTC in national legislation. That paper had been
> considered to be out of the scope of the colloquium by the organizers
> and therefore did not feature in his presentation.
The paper is available via my online bibliography.
% % %
I have previously posted about civil lawsuits that might arise when
the difference between GMT and "leapless civil time" diverge by more
than a few seconds. Subsequently I have privately mused that members
of the SRG might enjoy such litigiousness by becoming highly paid
expert witnesses in courts trying to determine which time was the
legal time. But then I considered that such testimony might be of
more risk than benefit, for the losers in such court cases could
pursue the SRG members with further lawsuits claiming that their
action to redefine UTC caused the economic loss. Now the whole
imaginary melodrama has collapsed, for if nothing happens for 20 years
the SRG members will not be worth pursuit.
% % %
The announcement that Galileo will fly implies that atomic time will
be available just about everywhere on the surface of the earth. We
are told that small receivers will be able to use both Galileo and GPS
to get time (and position). Indeed, for a price Galileo is willing to
put guarantees on accuracy and availability.
For applications deep within buildings and underground, the long wave
broadcasts such as WWVB could continue. They do not provide voice
modulation, and are for rather specialized applications. For the
purpose we use our WWVB receiver, I can say that we would not mind
if the coded modulations started reporting TAI instead of UTC.
For applications that can connect to the internet, it is easy to
imagine NTP-like services that provide either UTC or TAI.
Unless I'm mistaken, none of the above time broadcasts is regulated by
existing ITU recommendations. That leaves short wave radio, which has
historically been of great importance to navigators who actually need
UT1. If atomic time becomes available via all these other means, how
relevant are the recommendations of the ITU-R regarding "time signal
emissions"?
--
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 Mon Jun 02 2003 - 19:26:19 PDT