I just came back from the ITU-R SRG 7A Colloquium on the UTC timescale
at IEN Galileo Ferraris, Torino, Italy, 28-29 May 2003.
Here are few brief personal notes, which are by no means intended to be
complete, accurate or unbiased in any respect. This is just a quick
attempt to dump some of my scribblings and thoughts here, and I hope
other attendants will do the same, to preserve a more complete picture.
The meeting was organized and chaired by Ronald Beard (US Naval Research
Labs) and William Klepczynski, both of the ITU SRG. There were about 38
attendants, whose backgrounds can be summarized roughly as time &
frequency metrology, astronomy, aerospace industry, satellite navigation,
and one computer scientist.
I was in general pleasantly surprised by the high quality of the
presentations and the scope and comprehensiveness of the discussion.
The first few talks included reviews and some first-hand accounts of the
history of time standards, and in particular of TAI and UTC. The
astronomical background was also covered, and in particular the state of
the art in UT1 prediction, including speakers from BIPM and IERS.
We heard for example details of the definition of UT1 and how it
approximates GMT, but can't do so exactly because of rather complex
reasons. We heard how TAI was started in 1958 and was set to match UT2
(as it was defined then) on 1 Jan 1958 as much as was practically
achievable at the time. TAI and UT1 will have drifted apart by about 30
minutes in the year 2600 and by about 60 min in 3000. Judging from past
IERS data, it seems feasible at present to predict UT1 over 2 years with
an accuracy of better than 600 ms, and over 3 years with better than
1000 ms.
After the introductory talks that presented the scientific and historic
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. 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.
Throughout the presentations and discussions, it became clear that
people present were only aware of one single documented incident in
which a leap-second appears to have contributed to a severe disruption.
In the early 1990s, the Russian GLONASS navigation system, which is
internally based on the UTC timescale, became unavailable for 20 hours,
apparently due to a leap-second-related operator error. Details of the
incident seem not to be well documented and it appears that GLONASS had
no problems with more recent leap seconds.
Several of the talks proposed to set a dealine in the foreseeable future
(2022, the 50th anniversary of UTC was suggested), after which no
further leap seconds would be inserted into UTC. Several participants
expressed strong opinions on that after such a change, the resulting
time zone must not be continued under the name UTC, because the term
"Universal Time" in astronomical parlance clearly implies a close
coupling of a time zone to the earth's rotation angle. The name "Temps
International (TI)" was proposed, which is just TAI with the A dropped.
This new name has the advantage that it removes the technical
implementation of the time scale from TAI. If we stop adding leap
seconds to UTC in a decade or two and then rename the resulting time
into TI, we would get TI = TAI - x, where x is a small integer,
probabaly a bit above 40. A migration from UTC to TI would avoid the
about-one-minute jump that would be necessary if we moved from UTC to
TAI as the basis of civilian times.
One of the BIPM talks proposed to keep the UTC term in the interest of
diplomacy and merely relax the UTC tolerance to at least half an hour,
such that UTC could follow UT1 using leap hours. The first such leap
hour would be expected to become necessary near the year 2600. Various
people (including myself) voiced great concern about this variant in the
discussion and asked that if we really introduced a uniform (leap-free)
time scale, it should be _really_ uniform, and not just pile up the
problem into a larger one for future generations. In he
