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 [EMAIL PROTECTED] 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