On Sat 2016-07-23T12:36:21 -0700, Tom Van Baak hath writ: > I've always been curious about the conflict between accuracy and > stability with these various time scales. > > If the purpose of a UTx clock is long-term timekeeping, then I can > see that smoothing is helpful. OTOH, if the purpose of a UTx clock is > to pinpoint a telescope, then smoothing is exactly what you don't > want. That is, what you want is to know where to point the telescope > right now, not the average of where you would have pointed the > telescope each day over the past year. The former is a precise > actionable measurement; the latter is a filtered historical statistic.
I wrote this up for the 2011 Future of UTC colloquium in the context of what we would have to do if leap seconds were abandoned http://futureofutc.com/2011/program/presentations/AAS_11-678_Allen.ppt.pdf http://futureofutc.com/2011/preprints/38_AAS_11-678_Allen.pdf Of those telescopes the ones operated by humans do not care because the operators understand the quirks and will be able to adapt to whatever kind of time is provided. For the telescopes pointed by computer using software that we do not own we are fortunate to be able to tell their software lies about the longitude. (Basically the older telescopes are built like battleships, by the same foundries, and they point like battleships. The newer telescopes have software components which expect UTC with leap seconds as input.) > Can you shed light on this? Who / how / why was UT0 vs. UT1 vs. > UT2 used in practice? UT0 was the name coined around the end of 1955 for what had been called just plain UT before there was any consistent scheme for dealing with the fine details of nonuniform earth rotation. Except as a stepping stone in the calculations to produce a globally consistent UT1 my impression from reading the contemporary accounts is that references to UT0 bordered on disdain that anyone would use it. As is the case for lots of terminology when the original meaning evolves, literature continued to have examples of just plain UT found both in cases where the user did not know the distinctions and in cases where the user knew that the distinctions were not relevant. For all practical purposes UT1 has the same meaning now as it always did. It is the angle of earth rotation calculated using the conventional expressions and measured in the reference frame that are currently trendy. That remains the notion by which calendar days are defined. My impression of UT2 was that its invention was motivated by a desire not to be tweaking the frequencies used in radio broadcasts during a year by using conventional expressions for the mostly-predictable annual variations in earth rotation. Tweaking frequencies more often than that was not considered given that during the 1960s the BIH publications of "Heure Definitive" were coming out a year after the measurements which went into calculating that "time which should have been broadcast". Thus UT2 was the best that a radio engineer could try for frequency stability while keeping time broadcasts close to the earth. The agencies contributing to BIH improved their methods and the BIH got better computing capability. It became clear to astronomers and the BIH that the unpredictable variation of earth rotation, even within one year, could be bigger than the annual corrections. At the IAU general assembly in 1970 the usage of (just plain UT) and UT2 was still in active discussion. By the IAU general assembly in 1973 a specialist in atomic clocks and radio transmission opined that BIH should change its publications to tabulate discrepancies of radio broadcasts from UTC rather than UT2. I think this is basically saying that the introduction of leap seconds had already satisfied the underlying needs of folks who wanted frequency stability, so that purpose of UT2 was no longer relevant. BIH did change its tabulations soon after, and that was effectively the end of UT2 for any purpose other than re-interpreting historical observations. As a result of the radio-centric control over "what time is it?" the original draft of the CCIR leap second document which became Rec. 460 specified that the leap seconds should try to keep time close to UT (just plain UT), and in the context of earlier CCIR recommendations on broadcast time signals they meant UT2. At the IAU General Assembly in 1970 it was pointed out that the almanac expressions used for navigation were based on UT1, not UT2, so the radio broadcasts with leap seconds should track UT1. That suggestion and others from the 1973 IAU GA went into the first revision of CCIR Rec. 460-1. So what is the goal of a NTP server giving UT1 instead of UTC? If the goal is the best frequency smoothness over an interval of a year then UT2 makes sense now just as it did to the radio engineers in the 1960s. That would be close enough to earth rotation time and calendar days for almost any civil purpose, but I do not know if any existing system still needs that frequency smoothing goal. If the goal is the closest accuracy to instantaneous earth rotation then UT1 makes sense. That would be desirable if the intended use is as direct input to navigation or pointing, but I can't point to any existing system that can use such an input. -- Steve Allen <s...@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.