UT1 confidence
IERS Bulletin A gives an expression for the uncertainty of its UT1-UTC data predictions: S t = 0.00025 (MJD-today)**0.75 where today is the MJD of the bulletin's publication. The Bulletin only predicts a year ahead. Applying that formula gives an uncertainty a year ahead of 21 ms. It certainly ought to be possible, based on such a prediction, to decide with certainty whether a leap second will be required within that year. With the six-month scheduling cadence and a one-year prediction, we'd expect the kind of scheduling that we actually see: shortly after each leap opportunity they can look ahead to the next opportunity but one, and decide whether there needs to be a leap at the next opportunity. It seems to me that a switch to a monthly scheduling cadence, as Rob Seaman advocates, would have the immediate benefit of allowing a ten or eleven month scheduling lead instead of the current five or six months, without any advance in predictive ability. Immediately after each leap opportunity they could look *twelve* leap opportunities ahead, and thus decide whether the eleventh would be a good time for a leap. This is in addition to the ability to keep UT1-UTC within tighter bounds, which Rob's proposal describes. But I digress. I'm wondering how good UT1 predictions further ahead are. If the formula remains valid, it suggests that UT1 could be predicted to within 100 ms as far as eight years ahead. 100 ms is certainly a good enough prediction to schedule leap seconds on. My assumption there is highly suspect, though. Anyone know better? Does IERS publish any EOP predictions more than a year ahead? -zefram
Re: UT1 confidence
In message [EMAIL PROTECTED], Zefram writes: IERS Bulletin A gives an expression for the uncertainty of its UT1-UTC data predictions: S t = 0.00025 (MJD-today)**0.75 where today is the MJD of the bulletin's publication. The Bulletin only predicts a year ahead. Applying that formula gives an uncertainty a year ahead of 21 ms. The question is what domain of validity the above formula has ? In the builletin they only apply it up to 40d. For an argument of 10 years the result is 0.12 seconds. For an argument of 100 years the result is 0.66 seconds. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
Re: UT1 confidence
In message [EMAIL PROTECTED], Zefram writes: IERS Bulletin A gives an expression for the uncertainty of its UT1-UTC data predictions: S t = 0.00025 (MJD-today)**0.75 where today is the MJD of the bulletin's publication. The Bulletin only predicts a year ahead. Applying that formula gives an uncertainty a year ahead of 21 ms. The question is what domain of validity the above formula has ? In the builletin they only apply it up to 40d. In addition, since the drift is a higher order polynomial (13th order I recall hearing), a simple linear formula (or close approximation) is likely only going to fit the curve near the prediction date with wider variances the further one gets from today. It has been remarked that the current state of the art is that 100ms accuracy can be predicted about a year in advance only and that the models are constantly undergoing refinement. It has been estimated that IERS could issue leap seconds, with today's technology, about 3-5 years out and still be in a 95% or 99% band of certainty that the 0.9s margin is maintained. However, I can't find papers that show these models or point to any better data than hearsay... Warner
Re: UT1 confidence
On Wed 2007-01-17T12:31:14 -0700, Warner Losh hath writ: It has been remarked that the current state of the art is that 100ms accuracy can be predicted about a year in advance only and that the models are constantly undergoing refinement. It has been estimated that IERS could issue leap seconds, with today's technology, about 3-5 years out and still be in a 95% or 99% band of certainty that the 0.9s margin is maintained. However, I can't find papers that show these models or point to any better data than hearsay... The best that I know of were the ones presented at the Colloquium that the WP7A SRG held in Torino in May 2003. There was a time when the host institution (IEN) was providing the proceedings online, but the contents of that URL went away sometime around a year ago. (I wonder if they may not have liked the conclusion that was reached.) In the spirit of promulgation I provide what they once did at http://www.ucolick.org/~sla/leapsecs/torino/ITU.shtml The conclusion was originally a powerpoint drafted in real time, it is http://www.ucolick.org/~sla/leapsecs/torino/closure.pdf The indications of how well predictions of UT1 might be done are found in three presentations to which Felicitas Arias contributed. There are two which were powerpoint http://www.ucolick.org/~sla/leapsecs/torino/guinot.pdf http://www.ucolick.org/~sla/leapsecs/torino/arias_2.pdf and one which is a more verbose writeup of one of the powerpoints http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf The plots by Arias indicate how well UT1 could have been predicted over two and three year intervals for the 40 year interval starting around 1960. It is based on those plots that I have voiced no concerns for the pointing of our telescopes if leap seconds were published five years in advance. I'm not ready to go for ten. -- Steve Allen [EMAIL PROTECTED]WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99858 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06014 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m
Re: UT1 confidence
Steve, thank you for this enlightening report. In message: [EMAIL PROTECTED] Steve Allen [EMAIL PROTECTED] writes: : On Wed 2007-01-17T12:31:14 -0700, Warner Losh hath writ: : It has been remarked that the current state of the art is that 100ms : accuracy can be predicted about a year in advance only and that the : models are constantly undergoing refinement. It has been estimated : that IERS could issue leap seconds, with today's technology, about 3-5 : years out and still be in a 95% or 99% band of certainty that the 0.9s : margin is maintained. However, I can't find papers that show these : models or point to any better data than hearsay... : : The best that I know of were the ones presented at the Colloquium that : the WP7A SRG held in Torino in May 2003. There was a time when the : host institution (IEN) was providing the proceedings online, but the : contents of that URL went away sometime around a year ago. (I wonder : if they may not have liked the conclusion that was reached.) : : In the spirit of promulgation I provide what they once did at : http://www.ucolick.org/~sla/leapsecs/torino/ITU.shtml : : The conclusion was originally a powerpoint drafted in real time, it is : http://www.ucolick.org/~sla/leapsecs/torino/closure.pdf : : The indications of how well predictions of UT1 might be done are found : in three presentations to which Felicitas Arias contributed. : There are two which were powerpoint : http://www.ucolick.org/~sla/leapsecs/torino/guinot.pdf : http://www.ucolick.org/~sla/leapsecs/torino/arias_2.pdf : and one which is a more verbose writeup of one of the powerpoints : http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf I like figure 8, that shows that 20ms steps lead to 50ms steps lead to 100ms steps lead to 1s steps. :-) I also like the quotes: Dating in UTC is ambiguous when a positive leap second occurs in systems other than those using hours, minutes and seconds; in this latter system the use of second 60 may a cause of difficulty. and Thus UT1 is not, strictly speaking, a form of solar time Also, did I miss figure 9 in arias_3? Proposal II has gotten much press here (the leap hour one), but Proposal I sounds a lot like what I've suggested: Use TAI time and let countries move the time zones when they feel like they no longer are close enough, but it kinda omits that last part... Warner
