On Wed, Jan 04, 2006 at 07:36:17AM +0100, Poul-Henning Kamp wrote:
In message [EMAIL PROTECTED], Neal McBurnett writes:
On Tue, Jan 03, 2006 at 08:32:08PM +0100, Poul-Henning Kamp wrote:
If we can increase the tolerance to 10sec, IERS can give us the
leapseconds with 20 years notice and only the minority of computers
that survive longer than that would need to update the factory
installed table of leapseconds.
Do you have any evidence for this assertion?
It is an educated guess.
The IERS have already indicated that they belive they could do prediction
under the 0.9 second tolerance with two or three year horizon.
The Torino Colloquium had some discussion of this.
Proceedings of the Colloquium on the UTC Timescale held by
ITU-R SRG 7A
http://www.ien.it/luc/cesio/itu/ITU.shtml
Prediction of Universal Time and LOD Variation
D. Gambis and C. Bizouard, (IERS)
http://www.ien.it/luc/cesio/itu/gambis.pdf
After a bunch of nice graphs (not all of which were easy to interpret)
I found the periodogram (essentially a discrete Fourier transform of
the input data) interesting. The way I read it (expert advice
welcomed), the broad peaks at 26 years (0.6 ms/d) and 52 years (0.3
ms/d) suggest that the most common pattern is a gradual cycle a few
decades long of lengthening and shortening of the day, presumably
driven by movements in the earth's mantle and core.
Page 14 of the pdf has a table:
Skill of the UT1 prediction statistics over 1963-2003
Horizon Prediction accuracy in ms
3 years 308
2 years 163
1 year 68
180 days 36
90 days21
30 days 7
10 days 3
Perhaps these are worst cases? It would be nice to have confidence
intervals.
They presented these conclusions:
Possibility to predict UT1 with a 1s accuracy at least over 4 years
using a simple method : seasonal, bias and drift.
New prediction methods are under investigation (Singular Spectrum
Analysis, neural network,..)
Possibility to use Core Angular Momentum prediction for decadal
modeling
Steve Allen wrote:
http://www.ucolick.org/~sla/leapsecs/McCarthy.html
This deserves discussion and analysis and explanation.
I wrote Dennis McCarthy about that, and he said he'd look up the
details and get back to me next week. But he did remind me of this,
which I remember seeing in data they published via ftp years ago:
Regarding the accuracy of these long-term predictions, the IERS
Rapid Service and Prediction Center located at the U. S. Naval
Observatory does make predictions of Delta-T in the IERS Annual
Report. The algorithm for those predictions was determined
empirically by testing a wide range of possibilities. It is
essentially an auto-regressive model using the past ten years of
data. The accuracy based on comparison of observations with what
would have been predicted using that model is shown in the table
below. Note that the accuracy estimates are 1-sigma estimates and
that excursions of 2-sigma (or more) may not be unexpected.
+-+
|Year in the Future|Accuracy (1s) (seconds|
|--+--|
|1 | .04 |
|--+--|
|2 | .08 |
|--+--|
|3 | .3 |
|--+--|
|4 | .8 |
|--+--|
|5 | 1. |
|--+--|
|6 | 2. |
|--+--|
|7 | 2. |
|--+--|
|8 | 3. |
|--+--|
|9 | 4. |
+-+
The http://www.iers.org/ points eventually to
http://141.74.1.36/MainDisp.csl?pid=47-25786
but the links from there to the annual reports seem broken right now.
I still haven't seen any good data on predictions for periods of
longer than 9 years.
Neal McBurnett http://bcn.boulder.co.us/~neal/
