UT1 confidence

From: Zefram <zefram_at_fysh.org>
Date: Wed, 17 Jan 2007 12:24:30 +0000

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
Received on Wed Jan 17 2007 - 04:25:06 PST