On 2017-02-04 12:24 PM, Warner Losh wrote:
On Sat, Feb 4, 2017 at 9:41 AM, Clive D.W. Feather <[email protected]> wrote:
Looking only into the future, not historical data, what do people think the
probability is that TAI-UTC will ever be negative? Should data structures
be designed to handle this case or not bother?
Doubtful, but not impossible.
LoD would have to increase significantly for that to happen. The
current LoD delta is about 1ms. This looks to vary between -500us to
2ms on IERS' web page (http://hpiers.obspm.fr/eop-pc/index.php). It
also looks like the short term forecast is to return towards 0 in the
next 150 days or so. Looking at even longer term data from
http://www.usno.navy.mil/USNO/earth-orientation/eo-products/long-term
we see huge error bars for LoD just a few hundred years ago and a big
drop in deltaT from 1850-1900. So the data we have suggests that it is
unlikely, but not outside the realm of possibility.
It's quite likely that the first negative leap second would have a
much higher bug rate than the current positive leap code which has at
least been tested several times and is known to still have issues.
Yes, a negative Leap Second seems especially dangerous to me. How many
devices and applications have been exhaustively tested for that
condition? I'd think the IERS should be especially nervous about issuing
a negative Leap Second. The smears would have to go the other way too.
What if the negative Leap Second possibility was eliminated from the
specification so only positive Leap Seconds were allowed? If the
DUT1-to-UTC <.9s were relaxed, how far from DUT1 might it go? Would it
matter, how much, and to whom?
That would have a rather significant impact on simplification of
implementations, eliminating one whole set of cases and test conditions,
wouldn't it? That could lead to more reliable and uniform behavior,
couldn't it?
-Brooks
Warner
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