On Mon 2006-12-25T15:42:12 -0500, Keith Winstein hath writ:
> Even if a program is able to calculate TAI-UTC for arbitrary points in the
> past and near future, what should a library do when a program asks to
> convert between UTC and TAI for some instant further than six months in
> the future?
>
> - Signal an exception?
Something like that.
In the case of clocks "I don't know" has to be a valid answer, because
the alternative is effectively "I don't care".
This is something missing from most systems purporting to have clocks
that was there on almost all 19th century ships. The navigator has a
chronometer, and the navigator's log has an estimate of the offset and
rate of the chronometer. Nevertheless, until the ship next sails into
a port near an observatory, there's no way to be sure what time it
actually is. When the ship gets to port the navigator can go back and
correct the navigational measurements after the fact, and thus
establish better coordinates for the islands and reefs.
UTC is something which is calculated by consensus, that's what the U
in it means. It must be obtained from a source which is connected to
the other participants. No system which is disconnected, or
intermittently connected, can know what UTC is.
A GPS system which has been powered off may not know the number of
leap seconds, and thus the UTC, for up to 12 minutes, or maybe more,
until the next receipt of the frame with Delta t_LS. In the mean time
its best answer for the value of UTC has to be a guess.
I can't tell you what time it is, I can only tell you what time my
watch says. If I am diligent I have an estimate of how wrong I think
my watch is based a log of past history, and if I continue that practice
I can later give you an estimate of how wrong I was when I told you.
The people in the 19th century knew this. How have we forgotten it?
--
Steve Allen <sla_at_ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99858
University of California Voice: +1 831 459 3046 Lng -122.06014
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
Received on Mon Dec 25 2006 - 14:08:39 PST