[fitsbits] Re: leap second alert

[Patrick Wallace]

On Fri, 17 Dec 1999, Steve Allen wrote:
> I can't write a program for my computer which will cause it to emit a
> reliable value of TAI or TT -- at least, not unless I make it a rather
> savvy network-aware program which can several times annually navigate
> its way through a maze of firewalls, surf the supposedly-reliable URIs
> that give leap second information, and then update its own database.

If your system clock is set by a local time service there's clearly no
problem - you can choose whatever timescale you like.  But if it's set by
NTP (in which case it's a bit iffy, especially just after a leap second,
something not all data acquisition applications can tolerate) then, in
the absence of accessible information from the net, there needs to be a
place in your application where you can manually set the current UTC-TAI.
If your application has an configuration file of some kind, and I would
guess many big FITS-writing data acquisition programs do, that's the
place to put it.  The Gemini TCS, for example, doesn't attempt to get
leap-second information from the net.  The numbers are stored in an
initialization file (in a way so the update can be done as soon as the
leap second is announced and, months later, will be handled properly).
The file also contains the UT1-UTC and polar motion stuff, which is
updated nightly (not that you need to).

> I'll wager they're rather glad that the DATE keyword was specified as
> UTC.

Or perhaps this is a chicken waiting to come home to roost.  I certainly
agree we don't want to start creating examples where date and time are
in different timescales.


Patrick Wallace
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Starlink/HMNAO                                  Internet:  ptw at star.rl.ac.uk
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