Subject: FITS standard

From: jcm at urania.harvard.edu (Jonathan McDowell)
Date: Thu, 16 Apr 1998 15:33:03 -0400

The new FITS standard draft seems pretty reasonable. I may not have been
paying attention to the Y2K discussion enough, but I am puzzled by the
statement in 5.4.2.1 that UTC shall be used for the DATE keyword
'for all data sets created 
on earth'. Is it felt that there are practical difficulties in realizing
UTC for computers not on earth? This seems unlikely to the accuracy
required by the DATE keyword; it is also not a frivolous question;
there are enough laptops flying on Shuttle and Mir missions which might
in principle store their data in FITS. I suggest removing the 'on earth'
qualification (or replacing it by 'in the solar system', since I can
see there might be simultaneity issues for interstellar probes...)

  - Jonathan McDowell

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From: arots at xebec.harvard.edu (Arnold Rots)
Date: Thu, 16 Apr 1998 17:07:10 -0400 (EDT)

Jonathan McDowell wrote:
> paying attention to the Y2K discussion enough,

You got that right! ;-)

> but I am puzzled by the

You took the two easy examples: low earth orbit and outside the solar
system.  As you probably have guessed, the 'for all data sets created 
on earth' was added later.  It was argued (correctly, I believe) that
on the earth's surface, UTC is well defined and useful, but even on a
nearby planet like Mars it is not clear that UTC would be the system
of choice - TDB might be better.

What you see is the text as adopted by the IAU FWG displaying a
traditional FITS solution strategy: The convention is specific where
we know that it is sensible to do so.  In all other situations you are
free to follow the same convention if that works well; if not, then at
least we didn't paint ourselves into a corner - you can use something
that makes sense, as long as you document it in the comment field.

  - Arnold Rots

>   - Jonathan McDowell

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Arnold H. Rots                                         AXAF Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------

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From: William Thompson <William.T.Thompson.1 at gsfc.nasa.gov>
Date: Fri, 17 Apr 1998 19:26:21 -0400

> see there might be simultaneity issues for interstellar probes...)

I'm a little confused by the phrase "accuracy required by the DATE
keyword".  My understanding of the Y2K proposal is that it does not impose
any limitations on the accuracy of date/time values, other than the
limitation imposed by the 80 character limit of a header line.

That said, I agree with Jonathan that limiting the use of UTC to the Earth
is a little odd.  I work with the SOHO satellite, which is ~5 light seconds
out from the Earth, and we have no trouble relating the times of
observations to UTC.  Obviously, one needs to be aware that different
observers will observe the same phenomena at different times, because of
the speed of light, but that's true for Earth-based observers as well at
the millisecond level.

Perhaps the writers wanted to leave open the possibility that times might
be converted to light arrival times at the solar system barycenter, or some
other such scheme?

Bill Thompson

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From: sla at ucolick.borg (Steve Allen)
Date: 19 Apr 1998 23:53:23 -0700

In article <3537E51D.AD7C4BF8 at gsfc.nasa.gov>,
William Thompson  <William.T.Thompson.1 at gsfc.nasa.gov> wrote:
>observations to UTC.

Unless I am misinterpreting, the entire argument of this posting is
relating to the 'DATE-OBS' keyword, whereas the "on earth" clause
pertains to the 'DATE    ' keyword.

>be converted to light arrival times at the solar system barycenter, or some

See in the appendix to the Y2K agreement that this is explicitly the
case for the 'DATE-OBS' keyword, but not for the 'DATE    ' keyword.
-- 
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.borg     Voice: +1 408 459 3046     FAX (don't): +1 408 454 9863
WWW: http://www.ucolick.borg/~sla               PGP public keys:  see WWW
Junk mail is irrelevant -- my return address has been assimilated.

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From: sla at ucolick.borg (Steve Allen)
Date: 20 Apr 1998 00:28:05 -0700

In article <199804161933.PAA01740 at urania>,
>see there might be simultaneity issues for interstellar probes...)

Note that the "on earth" clause under discussion pertains to the 'DATE    '
keyword, not the 'DATE-OBS' keyword.  The 'DATE    ' keyword is the
time of creation of the FITS HDU, not the time at which the data were
obtained.

At the time of the insertion of this clause the Y2K agreement had
already left the cabal of Rots, Wells, and myself, passed thru the
WGAS vote, and was in the hands of the IAU FWG.  Although I wasn't
officially party to the evolution of the agreement at this phase, I
suspect that my suggestion was responsible for it.

My reasoning follows.  We usually presume that the equipment which
obtains the data has a clock suitable for defining a value of
'DATE-OBS' in whatever is the most convenient time scale.  However,
these data may not be stored immediately as FITS, but may be
transferred to some other computing device which casts them into
FITS.  It is the responsibility of that machine to create the 'DATE    '
keyword, and there need be no requirement that it has a clock which is
set to any particular timescale.  Furthermore, the 'DATE    '
keyword should be rewritten if the original data are reprocessed.

On earth the most available timescale may, without much loss of
generality, be presumed to be UTC.  Note that there is no prohibition
against using UTC for data sets created off earth.

It was my assertion that it was asking too much to presume that a
computer in the data processing center at Farside Lunar Observatory,
or Mars Base 1, or of a L5-based amateur who is simply playing with
the data, should have a clock which is set to UTC.  The clocks of such
non-terrestrial, non-data-acquisition computers should be permitted to
be set to whatever is the most available timescale.  In general, I do
not think that likely to be UTC.

I do not see any significant compromise in permitting the use of other
timescales for 'DATE '.  Indeed, my suggestion to members of the IAU
FWG indicated that I felt that, without this clause, such
non-terrestrial FITS creators would simply curse the Y2K amendment,
and then proceed to disregard it.  With this clause in place, the Y2K
agreement should be able to withstand all solar system use.
(Interstellar applications will beg the question of the use of the
earth-based Gregorian calendar, but that is a matter for a future FITS
working group ;-)
-- 
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.borg     Voice: +1 408 459 3046     FAX (don't): +1 408 454 9863
WWW: http://www.ucolick.borg/~sla               PGP public keys:  see WWW
Junk mail is irrelevant -- my return address has been assimilated.

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From: Peter Bunclark <psb at ast.cam.ac.uk>
Date: Tue, 21 Apr 1998 11:02:28 +0100

I would have thought that terran space-travellers who prefer to stay
synched to earth time,
perhaps because it suits their circadian rthyms, could stick with UTC,
probably running
ntp like the rest of us; or at lease, EXO-NTP when it comes out.  These
people can then quite
easily write their FITS tapes (have you ever tried loading a
6250-streamer in zero-G? Me neither)
using UTC.
	On the other hand, the Martian settlers may prefer to divide one Sol
into 24 hours etc etc.
Now these folk would naturally use MTC (Marsiversal Coordinated Time; 
the achronym is dyslexic
in honour of another terran language that used to be tbe lingua
franca...).  However, their
equipment is always dusty so archiving the data is about as far down the
pipeline as it goes.
	The Jovians don't matter because they're still waiting for their first
clear night, and
the Europeans just splash about under the ice all day. And night.

	And back in the real world, Voyager data can be clocked to
high-accuracy in UTC, so
what's the problem?

Peter.