WFC: Year-2000 issue (again)

[Don Wells]

Dear WGAS FITS Committee members,

I am an ex-officio (and non-voting) participant in WFC discussions.  I
am not speaking as Chair of the IAU FITS Working Group here: the remarks
below are my personal opinions.

"AR" == Arnold Rots <arots at xebec.gsfc.nasa.gov> writes:
AR> [re Bunclark] proposal for how to revise the definition of the
AR> DATExxxx keywords to provide a 4-digit year..
AR> .. proposed amendments..
AR> 1. Use of the keyword TIMESYS is strongly encouraged.  ..
AR> 2. The preferred value is "TT" which is considered equivalent to "TDT"
AR>    and "ET", though "ET" should not be used for data taken after 1976.
AR> 3. If the TIMESYS keyword is absent or has an unrecognized value,
AR>    a value of "UTC" will be assumed for dates starting in 1972.

This proposed change to the Bunclark DATExxxx proposal would create
unnecessary confusion about dates and time in FITS datasets passing in
interchange.  This confusion would be risky for our community because it
could result in improper calculations and might thereby compromise
astronomy research which depends on combining datasets from multiple
datasystems.

Our interchange format design will be more robust if we revise the
Bunclark proposal to specify that the DATExxxx timescale is (only) UTC.

The confusion of multiple timescales is unnecessary in interchange
because conversions between UTC and other modern uniform timescales are
well defined (e.g., SLALIB routines SLA_GMST(), SLA_DAT() and
SLA_DTT()). FITS reading and writing software can transform UTC to and
from local timescale conventions with no ambiguity or loss of precision.

In summary, I recommend that the WFC agree on a single timescale for
DATExxxx interchange purposes (UTC is the obvious choice) and that the
TIMESYS keyword be deleted from the proposal that WFC forwards to the
IAU-FWG for further consideration.

AR> 4. The use of the "Z" qualifier in DATE-OBS is discouraged..

I recommend that WFC modify the Bunclark proposal so that it either
_requires_ the appended 'Z' (to explicitly indicate the UTC convention)
or else _forbids_ the 'Z' (because the UTC convention is implicit). It
doesn't matter which option is chosen, the important thing is that the
unnecessary option should be removed from our interchange format design.

AR> The default interpretation of all DATExxxx keywords shall use the
AR> Gregorian Calendar.

Yes.

Regards,
Don
-- 
  Donald C. Wells         Associate Scientist         dwells at nrao.edu
                    http://fits.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA