[fitsbits] Proposed Changes to the FITS Standard

[Steve Allen]

On Wed 2007-08-01T17:51:47 +0200, Thierry Forveille hath writ:
> > * p92: the discussion at the bottom should indicate that the diurnal
> >  Doppler correction is only weakly dependent on position and therefore
> >  great accuracy is not required.
> >
> Depends on the context, and on what's meant by "no great accuracy": it's
> 1 m/s for 7 arcsec (worst case position on the sky), so for planet
> searches we need arcsecond precision or so (our best precision is under
> 1 m/s, and we want some safety margin to keep that particular contribution
> to the error budget negligible). Perhaps just give the 7" --> 1 m/s
> equivalence here and let readers make their own decision?

And for pulsar timing it's even more relevant to get the position right.

But even at the level of seven arcseconds on the surface of the earth
there are issues because some (pre-1980) geodetic datums can produce
topocentric positions which are off by many hundreds of meters.  In
Paper III we did not want to get into the specifics of geodetic datums.

For that reason we stopped at recommending post-1980 coordinates.
Whether that means WGS-84 or ITRF or ETRF or GTRF or any other
satellite-centered, VLBI-aligned reference frame is irrelevant at
the level of 1 m (and below that level everything is so heavily model
dependent that FITS WCS III didn't dare to tread).

For the quality and interpretability of any data with such precision
nothing can substitute for having read Lindegren and Dravins and being
familiar with the work of IAU Comm 52 (and all the efforts of its
predecessors RCMAM and others).

I'm not much more sure how all that would fit into the standard
than I was at the time we were considering Paper III.

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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