Standardising units and formats (and ref frames?) in transmission

[Arnold Rots]

At the risk of restarting the debate, let me summarize my conclusions
from this discussion as it relates to a minimum set for time, space,
spectrum, and redshift.
Needless to say, I fully agree with Anita on the Galactic coordinates
and Steve on time scales.

With all due respect, I don't think the question is: "how should services
store the data", but: "what can clients expect to be able to ask for
and receive".
And it may not be necessary to provide all combinations; for some data
it only makes sense to keep time in TT, for other data only TDB.
This goes back to another point Anita made: there is a difference
between the required accuracies for data discovery and for data usage.
I.e., a client may ask for time in TT and a service may return time in
TBD (well labeled!); the client can then decide whether the data are
usable for its purposes.

  - Arnold


Coordinate space   Reference system  Units    Reference positions
----------------   ----------------  -----    -------------------

Time               TT                *        TOPOCENTER
                                              GEOCENTER
                   TDB               *        BARYCENTER
                   **                *        HELIOCENTER

Space              ICRS              deg \  / TOPOCENTER
                   Galactic          deg  ><  GEOCENTER
                   Ecliptic          deg /  \ BARYCENTER
                   ***

Spectral           Wavelength        nm  \  / GEOCENTER
                   Frequency         GHz  ><  BARYCENTER
                   Energy            keV /  \ LSR

Redshift           z                 -    \ / BARYCENTER
                   Doppler optical   km/s / \ GALACTIC CENTER
                   Doppler radio     km/s     LSR


Notes:
Since TDB is directly tied to TT and both need to be accommodated,
it makes more sense to keep TT than UTC; besides, it's IAU standard.

We cannot escape the Doppler optical/radio duality in definition and
in reference position; it is inherent to the data.

* I'm afraid we have to accommodate time in ISO-8601 as well as MJD
  (or JD); both have specific application. Besides, there are times
  that cannot be expressed in ISO-8601 (since it has limited range).
  I believe that, as far as units are concerned, both 's' and 'd' have
  a role to play.

** I'm not sure what is most useful to the solar community; it could
   just be TT and TDB

*** The solar community may need its own spatial reference system



Anita M. S. Richards wrote:
> 
> 
> On Mon, 18 May 2009, Alberto Micol wrote:
> 
> > On 18 May 2009, at 11:58, Anita M. S. Richards wrote:
> >
> >> For example, I publish data in Galactic coordinates. If someone wants to 
> >> search in RA and Dec then a rough conversion is quite quick, as long as 
> >> they don;t mind getting back a region which may be slightly larger than 
> >> they asked for. But, we have had repeated complaints from users in the past 
> >> who want to search Galactic plane surveys in Galactic coordinates, since a 
> >> simple box (from an image or catalogue) will give them what they want, but 
> >> in RA and Dec it is horrid.
> > As Francois puts it, a galactic search box is nothing else than
> > a equatorial box plus an angle. Not that horrid.
> 
> It is not practical for images! And it is very non-linear for large 
> regions, taking sky curvature into account.
> 
> >> Or, I want data with a certain spectral resolution which I specify in 
> >> wavelength unts, but the data are in frequency units with a non-linear 
> >> conversion - i.e., the spectal resolution at one end of the bandpass is 
> >> different from that at the other, if the units are changed.
> > I'm asking for unification of units and formats: wavelengths always in 
> > meters, frequencies always in Hz. I was not pushing to have frequencies 
> > always in keV.  No complex conversions, no re-gridding, nothing subject
> > to interpretation, nothing that can corrupt the original piece of 
> > information.
> 
> Ah, that is better - sorry I missed that!
> 
> But why is it so difficult for us to apply SI prefixes, either?
> 
> Thanks very much Alberto,
> 
> a
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              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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