reference frame issues regarding units

[Steve Allen <sla@ucolick.org>]

Greetings to the Units ML group,

I received word of this group through the IVOA units discussion.
I note the existence of "astrononomical unit" in the list of units
and wish to inquire whether the UnitsML effort will be addressing
the reference frame issues involved in the values of measured quantities.

Over the past 40 years the IAU has devoted ongoing effort to a precise
meaning for measurements in a relativistic framework.  The current form
of that work in the IAU takes place in IAU Commission 52, Relativity
in Fundamental Astronomy.

The best reference for the questions that I am posing are found in
the publications of the IERS, specifically
IERS Technical Note 32 (2003), currently online at
http://www.iers.org/MainDisp.csl?pid=46-25776

My concern is the distinction between "TT units", "TDB units", "TCG units",
and "TCB units" which is detailed in that report in section 1.2 and
the beginning of chapter 6.

The issue in section 6 is spelled out:

The value of G*M_earth in TT  units is 398600.4415
The value of G*M_earth in TCG units is 398600.4418
The value of G*M_earth in TDB units is 398600.4356

Every application I know currently uses the "not SI" TDB/TT units, but
there are several issues of the ITRF by the IERS which used TCG units.

There is a draft paper exploring the issue and proposing nomenclatures
for these
http://arxiv.org/abs/astro-ph/0508292v3

Is this relevant to UnitsML, and if so, how will it be incorporated?

--
Steve Allen                 <sla@ucolick.org>                WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99855
University of California    Voice: +1 831 459 3046           Lng -122.06015
Santa Cruz, CA 95064        http://www.ucolick.org/~sla/     Hgt +250 m