The actual position of the antenna should in all cases be derived from the antenna FITS file. Co-ordinates provided through the GO FITS file are indicative only, and might be used, for example, to define the field center of an image.
A detailed description of GBT astronomical position handling is given in GBT/SPN/015, and so is only briefly reviewed here.
The keyword COORDSYS will be used to define the type of co-ordinate system in use. The possible values in FITS version 1.1. are ``GALACTIC'', ``RADEC'', ``HADEC'', ``AZEL'', or ``OTHER''. OTHER will be used for any situation where the precise specification of the co-ordinate system has not yet been decided; for FITS versions 1.1 this includes user defined co-ordinates and solar system objects. Later versions of the FITS definition may include a representation of these co-ordinates, in which case a new value will be provided for COORDSYS.
For COORDSYS = ``RADEC'', the standard (WCS) keyword RADESYS (note missing ``C'') will be used to define the specific type of system in use.
Note that the standard FITS keyword EQUINOX is used to provide the epoch (i.e. instant of time) of the mean equator and equinox. Prior to version 1 of the FITS standard, the keyword EPOCH was used for this quantity; it is illegal to use the keyword EPOCH in new FITS files. For GAPPT, WCS paper II recommends use of the start time of the observation (via DATE-OBS or MJD-OBS) to provide the epoch of the equator and equinox.
The actual co-ordinate value used will be provided by the appropriate (long, lat) pair, depending on the value of COORDSYS.
The following example shows how the target positional information might appear in the GO FITS file:
COORDSYS= 'RADEC ' / co-ordinate system used to command telescope RADESYS = 'FK5 ' / Type of equatorial system in use EQUINOX = 2.00000000000000E+03 / Equinox of selected coordinate reference frame RA = 3.30375000000000E+02 / Commanded Major Axis Value in Degrees DEC = 4.35000000000000E+01 / Commanded Minor Axis Value in Degrees