Introduction

The GBT supports a variety of co-ordinate systems, for both input (specification of command positions) and output (reporting of indicated positions).

The fundamental equatorial co-ordinate system supported is mean FK5, J2000. Other supported equatorial co-ordinates are FK5 for the mean equator and equinox of a specified date, and true, geocentric apparent. Provision is also made for mean FK4 B1950, co-ordinates, albeit with some caveats as discussed in Section 2.1. In addition to equatorial co-ordinates galactic, local equatorial and horizontal co-ordinates systems may be used. Finally, the GBT provides support for ``user-defined'' co-ordinate systems, and specification of solar-system objects. Indicated positions are reported in mount (encoder) co-ordinates, mean FK5 J2000, and the co-ordinate system in which the telescope was commanded.

The purpose of this document is to provide a precise definition of how the GBT implements these co-ordinate systems, and the transformations between them. It also reviews how these are specified in the GO and antenna FITS files. Useful reference material includes:

• Explanatory Supplement to the Astronomical Almanac, ed. P.Kenneth Seidelmann (1992), University Science Books.
• Spherical Astronomy, Robin M.Green (1985), Cambridge University Press.
• SLALIB - Positional Astronomy Library, Patrick T. Wallace, Starlink User Note 67, Starlink Project.

The philosophy of the system is as follows:

• The observer provides a commanded position via GO. The antenna then either tracks this position, or performs offets (for example cross-scans) relative to this position. The observer can specify this position in a variety of co-ordinate systems, some of which have precise, well defined meanings (i.e. FK5 J2000), others of which are more telescope-specific (e.g. ``AZEL'').

  The GO FITS file records only limited information about the commanded position. This information is only intended to be indicative of the observer's intent, and might be used, for example to provide the reference position for an image made from the data. Precise position information is provided by the antenna FITS file

• for each scan, the antenna FITS file contains the actual position of the antenna, sampled at 10Hz throughout the scan, and provided in a variety of co-ordinate systems (including FK5 J2000). One of the main functions of the aips++ filler is to average the antenna positions over the times corresponding to the backend integrations, so that a single position can be associated with each integration in the scan.

• The antenna FITS file also contains detailed information on how various pointing and other corrections were applied.