A Summary of the Minimum Requirements for the Control
            and Operation of the GBT in the Initial Phase

                             D. E. Hogg
 
                         December 22,1992

	Although it is expected that the GBT will eventually have advanced
hardware and software which will enable it to work at long millimeter 
wavelengths, it must be recognized that these developments will require
experience with the telescope itself, and it will not therefore be possible
to provide them in the first days of operation.  However, a system must be
available which will provide the basic capabilities needed for calibration of
the telescope and for the initiation of astronomical observing.  The following
outlines the requirements of this basic system.  The detailed specification
of some of the features will be developed elsewhere.

1. Antenna Pointing.  At the outset, the antenna pointing will be measured
   relative to the encoders on the two rotation axes.  Observations will be 
   made to develop an estimate of the pointing errors as a function of azimuth
   and elevation, relative to the encoder output.  Access to relevant weather
   information will be needed.  The pointing errors could be modelled in the 
   conventional manner, as coefficients of various trigonometric functions of 
   azimuth and elevation, or they might exploit the more advanced concepts 
   described by Condon in GBT Memo No.75.  In either case, provision will be 
   made within the Monitor and Control system to apply these corrections to 
   ensure that the telescope is properly pointed using encoder readings only.

2. Antenna Calibration.  During the evaluation of the antenna, and for
   general astronomical observations, there must be available a number of
   standard calibration procedures.  These have been enumerated by R. Fisher
   (Aips++ Specifications for Single-Dish Data Reduction, November 11,1992)
   in Section 3.6.  Of particular relevance to this memo are the first
   eight items, repeated here for completeness.  A requirement about focussing
   has been added.  
   a) Tipping observations to measure atmospheric opacity.
   b) Determination of system temperatures from astronomical observations.
   c) Pointing offsets and flux density calibration from
      i. five-point maps
      ii.crossed continuous scans (gaussian fitting).
   d) Full sky determination of pointing coefficients by obtaining
      a set of pointing offsets.
   e) Full sky observations to determine flux density calibration.
   f) Beam map for application in telescope evaluation and deconvolutions.
   g) Polarized source measures for development of instrumental
      polarization matrices.
   h) Holographic measurements to estimate the surface accuracy.
   i) Focus of the telescope, at both prime and gregorian positions.
   It is important to note the complex relationship between pointing and
   focus for the GBT. Provision must be made to update the pointing as the
   focus is changed.  The focus will have to be tracked as the elevation
   changes.  The motion of the subreflector will have to be calibrated.
	It is perhaps useful to mention other types of observations that
   will undoubtedly be made in the course of the early telescope evaluations.
   These include searches for standing waves, studies of the spectroscopic
   baselines, measurements of far sidelobes, and the study of the off-axis
   feed performance.  However, I believe it will be possible to make these
   observations if the capabilities specified above are in place.

3. Astronomical Observations
   The GBT will be required to perform a wide range of observing techniques
   in support of the diverse observational programs it will undertake.  A
   general discussion of the Control system has been given by Fisher and
   Lockman in GBT Memo No. 81, with specific examples of observing functions
   shown in Appendix B.  Not all of this power will be available at the 
   outset.  However, as a minimum, the following capabilities, in addition 
   to those required for the calibration observations above, must be available
   from the outset:
   a) Tracking a source, with pointing corrections updated as appropriate.
      Provision to maintain a fixed parallactic angle is necessary.
   b) Switching the telescope between two positions, with pointing corrections
      included as required.
   c) Making observations at each point in a grid, specified in az-el or in
      standard astronomical coordinates ( Ra-Dec;l,b).  The matrix of points 
      should be generated by the Control system from simple specifications of 
      number and spacing.
   d) Taking observations "on-the-fly", over a specified track.  The track
      could be a function described in either az-el or RA-Dec coordinates.
      The speed of motion, and the frequency of sampling, should be options
      within the range provided by the telescope drives.
   e) Making observations with either the frequency-switching or beam-switching
      technique.