Objectives of the GMRT Servo System

The servo systems used for position control of the radio telescopes must meet following objectives.

1. Ability to point anywhere in the sky.
2. High pointing & tracking accuracy.
3. Able to accelerate rapidly in the direction of source.
4. Able to manoeuver remotely

The first requirement is met by making a two axes mount for the antenna. For large antennas like those used in the GMRT (i.e. with weight in excess of 80 tones) an alt-azimuth mount is preferred. In such a mount the antenna can be moved in two axes viz. azimuth and elevation. The azimuth axis movement is parallel to the horizon, while elevation axis movement is normal to the horizon. Alt-az mounts are mechanically simple, yet very stable.

Radio telescope antennas are required to point within +/- 10HPBW at any given wavelength of operation of the antenna. This means that the pointing accuracy of the antenna should be fairly good. The following issues are of concern when trying to achieve high accuracy pointing or tracking:

1. Structural deformation due to gravity.
2. Structural vibrations/deformations due to wind forces.
3. Servo positioning error.

Note that not only can the reflecting surface of the antenna be affected by gravity, the feed support legs too could deform, leading to a displacement of the feed from the focus of the antenna. The GMRT antennas are built using a novel technique (nicknamed ``SMART'') involving a stainless steel mesh which is attached to rope trusses by wires which are are tensed appropriately in order to achieve the desired parabolic reflecting surface. This results in a dramatic reduction in the gravitational and wind loading on the structure, as well as in the total weight of the dish.