Zenith transit problems

It has been noticed that in some of the GMRT antennas the poiting changes after the transit.

Here is an example of good antennas, where the gains does not change after the transit. If we interpret the gain change as a consequence of change in the pointing offset, this means that in these antennas the pointing does not change significantly before annd after the transit.

However, it is not the case with these set of antennas. The gains for these antennas are significantly different after the transit. Past studies have shown that this anomaly is due to change in the pointing offset after the transit. In one classic example, the antenna S02, had the largest pointing offset before and after transit. Mechanical team has replaced the elevation bearings and the problem disappeared. It appears that in these antennas also, the bearings are at fault.

There are a third catagory of antennas, such as C09, where the transit problem is not so significant, but the gain changes significantly as a function of elevation. It is not clear as why.

Please wait for detailed write-up