This is to inform all GMRT users that the problem reported earlier on the offset
in the calibrated visibilities on baselines between GMRT central square antennas
(i.e. baselines of the form CXX - CXX) has been identified (as arising from a
grounding issue in the backend receiver system) and fixed. The issue affected
GMRT Wideband Backend (GWB) data between October 2018 and 3 December 2020; GWB
data taken after 3 December 2020 should not be affected by this issue. Further,
data from the GMRT Software Backend (GSB) were not affected by this issue. If you
do see offsets in the calibrated visibilities in GWB data taken after 3 December 2020,
please write to the GMRT operations team at gmrtoperations@ncra.tifr.res.in .

We have characterized the offsets over the period 10/2018 - 9/2020, and find that
the observed offsets are highest in Band-5, and lower in Band-4 and Band-3; the
typical offset in Band-5 is ~15%, while that in Band-3 and Band-4 is ~5-10%. The
offsets can be seen most easily by excluding the CXX - CXX baselines (e.g. using
UVRANGE > 7 klambda in Band-5 data) while determining the antenna-based gains.
Applying the derived gains to the CXX - CXX baselines would then yield an offset
in the calibrated visibilities.

For GWB data taken during the period 10/2018 - 9/2020, we recommend the following:

1. If your science goals are based on compact sources, then we advise that you
   simply flag out the 91 baselines between central square antennas (CXX - CXX)
   throughout; this would affect your RMS noise by only ~10%, and so should not
   seriously hinder your science goals. This approach should work if the offsets
   are <~15%.

2. For spectral line users, if you have recorded GSB data, and these are sufficient
   for your science goals, we advise that you simply use the GSB data for your
   science. The GSB data are not affected by the issue in any way.

3. For continuum observers interested in extended emission, we have an experimental
   scheme that may allow you to fully make use of the GWB data. For more information
   on this, please write to  C. H. Ishwara-Chandra  at ishwar@ncra.tifr.res.in , and
   we will try to help you with the analysis.

The offsets may be larger than 15% for Band-5 data taken in October - November 2020
(and possibly even earlier), and possibly even for Band-3 and Band-4 data. We advise
all users to check their data carefully and to write to C. H. Ishwara-Chandra at the
address above in case of large offsets.