This is to inform all potential users whose data might have been affected by a recently discovered issue about +10 KHz offset in the local oscillator settings in the observations conducted through the new control and monitor system of GMRT, the TGC, till 29th July 2021. This notice is to let you know of the problem, how to identify if your data suffered from this issue, and our current advice on how to handle it.

In essence, we have found that for the upgraded GMRT system, the local oscillator is set with an additional 10 KHz value for all observations carried out with the TGC (the new monitor and control system for the upgraded GMRT), from 18th October 2019 till 29th July 2021. It has to be noted that between 18th October 2019 and 5th January 2021, not all observations were done with TGC. Users can check the ONLINE Machine entry in the respective GTAC Logs to see if the observations were done using the TGC. Observations for which ONLINE Machine is not TGC (but is for e.g. Shivneri/Lenyadri) are not affected by this problem.

The offset is always positive. For example, if the LO frequency is set to 500 MHz, the actual LO set is at 500.01 MHz.That is the offset is always positive. We have fixed the issues of 29th July 2021 and all observations carried out after this date don't have this problem. However, the implications of the offset vary from observations to observations, we describe below the possible implication of the above problem, for the 3 main modes of observations with the GMRT. In case your data needs correction, the best option is to add +10 KHz to the frequency in the header before carrying out any data reduction.

1. For continuum imaging observations: The LO offset of 10 KHz is unlikely to make any significant effect, and no corrective action may be needed except for the user to be aware of the slight shift in the RF band.
2. For spectral line observations: The LO offset of 10 KHz corresponds to a velocity offset of 2.1 km/s at ~ 1420 MHz and higher at lower frequencies by a factor of 1420/f, where f is the central frequency of the spectral line of your interest in MHz. To correct this offset, it is recommended to add +10 KHz to the frequency in the header before carrying out any data reduction.
3. For beam mode observations: The LO offset of 10 KHz introduces a systematic shift in frequencies interpreted by the analysis software. This can lead to an error in the DM estimates between 0.0005 to 0.001 pc-cm^-3 and a residual DM smear in the time-series due to dedispersion to incorrect frequencies. This can also introduce a small shift in the time-of-arrival of data. The magnitude of these errors depend on DM of the source being observed and will vary from one source to another. It also depends on observing frequency. The most significant effect is likely in Band 3 and 4, while Band 5 data is not affected except for very large DM sources. The header for all data taken with TGC system prior to 29 July 2021 needs to be corrected by adding +10 KHz to the frequency in the header before carrying out the data reduction.

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