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SPOTLIGHT for detecting FRBs and pulsars using the GMRT: Dec 2024

A project for developing an HPC facility with PetaFlop computing capacity for conducting a real-time commensal search for Fast Radio Bursts (FRBs) and pulsars with the GMRT is funded under the National Supercomputing Mission (NSM). The data centre hosts CDAC's indigenously developed Param Rudra servers, which are equipped with multiple Nvidia A100 GPUs. This facility is called Param Brahmand. SPOTLIGHT is a project developed using the Param Brahmand facility at the GMRT primarily supporting 24/7 commensal observations of FRBs and pulsars. In open-sky mode (for targeted observations), a maximum of 100 hours with the SPOTLIGHT system is available for GTAC Cycle 48. For more details please refer to this document.

Walsh Scheme implemented in uGMRT: 06Dec2023

The Walsh scheme to reduce cross-talk in the GMRT receiver chain is implemented with Walsh modulation in the front-end system and demodulation in the GMRT Wide-band Backend (GWB). The scheme is extensively tested in Band-3, Band-4, and Band-5, and it was released for observation in April 2023 (GTAC Cycle 44) on a shared-risk basis and will be available on full release from April 2024 (GTAC Cycle 46) onwards. Testing in Band-2 is currently in progress. While using the Walsh scheme, GSB data will not be useful and also Walsh scheme cannot be combined with RFI filtering as of now. For more information and operating procedure, please refer to this document

Post Correlation (PC) beam for beam mode observations: 17Jul2023

The post-correlation (PC) beam has been added for the total intensity mode (stokes = 1). The PC beam is formed by subtracting the IA beam from the PA beam, both made with the same antenna combination. The subtraction is implemented for each time and frequency sample. The process of removing the self-power contribution from the PA beam and retaining only the cross-power can reduce the effect of interferences that are mostly contributed by the self-terms. The PC beam can be selected from the GUI like other beams (IA, PA, or voltage beam). The input/output data rate weight of the PC beam in the GWB is the same as the IA or PA beam. Thus the PC beam is supported for the same time and frequency resolution as the IA and PA total intensity beams. For more details please refere to this document.

Updated coefficients of the beam shape parameters for the uGMRT: 29Nov2023

Updated coefficients of the beam shape parameters for the uGMRT for bands 2, 3, 4, and 5 are available now. This note (dated 29 November 2023) supersedes all the earlier notes (dated 01 December 2018 and dated 09 September 2022).

Updated coefficients of the beam shape parameters for the uGMRT: 29Nov2023

Updated coefficients of the beam shape parameters for the uGMRT for bands 2, 4, and 5 are available now. This note (dated 9 September 2022) supersedes the earlier note (dated 1 December 2018). Currently, we are working on the coefficients of the beam shape parameters for band 3.

Real-time RFI Filtering for GTAC Observations: 04May2023

Real-time RFI filtering in the GMRT Wideband Backend (GWB) has been implemented for uGMRT Bands-3 and 4 since April, 2022 (GTAC Cycle 42). For uGMRT Band-2, it is currently available on a shared-risk basis. For more information on RFI filtering and its settings, please visit http://gmrt.ncra.tifr.res.in/doc/Real_time_rfi_filter_doc.pdf

Release of PASV mode: 17Nov2022

The new(1.4) version of the command file creator is released on 17Nov2022. In this version, a new beam mode is added i.e. PASV - Phased Array Spectral Voltage. The detailed documentation is available at http://www.gmrt.ncra.tifr.res.in/doc/PASV_update.pdf

Polarisation Observations SOP release : 12Aug2022

The document below covers points to be taken into consideration when planning polarimetric observations with the uGMRT. Clarification and help is given for computing exposure time calculator and in preparing command files for polarimetric observations. The detailed documentation is available at polarisation_observation_sop.pdf

GBMON(gptool) release : 04Aug2022

GBMon monitors current state of beamformer data recording using the GMRT. This tool can be effectively used even by the external users to monitor the data quality. Please contact control-room (ctrlgmrt@gmail.com) to activate the web-UI mode during the observations. For remote monitoring of beamformer data recorded in 16-bit mode use the link www.gmrt.ncra.tifr.res.in/~astrosupp/gbmon/psr.html

On 10 KHz offset in the Local Oscillator in observations done with TGC : 24 Sep 2021

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.

Issues with the GMRT central square baselines Fixed : 20 Aug 2021

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.

Issues with the GMRT central square baselines : 23 November 2020

This is to inform all users of a problem that we have recently discovered that may affect your data. We are currently trying to fix the issue; however, this notice is to let you know of the problem and our current advice on how to handle it. We will be in touch with more details.

In essence, we have found that for the upgraded GMRT system, the GMRT Wideband Backend (GWB) visibilities show an offset which affects the baselines between central square antennas (i.e. baselines of the form CXX - CXX). Baselines between central square antennas and arm antennas, or between arm antennas, appear to be affected much less, or not at all. The offset is most severe in Band-5, and less in Band-4 and Band-3; the typical offset in Band-5 is ~15% on the central square baselines, while that in Band-3 and Band-4 is ~5-10%. The issue appears to have affected the GWB data since October 2018. Data from the GMRT Software Backend (GSB) of the legacy GMRT system are not affected by this problem.

As of now, we recommend the following:
1. Spectral line observers at all bands, but especially in Band-5, should make sure to ask for the GSB data to be recorded, in parallel with the GWB data. We expect that, in most cases, the GSB data would be sufficient for meeting the spectral line science goals. In case of questions on how to set up the GSB for your observations, please write to Nissim Kanekar at the address nkanekar@ncra.tifr.res.in . 2. 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. 3. Continuum observers interested in extended emission are advised to exclude the UV range corresponding to antenna separations <~ 1 km, while calibrating their data, so that the offset does not affect the calibration. The extent of the offset would then be visible on plotting the calibrated visibilities on the flux or phase calibrators versus UV distance (e.g. using UVPLT in AIPS). If you do see an offset in the central square baselines on doing the above, please write to C. H. Ishwara-Chandra at ishwar@ncra.tifr.res.in , and we will try to help you with the analysis. We will update you as soon as we have resolved the issue, at which time normal procedures for using the GWB data can be followed.