Introduction to the GMRT data

GMRT can record data in continuum, spectral line and pulsar/beamformed (phased/incoherent array, primarily to observe pulsars/transients) modes.

Continuum and spectral line data

These are data typically recorded with the full array and contain visibilities. The raw data are in the “lta” format. The continuum and spectral line data will both be in the same format. The data in lta format is converted to FITS format using the in-house codes listscan and gvfits. The FITS file is further converted into the measurement set (ms) format in order to analyse the data using CASA.

The GMRT online archive (GOA) allows you to download data in FITS and lta formats. Thus if you downloaded your data in FITS format you can skip the step of converting the format from lta to FITS.

The files downloaded from GOA having ‘gsb’ in their filenames are the data recorded with the narrow band backend which was called the GMRT Software Backend (GSB). The files with ‘GWB’ in their names are the data recorded using the GMRT Wideband Backend (GWB) which is the new wideband system (Upgraded GMRT).

Pulsar/Beam data

These are data typically recorded after adding the signals from multiple dishes or the full array, either incoherently (i.e., the incoherent array mode, IA) or coherently (i.e., the phased-array mode, PA). The native data are in filterbank format, i.e., a spectrum with the specified number of frequency channel is recorded for every time sample, for a specified sampling time. The data recorded by GSB and GWB are of same format.

The native beam (PA/IA) data from GMRT need to be converted to SIGPROC filterbank format, after which it can be used with softwares such as SIGPROC, PRESTO, DSPSR, etc. The format conversion can be performed using the filterbank command from SIGPROC. Alternatively, one can use the rficlean command from RFIClean to do the format conversion, in addition to mitigating some radio frequency interference (RFI). If desired, rficlean can also be used only for the format conversion, i.e., without performing any RFI excision.