Speaker
Description
The upgraded GMRT (uGMRT) offering nearly seamless wideband coverage from 50 MHz to 1500 MHz is one of the most sensitive low frequency radio telescopes in the world. Its enhanced capabilities along with its unique geographical location offers tremendous potential for VLBI. Previous fringe test experiments with the legacy GMRT and the European and Australian VLBI networks showed that the Rubidium clock, though adequate for low frequencies, could not provide sufficient clock stability to maintain fringe stability across longer durations. The difficulty was compounded by the lack of seamless frequency coverage and the requirement of converting the 16.666/33.333 MHz GSB baseband data to VLBI-ready bandwidths. However, with the H-maser reference and the larger bandwidths provided by the GWB along with new software instrumentation enabling selectable bandwidths, VLBI can now be done routinely with the GMRT.
In this talk, we describe the three different digital backend modes of recording baseband data with the uGMRT : (i) voltage data from a single GMRT antenna with 16 MHz bandwidth, captured via a stand-alone set-up; (ii) voltage data from up to 2 GMRT antennas, tapping the signal from the main uGMRT back-end, for bandwidths up to 100 MHz; and (iii) spectral voltage data from the tied array output of the uGMRT back-end with any number of GMRT antennas added in phase (PASV), for bandwidths of 50/100/200/400 MHz. Off-line processing routines were developed and tested for : (a) converting spectral voltage data to time domain voltage data with provision for converting to VLBI friendly bandwidths (32/64/128/256 MHz); (b) filter and resample voltage data for bandwidths such as 100 MHz, to VLBI friendly bandwidths; and (c) routine to translate the final voltage data to VDIF format, including options for setting the number of bits to 8, 4 or 2. Out of the three modes of baseband recording, the PASV or the tied array mode along with the offline processing routines offers better suitability because of higher sensitivity, flexibility to select any part of the band and record multiple baseband beams concurrently. We will also briefly present the results of the tests conducted with the European VLBI network and the future growth path to make regular VLBI observations with the uGMRT.
| Presentation type | Poster |
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