GMRT Report

The upgraded GMRT moves to the next level of release

The work on the upgrade of the GMRT continues to make good progress. Following up on the release of the 2nd phase of the upgraded GMRT (uGMRT) in October 2016, the 3rd phase of the release is nearing completion, and will be available for use from the 32nd GMRT observing cycle starting in mid-April 2017. Proposals for Cycle 32, submitted in January 2017, have seen an oversubscription of a factor of 2.5 and there are a significant number of proposals for using the uGMRT systems, compared to the legacy GMRT systems, which is very promising.

The main new features of the uGMRT that will become available in this cycle — in addition to all 30 antennas fitted with the Band-3 (250 to 500 MHz) and Band-5 (1000 to 1450 MHz) receivers which are already available — are 16 antennas fitted with the Band-4 (550 to 850 MHz) receiver systems, 8 antennas fitted with the Band-2 (120 to 250 MHz) systems and the complete 30-antenna back-end with full polar interferometry and beamformer modes available.

This should provide for some exciting new science results from this cycle of observations!

Sample first light result with Band-3 of 30-antenna uGMRT

Meanwhile, interesting first results are already coming from the uGMRT systems. One of the first light results using the full complement of 30-antennas at Band-3 (250 to 500 MHz) that has recently been obtained is shown in the accompanying figure.

Picture1

Sample first light image from the uGMRT using the full 30-antenna system at Band-3 (250 to 500 MHz). Source is the Coma Cluster, observed with a bandwidth of 225 MHz centred at 378 MHz, for a duration of 4 hours. The image has a resolution of 7.5” and achieves a rms noise level of 30 microJy. A few new sources have been identified, in addition to much better clarity for the known sources.

This is an image of the Coma cluster made with the uGMRT using only 4 hours observing time using a bandwidth of 225 MHz around a centre frequency 378 M of 378 MHz. The image achieves a resolution of 7.5″, and a rms noise of 30 microJy. This is a promising first light result that nicely demonstrates the potential of the uGMRT for such studies.

Mitigating Radio Frequency Interference at the uGMRT

One of the major threats to the upgraded GMRT is Radio Frequency Interference (RFI) from different kinds of man-made sources. Of these, transmissions from satellites are a major contributor, as there is an ever growing number of satellites, transmitting over a wide range of frequencies. Recognising this, a novel program for predicting and avoiding RFI from satellites has been developed at the GMRT. For this, first a complete database of all satellites visible in the GMRT sky and having transmissions in the range of frequencies covered by the GMRT has been generated.

Next, the response of each satellite has been characterised by studying it with the appropriate GMRT receiver system and working out an angular zone of avoidance that should be maintained from the satellite to prevent saturation of the GMRT receiver chain. Finally, using a real-time ephemeris for each satellite, a program has been developed that does a real-time calculation of the position of each satellite and then generates a real-time warning signal in the GMRT control room whenever the beam of any GMRT antenna comes within the zone of avoidance. A real-time display allows the operator to see all the relevant satellite visible in the sky at any given time, in relation to the astronomical source being observed by the GMRT antennas (as illustrated in the figure).

Picture2

Sample screen shot from the real-time satellite warning system at the GMRT control room, showing the sky positions of all known relevant satellites visible in the sky at a given moment of time, alongwith the position of the source being observed.

Modified versions of the above program have been developed for generating predictive warnings for planned observations that the astronomer can use for checking against their observing schedule. The result can also be visualised in graphic form as illustrated in the second figure which shows a plot of the sky tracks of the visible satellites over the planned duration of the observation, along with the sky track of the GMRT antennas for the source being observed. Similarly, for observations done in the past, the program can read the details from the recorded data file and work out whether any parts of the observations would have been affected by satellite RFI and generate post facto warnings that the astronomer can use while analysing the recorded data.

Picture3

Sample screen shot from the satellite warning system in the GMRT control room, showing the predicted trajectories of the relevant satellites over the duration of the planned observation, alongwith the trajectories of the target sources to be observed, showing that the observations would be affected by satellite crossings at two different times, by two different satellites.

This package has been installed and running at the GMRT for a few months now, and has already proved its usefulness on several occasions where users have been warned in real-time about RFI from satellites and have also used the tool to plan their observations to avoid the effect of satellites.

This package can, without much effort, be extended for other observatories.


Report provided by GMRT