Upgrading the GMRT: Towards a deeper look at the Universe

The Giant Metrewave Radio Telescope (GMRT), the world’s largest radio telescope facility operating at low frequencies, is an array of 30 fully steerable, 45 metre diameter antennas, spread out over a 30 km region around Khodad, near Narayangaon town of Pune district in western India. The GMRT has been in operation since 2002, and is used by astronomers across the world for cutting, front-line research in numerous areas of astronomy and astrophysics. It was designed to operate at a range of spot frequencies from 50 MHz to 1420 MHz, in five distinct narrow frequency bands with a maximum bandwidth of 32 MHz, primarily due to technology limitations of that time. Currently all the systems of GMRT are going through an upgrade using latest analog and digital technologies, servo systems etc to enhance the telescope performance and features, along with matching improvement in infrastructures such as civil, electrical and mechanical. These improvements will increase the sensitivity of the GMRT by more than a factor of three, and keep it on the forefront of the international scenario in the field, for many more years to come.

 Feeds with wide bandwidths are being developed using novel designs to have seamless frequency coverage from 130 to 1500 MHz, to enable astronomers to receive signals at any frequency in this range, just like tuning a commercial radio receiver to listen to any station of our choice. These designs include dual-ring dipole feed for 130-260 MHz and cone-dipole feed design for the 250-500 MHz band. For frequencies from 1000 to 1500 MHz, the current L-band feed design is retained as it provides a wide bandwidth of over 400 MHz. To match these feeds new wideband, high dynamic range Low Noise Amplifiers (LNA) working at room temperature have been successfully designed in house, which can withstand strong unwanted interference signals that are likely to creep through the wide band. The L-band front end receiver is upgraded for better dynamic range and is equipped with efficient filters to reject interference signals from CDMA and GSM mobile transmitters.

 The current signal transport system uses analog fiber optic links to bring the radio astronomy signal of 32 MHz bandwidth from remotely located antennas to the central electronics building (CEB) for further signal processing. To meet the requirements of the broadband antenna feeds and frontend systems, this signal transport system has been upgraded to bring the full, broadband signal directly from the frontend without any frequency translation. The biggest challenge was to allow the existing mode of signal flow as well as new broadband signal through single fibre. To achieve this, an indigenous configuration was developed at GMRT, using 4 channel Dense Wavelength Division Multiplexing (DWDM) based RF over fiber (ROF) optic system which allows the coexistence of the existing and upgraded systems at the GMRT.

 Since the existing backend electronics at the central building caters to only a narrow bandwidth of 32 MHz, a new backend system which can handle broad band signals is needed. Such a new backend system is being developed by NCRA engineers and scientists in collaboration with nVidia-India, the US based CASPER group and Swinburne University of Australia, which uses the latest and best technologies in digital electronics and computer engineering to achieve a ten-fold increase in the bandwidth of the receivers (to 400MHz). The Graphics Processing Units (GUP) cards, which are widely used in video gaming industry, are being used here to achieve this. These cards, along with Field Programmable Graphics Array (FPGA) boards do the complex signal processing required for combining the signals spread over in thousands of channels within the 400 MHz bandwidth, from the 30 antennas of GMRT. The full backend is equivalent to a 20 Teraflops machine, i.e. a supercomputer. This highly versatile design will use sophisticated local oscillators and extremely stable time-frequency standards to make GMRT capable of being part of a global telescope configuration like Very Long Baseline Interferometry, involving precise synchronising of telescopes across continents.

 The GMRT monitor and control (M&C) system is the central supervisory system which controls and monitors telescope subsystems. The current system is over 20 year old and lacks advanced features and automation. To control and coordinate the upgraded subsystems of the telescope, the GMRT M&C system is being upgraded with enhanced capabilities and automation, exploiting the advances in micro-controllers running embedded control software.

 To benefit from the major improvement in signal quality and sensitivity, rest of the subsystems should also match. For example, the antennas must now point and track much more accurately and precisely than before. The upgraded GMRT Servo system implements Brush Less DC (BLDC) Motors and Drives which are compact in size and has improved features to implement modern pointing algorithms for precise positioning of the Telescope. The controller is being modified to enhance self diagnostic features and to improve the accuracy of pointing and dynamic range of the system. This also takes care of the problem of obsolescence that the existing servo system of the GMRT is facing.

 Various other infrastructural facilities like mechanical, electrical and civil are also being upgraded to provide better support to ongoing developmental activities. Antennas are being painted with high quality all-weather protective coating to increase the life span of the steel structure. The antenna reflecting surface is also being improved to enhance the surface accuracy. Two numbers of a new hydraulic high lift platform which can reach upto a height of 100 ft, are being procured to support the maintenance, upkeep and installation activities at the foucus of the giant antenna.

 All 30 antenans are provided with diesel generator back-up to ensure zero down time of the antenna systems in case of power failure. The electrical back-up facility is improved by providing diesel generators with synchronizing panels for smooth load transfer, which also help to reduce the fuel consumption. The temperature of the antenna shell is being regulated using advanced air-conditioning system with digital temperature controller. Efforts are on to reduce the Radio Frequency Interference (RFI) due to high tension electrical lines, breakers and insulators to help in improving the data quality from telescope. Latest Uninterrupted Power Supply systems (UPS) are being installed for making the environment RFI free.

 Once these upgrade activites are over in the coming years, the GMRT will once again be the leading instruments in this field, helping astronomers to do frontline research.