RFI characterisation and mitigation at upgraded GMRT

Abstract

A modern radio astronomy observatory needs continuous monitoring of the radio spectrum to detect and characterise the sources of RFI, and ever improving plans for mitigation against such sources. With increasing population densities, accompanied by modernization in transport and communication facilities, the sources of external RFI affecting a radio observatory increase with time. The problem is aggravated by the increase in sensitivity of the observatories, as well as by the increase in the frequency range and bandwidths of observations. An additional threat comes from self-generated RFI produced by the increasingly computer controlled and data intensive operations of a typical radio telescope. These aspects of RFI significantly affect the Giant Metrewave Radio Telescope (GMRT), working at low frequency. The ongoing upgrade of the GMRT, which provides near-seamless frequency coverage from about 120 to 1500 MHz, with improved sensitivity receivers, makes it even more prone to the threat of RFI. We present here an overview of the different kinds of RFI that affect the GMRT, and the schemes used for regular monitoring to detect sources of RFI, their characterization, and the steps taken towards mitigation. The different measures for mitigating the various kinds of RFI are described. These range from coordination with various external agencies to maintain the restrictions in and around the GMRT array and reduce the impact of RFI with better shielding for equipments at the observatory. Significant work has been done for design of shielded enclosures for various kinds of equipment in the observatory, which helps to achieve 40 to 60 dB additional isolation to mitigate leakage of RFI. We present our learning and improvements achieved from these exercises. We also outline our plans for future work in RFI mitigation at the GMRT. A distributed RFI monitoring antenna array along the GMRT antenna array is planned with a dedicated analog fiber optic link for real-time RFI monitoring and software mitigation in future.