Radio Frequency Interference (RFI) is a growing concern for contemporary radio telescopes. This paper describes techniques for real-time threshold-based detection and filtering of broadband and narrowband RFI for the correlator and beamformer chains of a telescope back-end, with specific applications to the upgraded Giant Meterwave Radio Telescope (uGMRT). The Median Absolute Deviation (MAD) estimator is used for robust estimation of dispersion of the received signal in temporal and spectral domains. Results from the tests carried out for the GMRT wide-band backend (GWB) using this technique show 10 dB improvement in the signal-to-noise ratio. MAD-based estimation and filtering was also found to be useful for filtering beamformer data. The RFI filtering technique demonstrated in this paper will find applications in other radio telescopes as well as receivers for digital communication and passive radiometry.
Radio Frequency Interference (RFI) excision in wideband radio telescope receivers is gaining significance due to increasing levels of manmade RFI and operation outside the protected radio astronomy bands. The effect of RFI on astronomical data can be significantly reduced through real-time excision. In this paper, Median Absolute Deviation (MAD) is used for excising signals corrupted by strong impulsive interference. MAD estimation requires recursive median calculation which is a computationally challenging problem for real-time excision. This challenge is addressed by implementation of a histogram-based technique for MAD computation. The architecture is developed and optimized for Field Programmable Gate Array (FPGA) implementation. The design of a more robust variant of MAD called Median-of-MAD (MoM) is described. The architecture of MAD and MoM techniques and subsequent optimization allows for four RFI excision blocks on a single Xilinx Virtex-5 FPGA. These techniques have been tested on the GMRT wideband backend (GWB) processing a maximum of 400[Formula: see text]MHz bandwidth and the results show significant improvement in the signal-to-noise ratio (SNR).
The Giant Metrewave Radio Telescope (GMRT) is being upgraded to increase the receiver sensitivity. This makes the receiver more susceptible to man-made Radio Frequency Interference (RFI). To improve the receiver performance in presence of RFI, real-time RFI excision (filtering) is incorporated in the GMRT wideband backend (GWB). The RFI filtering system is implemented on FPGA and CPU-GPU platforms to detect and remove broadband and narrowband RFI. The RFI is detected using a threshold-based technique where the threshold is computed using Median Absolute Deviation (MAD) estimator. The filtering is carried out by replacing the RFI samples by either noise samples or constant value or threshold. This paper describes the status of the real-time broadband RFI excision system in the wideband receiver chain of the upgraded GMRT (uGMRT). The test methodology for carrying out various tests to demonstrate the performance of broadband RFI excision at the system level and on radio astronomical imaging experiments are also described.
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