Mitigation of self-generated RFI using ASKAP's phased array feeds

TL;DR

This paper demonstrates that ASKAP's phased array feeds can effectively mitigate self-generated RFI using spatial nulling, with minimal impact on sensitivity and beam quality, enabling better scientific observations during interference.

Contribution

First demonstration of RFI mitigation via spatial nulling with PAFs on a large aperture array, quantifying impact on sensitivity and beam shape.

Findings

Suppressed RFI by 31dB to noise floor with 1.5% sensitivity degradation.

Overall array sensitivity degraded by only 3.1%.

Main beam gain reduced by 0.4% at half-power point.

Abstract

This paper presents the effects of radio frequency interference (RFI) mitigation on a radio telescope's sensitivity and beam pattern. It specifically explores the impact of subspace-projection mitigation on the phased array feed (PAF) beams of the Australian SKA Pathfinder (ASKAP) telescope. The goal is to demonstrate ASKAP's ability to make science observations during active RFI mitigation. The target interfering signal is a self-generated clock signal from the digital receivers of ASKAP's PAFs. This signal is stationary, so we apply the mitigation projection to the beamformer weights at the beginning of the observation and hold them fixed. We suppressed the unwanted narrowband signal by 31dB, to the noise floor of an 880s integration on one antenna, with a typical degradation in sensitivity of just 1.5%. Sensitivity degradation over the whole 36 antenna array of 3.1% was then measured via interferometric assessment of system equivalent flux density (SEFD). These measurements are in line with theoretical calculation of noise increase using the correlation of the beam weights and RFI spatial signature. Further, degradation to the main beam's gain is 0.4% on average at the half-power point, with no significant change to the gain in the first sidelobe and no variation during extended observations; also consistent with our modelling. In summary, we present the first demonstration of mitigation via spatial nulling with PAFs on a large aperture synthesis array telescope and assess impact on sensitivity and beam shape via SEFD and holography measurements. The mitigation introduces smaller changes to sensitivity than intrinsic sensitivity differences between beams, does not preclude high dynamic range imaging and, in continuum 1MHz mode, recovers an otherwise corrupted holography beam map and usable astronomical source correlations in the RFI-affected channel.