Removing Radio Interference from Contaminated Astronomical Spectra Using an Independent Reference Signal and Closure Relations

TLDR

Radio frequency interference is a growing problem in radio astronomy, and future large telescopes such as LOFAR and SKA will require advanced suppression techniques. The paper proposes an intuitive, powerful RFI cancellation technique for radio spectroscopy that operates on time‑averaged data. It constructs an RFI reference signal from the cross‑power spectrum of two polarizations of a reference horn, applies phase and amplitude closure relations to compute and correct contamination, and can be generalized to interferometer arrays, solar‑radiation correction, and pulsar surveys. The method is immune to multipath scattering in both astronomy and reference channels, removing the need for clean RFI copies.

Abstract

The growing level of radio frequency interference (RFI) is a recognized problem for research in radio astronomy. This paper describes an intuitive but powerful RFI cancellation technique that is suitable for radio spectroscopy where time-averages are recorded. An RFI "reference signal," is constructed from the cross power spectrum of the signals from the two polarizations of a reference horn pointed at the source of the RFI signal. The RFI signal paths obey simple phase and amplitude closure relations, which allows computation of the RFI contamination in the astronomical data and the corrections to be applied to the astronomical spectra. Since the method is immune to the effects of multipath scattering in both the astronomy and reference signal channels, "clean copies" of the RFI signal are not required. The method could be generalized (1) to interferometer arrays, (2) to correct for scattered solar radiation that causes spectral "standing waves" in single-dish spectroscopy, and (3) to pulsar survey and timing applications where a digital correlator plays an important role in broadband pulse dedispersion. Future large radio telescopes, such as the proposed LOFAR and SKA arrays, will require a high degree of RFI suppression and could implement the technique proposed here with the benefit of faster electronics, greater digital precision and higher data rates.