Removing radio interference from contaminated astronomical spectra using an independent reference signal and closure relations

The growing level of radio frequency interference (RFI) is a recognized pro blem for research in radio astronomy. This paper describes an intuitive but powerful RFI cancellation technique that is suitable for radio spectroscop y where time-averages are recorded. An RFI "reference signal," is construct ed 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 comp utation of the RFI contamination in the astronomical data and the correctio ns to be applied to the astronomical spectra. Since the method is immune to the effects of multipath scattering in both the astronomy and reference si gnal channels, "clean copies" of the RFI signal are not required. The metho d could be generalized (1) to interferometer arrays, (2) to correct for sca ttered solar radiation that causes spectral "standing waves" in single-dish spectroscopy, and (3) to pulsar survey and timing applications where a dig ital correlator plays an important role in broadband pulse dedispersion. Fu ture large radio telescopes, such as the proposed LOFAR and SKA arrays, wil l require a high degree of RFI suppression and could implement the techniqu e proposed here with the benefit of faster electronics, greater digital pre cision and higher data rates.