RFI subtraction from time-averaged spectroscopy using a reference horn and cross power spectra

Effective radio frequency interference (RFI) cancellation for spectroscopic applications can take place using time averaged spectra. An RFI “reference signal,” 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, in combination with cross power spectra with the signals from the Parkes telescope, allows computation of the RFI contamination in the astronomical data, as well as construction of the corrections to be applied to the astronomical spectra. 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. This technique does not require “clean copies” of the RFI signal, since the method is immune to the effects of multipath scattering in both the astronomy and reference signal channels, provided the multipathing does not lead to near perfect cancellation of the reference signal through destructive interference. The RFI signal paths obey simple phase and amplitude closure relations. Tests made at Parkes, demonstrate that a specifically designed reference sensor provides a higher signal-to-noise ratio reference signal – and consequently cleaner cancellation – than that obtained from a second horn feed at the Parkes Telescope focus, whose principal function is to illuminate the Parkes dish. Subject headings: instrumentation: detectors — Methods: analytical, interference suppression