Excising terrestrial radio interference in low frequency radio astronomy

Interference from terrestrial transmitters is a major problem in low-frequency radio astronomy. Experimental work is described in which a 22.25-MHz interferometer provided baseband signals from which 128- channel spectra were calculated in hardware by FFT and processed by a microcomputer. Narrow-band interference was identified and excised from the in-phase and quadrature cross-spectra and the auto-spectra in a 52-kHz band in real time. The observations were performed during winter near sunspot maximum with the on-line microcomputer performing ± 4 σ iterative deletion of interference. Off-line, an algorithm was developed in which robust estimation was used to give protection from statistical outliers. The results showed consistently that 2–3 hr extra observing time were made possible per night. Low-level interference, which would not have been noticeable in records taken with a conventional receiving system, was detected and excised on most nights.