Deterministic RF nulling in phased arrays for the next generation of radio telescopes

A requirement of the next generation of radio telescopes for astronomy is the ability to cope with the forever increasing problem of radio-frequency interference (RFI). Unlike conventional fixed parabolic receivers, currently used in astronomy, the application of phased-array beamforming techniques opens up the possibility to spatially null RFI in the RF domain, prior to signal digitization. This paper presents results from the second phased-array experimental demonstrator, the One Square Meter Array, on calibration and RF nulling performance. The approach is to deterministically null known RFI in the RF beamforming domain, and to adaptively remove the remaining RFI in the digital beamforming domain. A novel array-calibration technique, called the multi-element phase toggle technique (MEP), is presented. This technique allows a fast and very accurate calibration of wide-band phased-array antennas. Array calibration is shown to determine the extent to which RFI can be removed by experimental verification of simulated null depths.