Improved interference cancellation in synthesis array radio astronomy using auxiliary antennas

This paper discusses how using a few low-gain auxiliary antennas as pan of a radio astronomy (RA) imaging m a y makes it possible to obtain outstanding rejection of interfering sources. We are particularly interested in canceling interference from orbital satellite downlink signals such as from GLONASS and IRIDIUM. These very strong interferers affect critical observation spectral bands used in radio astronomy [3]. For example, the Russian GLONASS positioning satellite signal produces strong spectral sidelobes that extend through the important hydroxyl ion (OH) emission band. In many practical scenarios with satellite interference, we have found poor cancellation performance when using existing array processing and subspace projection methods. Shallow, unstable beamforming nulls and poor interferer subspace estimates are common. This is usually due to the relatively low interference-tosignal-plus-noise ratio (ISNR) at antenna feeds of the high gain antennas used in imaging mays. This problem occurs even when interfering sources have flux densities which are many tens ofdecibels higher than the desired signal, because of the low sidelobe response for the high gain antennas. It must be remembered that in RA, signals of interest are usually well below the noise floor, so an interference level at antenna feeds which is much higher than the signal may not be significantly higher than the noise. In this case it is difficult to form the accurate interference signal parameter estimates necessary for high performance adaptive cancellation or subspace projection. In other words, the interference is often sufficiently strong to corrupt signal reception, but not enough higher than noise to permit effective adaptive cancellation. By adding a few (say one to three) low gain "auxiliary" antennas to an imaging may, it is possible to overcome poor interference parameter