The response of a radio-astronomy synthesis array to interfering signals

Multiplying interferometers, and the more complex synthesis arrays that have been developed from them, possess the advantage of greater discrimination against interfering signals than can be obtained with single-antenna radio telescopes. The major contribution to this effect results from the relative changes of the phases of the signals received in spaced antennas, associated with the sidereal motion of a cosmic source across the sky. Signals that do not show this predictable phase behavior are substantially suppressed in the data processing. In this paper an approximate general expression for the interference response is derived and compared with results of an experimental test. The major uncertainty results from the variability of the gain of the antenna sidelobes in which the interferenee is received. Threshold levels at which interference becomes harmful are derived for the very large array (VLA) of the National Radio Astronomy Observatory. In the case of broad-band interfering signals, further rejection occurs because inequalities in the time delays of the signal paths via the individual antennas result in decorrelation. The magnitude of the decorrelation depends upon the position of the source of interference, and as an example, computations are given for broad-band signals from a satellite in geostationary orbit. General thresholds for harmful interference to radio astronomy are given in International Radio Consultive Committee (CCIR) Report 224, and the present results are not intended to supercede them. Rather, the intention is to provide data specific to synthesis arrays to allow more accurate coordination in certain frequency ranges shared with active spectrum users. A possible application to the search for extraterrestrial intelligence is mentioned.