Discrimination between sidelobe and main beam reception of interference signals in a lobe on receive only radar is accomplished by a method of operation in which interference signals are stored and averaged for the radar receive beam positions of boresight, up, down, left and right. Interference signals received at boresight are compared with interference signals received at each off-boresight beam position. Interference signals received at boresight are also differenced with interference signals received at the off-boresight beam positions and the results are compared with beam width defining threshold factors. The determined values are applied to logic functions that indicate reception of interference signals outside of the main beam pattern.