The present invention relates to improvements in radar systems and more particularly to improved techniques for eliminating or cancelling interference introduced into the side lobes of an ECCM radar antenna from multiple interfering signal sources.
Radar antenna systems, including those adapted for ECCM techniques, have characteristics that include a main lobe for receiving the desired information, and a plurality of side lobes at various angles relative to the main lobe. Due to the nature of an antenna, information received in a side lobe is indistinguishable from information received in the main lobe, and thus renders the antenna highly susceptible to interference from unwanted signals or information. The problem is particularly acute in radar systems where the presence of side lobes makes it possible for a single noise jammer to be effective against a radar from any angle of azimuth. The problem becomes even more acute when multiple interference or jamming sources are used against a radar and directed from a variety of directions simultaneously. Side lobe cancellation is a fundamental approach to eliminating interference in received signals, and has been used successfully to eliminate the interference introduced from a single jamming source. One such system uses an adaptive side lobe canceller system well known in the art. In general, such systems use a signal received by an auxiliary omnidirectional antenna to cancel the interference signal received in the side lobe of the primary directional antenna. In ECCM radar systems where the desired signal is usually broadband, such as a short radar pulse, chirp pulse, or spread spectrum signal, and interference is caused by a narrow band signal, a channelized frequency selective limiter circuit is utilized for suppressing a cw or narrow band interfering signal. However, in such prior art broad band ECCM systems there is no provision for responding to wide band interfering signals which are unaffected by the frequency selective limiter circuit. Accordingly, the present invention has been developed to provide an ECCM radar system wherein both wide band and narrow band interfering signals are readily cancelled or eliminated.