Principles of Modern Radar - Volume 2 1891121537

12.5 Antenna-Based EP 557are adjusted with complex (amplitude and phase) weights adaptively derived from theinterference signals being received such that the jamming interference received throughthe main antenna is minimized. This effectively generates spatial nulls in the directionsof the interference, with the intent of simultaneously preserving the underlying mainlobe target signal. The cancellation ratio, which is the ratio of the uncanceled signal tothe canceled signal, may be tens of dB and depends on factors such as the waveformbandwidth and the accuracy with which channels are gain and phase matched. Anothermeasure of performance is the change in signal-to-interference-plus-noise ratio (SINR),which reflects both the reduction in jammer interference as well as possible unintendedloss in desired signal. In general there must be at least one auxiliary channel for eachspatially separated interference source in the environment, although multipath scatteringmay require additional channels.Early, analog implementations of SLC featured closed-loop feedback control circuitsthat adjusted the weights of the auxiliary channels within a certain time period. The SLCweights therefore had an associated settling time that introduced a lag to changes in theenvironment. Many modern radars employ a digitally based approach whereby the gainand phase adjustments are applied numerically in the signal processor and are determinedindependently on each data batch being processed. Adaptive digital beamforming (DBF)is a design approach in which an electronically scanned array (ESA) antenna is dividedinto subarrays, with each subarray feeding an independent receiver channel whose outputis digitized through an ADC. The subarrays may include hundreds or even thousandsof radiating elements; or, in the limit, there may be only one element per channel. Thedigitized channels can be combined in multiple ways on the same sample of data toextract the desired information without consuming additional radar resource time. Referto Chapters 9 and 10 for a more complete discussion of adaptive cancellation of sidelobeinterference.12.5.5 Main Lobe CancellationMain lobe cancellation (MLC) is used to adaptively cancel one or more escort noisejammers in the main beam. The MLC places an adaptive spatial null in the direction ofthe escort jammer(s) while trying to maintain acceptable pattern gain elsewhere in themain lobe region to allow detection of the nearby target. Although the pattern gain towardthe target may be reduced in the process, the resulting SINR still improves due to theeven greater attenuation of the jammer(s). The SINR improvement resulting from MLCdecreases as the jammer-to-target angular separation decreases; for separations on theorder of 0.1 beamwidth, it may be difficult to cancel the jammer without also significantlyattenuating the target. Typically MLC apertures must be much larger than SLC auxiliaryantennas because the target signal level is much stronger in the direction of main lobeinterference than for sidelobe interference.The process of generating a null in the main beam may cause the average sidelobelevels to increase. This might be a concern if strong sidelobe clutter were present or ifthere were other jamming or EMI in the sidelobes. Another potential consequence ofMLC is the degradation in the angle estimate of the underlying target. For example, anangle estimate formed from a monopulse antenna is based on the designed and calibratedrelationship between the complex monopulse ratio of the difference () and sum ()beams as a function of the angle off boresight, as described in Section 12.5.9. If the and patterns are each affected differently by the MLC process, their ratio will in turn also