Principles of Modern Radar - Volume 2 1891121537

558 CHAPTER 12 Electronic Protectionbe different, thereby resulting in an erroneous angle measurement. Additional calibrationand processing algorithms may be needed to enhance MLC if such errors are unacceptablyhigh. Refer to Chapters 9 and 10 for a more complete discussion of adaptive cancellationof main lobe interference.12.5.6 Narrow BeamwidthA narrow antenna beamwidth reduces the angular region of influence of a main lobe supportjammer. An ESJ will have to fly closer to the target(s) being protected to remain in the mainbeam; this may impact tactics or require additional ESJs to screen multiple targets. Theangular region in which an SOJ or SIJ is in the main beam simultaneously with the targetis also reduced. The narrow beamwidth also implies high transmit and receive antennadirectivity. The transmit directivity increases the radar ERP which decreases the JSR asshown in Equations 12.9, 12.13, and 12.14. The receive directivity increases the mainlobe to sidelobe ratio, thereby making it more difficult for a sidelobe jammer to penetratethe radar sidelobes. Such EP benefits notwithstanding, the selection of beamwidth in aradar system design is rarely if ever motivated by EW considerations but rather by searchvolume coverage, sensitivity, and angle estimation accuracy requirements.12.5.7 Random Beam ScanningRandom beam scanning is a capability afforded by ESAs that can reduce the effectivenessof transponder and repeater jammers attempting to operate in the radar sidelobes.Electronically scanned array radars have the ability to generate arbitrary scan patternswhen performing search. In contrast, mechanically scanned antennas generally followpredictable raster or circular scan patterns with regular, periodic revisits to a specific beamposition. A jammer operating in the sidelobe can potentially exploit such predictabilityand generate its interference, whether noise or false targets, at specific angles by synchronizingto the periodic radar main beam illumination. For example, a mechanically scannedantenna with a 0.2-Hz circular scan pattern will paint the jammer with a main lobe illuminationonce every 5 seconds. If the jammer knows that the radar uses a clockwise scan, itcan place its SLFTs at, say, 36 degrees further clockwise from its own position by timing itsresponse to occur 0.5 seconds after receiving the main lobe illumination (36/360×5.0 sec= 0.5 sec). The ESA randomization of the scan pattern denies the jammer this capability,forcing it to employ a potentially less effective EA mode.12.5.8 Passive Conical ScanningPrior to the arrival of monopulse antennas, radars often employed conical scan (CON-SCAN) antennas to perform angle tracking. In a traditional, active CONSCAN approach,the antenna beam is nutated about an axis that is intended to be at or near the directionof the target being tracked. As the center axis drifts away from the target position, theamplitude of the target return is stronger for beams closest to the target and weaker forbeams farther from the target. This produces an AM on the received target signal. Thephase of this AM signal relative to the CONSCAN modulation control voltage indicatesthe direction of the angle tracking error, and the relative amplitude variation indicates themagnitude of the correction required. The angle error estimate can only be obtained bytaking measurements from multiple pulses over the modulation scan period.