Principles of Modern Radar - Volume 2 1891121537

574 CHAPTER 12 Electronic ProtectionThe OS-CFAR computes the mean interference based on a subset of the sampled cells thatexclude the k cells with the highest values, with the value of k being a design parameter.The CC-CFAR picks a specific cell of the rank-ordered sequence as representative of thetrue, EA-free noise, analogous to picking the median instead of the mean. These algorithmshelp ensure that the true target is detected, although the false targets (or clutter discretes)may also be detected in the process.12.9.3 Angle VarianceA high variance in the angle measurements made under conditions of main beam noisejamming may be indicative of multiple jammers in the main beam. The relative amplitudeand phase of the two noise jammers will vary in a random fashion because theirnoise waveform generators are independent. The angle measurement on average will benear the power centroid of the multiple jammers, but individual measurements will varyconsiderably due to random variations of their relative amplitude and phase. For the casein which two noise jammers are blinking, either synchronously or asynchronously, suchthat there are intervals during which only one jammer is on at a time, the distributionwill tend to have a more bimodal characteristic to it. In contrast, a single jammer with ahigh JNR will be characterized by a narrow distribution of angle measurements arounda single angle. Noise jammers will cover many if not all of the range-Doppler cells usedfor detection. The signal processor can form the monopulse ratio on a large number ofthe range-Doppler cells in a single CPI and compute the statistical properties of the noisejamming angle distribution. The angle variance is more reliable for high-JNR conditionsbecause the angle measurements are less influenced by thermal noise.12.9.4 Data EditingRadars whose CPIs are long compared to the ADC sampling interval may be able to employdata editing, or excising, to minimize effects of intermittent jamming. This concept issimilar in spirit to wideband limiting, except in this case, the jammer sweep time islong compared to the impulse response time of the band-limiting IF filter. As a result, ajammer that is either pulsed or sweeps in and out of the radar band will manifest itselfas an intermittent on–off modulation during the radar CPI. Ordinarily, the radar signalprocessor coherently integrates the jammer spikes over the CPI, and the resulting effectis comparable to a continuous jammer with an average power equal to the intermittentjammer’s peak power multiplied by its duty factor. The data editing technique is applicableif the jammer has a low duty factor and high peak power relative to the target signal, thermalnoise, and clutter.The signal processor must recognize unnaturally large increases and decreases inpower over the CPI and then excise these data intervals from the data before performing thecoherent signal integration. There is an accompanying loss in signal energy and degradationof range or Doppler image quality. The signal energy in the detection cell decreases by afactor (1 – d j ) for a given jamming duty cycle d j within the CPI; this should be more thanoffset, however, by a much greater decrease in jamming energy under intended conditions.One of the likely consequences of the data editing is elevated range or Doppler sidelobes.These sidelobes may increase the amount the clutter interference in the target detectioncell in clutter-intensive environments. This cost must be weighed against the potentialbenefit of reducing the EA impact. Signal processing algorithms that partially restore