Principles of Modern Radar - Volume 2 1891121537

16.3 Spectrogram Analysis of Human Returns 719TABLE 16-3Summary of Swerling Target Fluctuation Model CharacteristicsSwerlingModel Target Type Fluctuation PDF of RCSIscan-to-scanMany independent scatterers of equal sizeIIpulse-to-pulseIII One large scatterer surrounded by many scan-to-scanIV smaller scattererspulse-to-pulseRayleighChi-square, degree 4However practical the nonfluctuating Marcum model for human RCS may be, resultsfrom a 2007 study [79] show that fluctuating models may better represent the true humanRCS. This result should not be surprising, as humans are not rigid point targets but in facthave their own kinematics, namely, the periodic motion of the arms and legs, which is separatefrom gross motion along a path. Consider the classification criteria for the SwerlingModels, summarized in Table 16-3. A signal is considered to be scan-to-scan decorrelatedif the value measured over one coherent processing interval (CPI) is independent of valuesmeasured over previous CPIs. For typical radars, each CPI is composed of M pulses, sothat the CPI may be computed as M times the PRI. If each individual pulse in a CPI resultsin an independent value for the RCS, then the signal is referred to as being pulse-to-pulsedecorrelated. Because typical PRIs are on the order just fractions of a second, during thecourse of a single PRI slow-moving targets, such as humans, exhibit little fluctuation.Thus, one might expect that human targets would be scan-to-scan decorrelated.The results in [79] verify this intuition. In particular, it was found that for low frequenciesthe RCS followed a Swerling III distribution, while for high frequencies thedistribution more closely matched that of a Swerling I distribution. In both of these casesthe decorrelation occurs scan-to-scan. Moreover, the transition between these two casesoccurred between 1 and 2 GHz, although body position did affect the exact transitionfrequency. For example, the transition for a kneeling man occurred at a lower frequencythan that of a standing man, who continued to exhibit Swerling III properties at 1.8 GHz.This too matches our intuition on human RCS, as when a human stands, the reflectionsfrom the torso act as a single dominant scatterer in addition to the many scatterers fromother body parts. In the kneeling position, the torso is less prominent and thus on parwith reflections from other body parts, which is consistent with the description matchinga Swerling I target.Although the modeling of human RCS is gaining increased attention [80-83], results sofar are just beginning to provide an understanding of the RCS characteristics of humans.Much research remains to be performed to completely define and model human RCS,especially for the cases involving airborne applications, where elevation look angle hassignificant impact, as well as for a variety of target activities.16.3 SPECTROGRAM ANALYSIS OFHUMAN RETURNSThe spectrogram for a human target may be derived from the mathematical expressionfor the expected human radar return given in (16.3). First, the received data are organizedinto slow-time, fast-time matrix. Fast-time refers to the samples from each pulse within a