Principles of Modern Radar - Volume 2 1891121537

16.3 Spectrogram Analysis of Human Returns 721Freq (Hz)−300 FIGURE 16-630Simulated−250spectrogram for a20typical human target.−20010(From Gürbüz [84].−150With permission.)0−100−5005010011.522.5Time (sec)33.5−10−20−30−40−50Dopller Frequency (Hz)500450400350300250200150100500Human10.5 11 11.5 12 12.5 13 13.5Time (s)500450400350300250200150100500Dog70.5 71 71.5 72 72.5 73 73.5Time (s)FIGURE 16-7Measuredspectrograms for ahuman and dog.(From Otero [85].With permission.)characteristics to analytically distinguish humans from dogs based on their spectrograms.In particular, the appendage-to-torso ratio (ATR) was proposed as a statistic that couldbe useful for such a purpose. The ATR is computed from what Otero terms a cadencefrequency plot. This is basically the Fourier transform of the spectrogram, which exhibitspeaks at frequencies corresponding to the constant-rate torso motion, the fundamental frequenciesof appendage oscillations, and their harmonics. The amplitudes of these peaksare interpreted as being proportional to RCS. Thus, the ATR is defined as the ratio of thesum of the amplitudes of the peaks at the fundamental, second, and third harmonics to theamplitude of the torso peak:ATR ≡3∑RCS nn=1RCS 0(16.24)Otero postulated that the ATR for each animal could be unique and that, in particular, dogswould have a smaller ATR due to their significantly shorter legs relative to human legs.Spectrogram analysis continues to be the primary technique for target characterization,discrimination, and classification today. Applications that have been investigated includegender discrimination [85, 86], suicide bomber detection [87], human activity and targetclassification [66, 88–91], gait analysis [92, 93], and through-the-wall detection [60].