23 June 1997 Analysis in the joint time-frequency domain of the identifying signatures of submerged targets insonified by dolphin clicks
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We study the scattering interaction of dolphin-emitted acoustic pulses ('clicks') with various elastic shells located, underwater, in front of the animal in a large test site in Kaneohe Bay, Hawaii. A carefully instrumented analog- to-digital system continuously captured the emitted clicks and also the returned, backscattered echoes. Using standard conditioning techniques and food reinforcers, the dolphin is taught to push an underwater paddle when the 'correct' target -- the one he has been trained to identify -- is presented to him. He communicates to us his consistently correct identifying choices in this manner. By means of several time- frequency distributions (TFD) of the Wigner-type, or Cohen class, we examine echoes returned by three types of cylindrical shells. The time-frequency distributions we compare in this survey are the pseudo-Wigner distribution (PWD), the Choi-Williams distribution (CWD), the adaptive spectrogram (AS), the cone-shaped distribution (CSD), the Gabor spectrogram (GS), and the spectrogram (SPEC). To be satisfactory for target identification purposes, a time- frequency representation of the echoes should display a sufficient amount of distinguishing features, and still be robust enough to suppress the interference of noise contained in the received signals. Both these properties in a time- frequency distribution depend on the distribution's capability of concentrating the featuers in time and frequency and of handling cross-term interference. With some time-frequency distributions there is a trade-off between the concentration of features and the suppression of cross-term interference. The results of our investigation serve the twofold purposes of (1) advancing the understanding of the amazing target identification capability of dolphins, and (2) to assist in assessing the possibility of identifying submerged targets using active sonar and a classifier based on target signatures in the combined time-frequency domain.
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Hans C. Strifors, Hans C. Strifors, Guillermo C. Gaunaurd, Guillermo C. Gaunaurd, Patrick W. Moore, Patrick W. Moore, } "Analysis in the joint time-frequency domain of the identifying signatures of submerged targets insonified by dolphin clicks", Proc. SPIE 3069, Automatic Target Recognition VII, (23 June 1997); doi: 10.1117/12.277106; https://doi.org/10.1117/12.277106


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