G. C. Gaunaurd
Naval Surface Warfare Ctr., Res. Dept., Code R42, White Oak, Silver Spring, MD 20903
H. C. Strifors
Natl. Defense Res. Estab., FOA-2, S-17290 Sundbyberg, Sweden
Significant features in the backscattered echoes of shells insonified by idealized pulses were studied earlier [J. Acoust. Soc. Am. 91, 2384 (A) (1992)], in the frequency, the time, and particularly in the combined time--frequency (t--f) domain. In this last case, (Gaussian) time-windowed Fourier transforms (TWFT) and pseudo-Wigner distributions (PWD) of the returned pulses were used. This analysis is now extended here to finite, realistic, incident pulses, with either rectangular or sinusoidal envelopes. The above distributions extract from the returned pulses the time-evolution of the resonances the target may have in the chosen (broad) band of operation. This band includes the so-called medium-frequency range. The selection of suitable carrier frequencies produces t--f-domain displays with recognizable features linked to specific target characteristics. One of these is, for example, the mid-frequency enhancement ``hump,'' present at ``coincidence,'' which determines the shell thickness. Adjusting the time-window widths controls the amount of detail in the displays, which will be shown in an informative colorized format in 3-D.