I. Introduction
The problem of classifying underwater targets from the acoustic backscattered signals involves discrimination between targets and nontarget objects as well as the characterization of background clutter. Several factors that complicate this process include: nonrepeatability and variation of the target signature with aspect angles, range and grazing angle, competing natural and man-made clutter, highly variable and reverberant operating environment, and lack of any a priori knowledge about the shape and geometry of the nontargets.