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Traditionally, the nost important military use of underwater sound has been the detection of subnarines at as great a distance as technology and costs will allow (sonar). Nowadays, these distances are as great as hundreds of miles for passive sonars, utilizing the noise radiated by the target, and tens of niles for active sonars, utilizing echoes from their targets. Detection ranges of these nagnitudes require the naxinun state-of-the-art enployment of techniques in signal processing, array designs and displays. The purpose of this paper is to show that these techniques are limited - in their possible gains and their degree of sophistication - by the peculiar and disadvantageous peculiarities of the ocean medium. These characteristics are the result of the presence of multipaths in the ocean - that is, the fact that there is always nore than one ray path between source and receiver. Here, some of the consequences of multipath propagation in deep water will be pointed out, and the limitations imposed on processing and the gain of arrays will be mentioned.