ABSTRACT

This paper takes a look at the subject of resolution, a term much misused in the sonar field, as applied to continuous scattering fields It proposes the adoption of a more accurate term, the Modulation Transfer Function, to describe the imaging performance of sonars, borrowing from the field of optics. Finally a look is taken at the effects of the diffuse component of reflected energy on the appearance of sonar images and hence the ability to detect small manmade artefacts

1 INTRODUCTION

Mankind's use of underwater acoustics in a systematic way dates back less than a century, but in the last couple of decades it has come to be regarded almost as a mature technology. This is but an approximation to the truth, there are surely significant developments yet to come. What is not in doubt is that all of mankind's activities in the ocean are underpinned either directly by the use of sonar or indirectly by the products of its use, right down to the weekend yachtsman Through most of the decades of development of sonar science and techniques the leading force has been defence needs, and only in the last 20 years has there been a significant push for development in the civilian field With the apparent end of the Cold War the civilian development effort can only grow until it dominates the field.

This fairly recent and very rapid acceptance of sonar techniques has allowed some undesirable simplifications to become current as the subject develops its own jargon Jargon, in any specialist field, is a valuable short cut to communication between its practitioners, but it all too easily spreads beyond its proper boundaries and becomes at best a substitute for thought and at worst a channel for actual confusion in the mouths of would-be experts The term resolution is in danger of becoming an example of such jargon It is becoming increasingly common to hear people referring to the ‘resolution’ of a sonar imaging system as if it were a single well defined number without any thought being given to the processes behind the term. This contribution is an attempt to clear away some of the confusion surrounding the specification of imaging sonars It examines the nature of the backscattered signal received by a sonar from the seabed and the mechanisms of image formation. In the course of the article the author proposes, without much hope that it will be widely accepted, a different measure of sonar performance for a two-dimensional scattering field. This measure is called the Modulation Transfer Function and it is taken, with due acknowledgement, from the field of optical engineering No originality is claimed for what follows, but as far as the author is aware the material has not previously been gathered and presented in this way with a view to clarifying a topic which is getting increasingly muddled.

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