Underwater Acoustic Telemetry
Authors: Riter Stephen 
Published: October 2008

In this paper basic considerations in the design of a practical acoustic telemetry system are developed. These considerations are then applied to three representative modulation schemes: pulse position modulation [PPM], frequency modulation [FM] and frequency shift keying [FSK]. These modulation techniques when used in an underwater telemetry system are then compared on the basis of system complexity, performance and reliability.

INTRODUCTION

Reliable transmission of data under water is a necessary prerequisite for the observation and development of undersea resources. In many cases this can be accomplished by using hardwire links between transmitters and receivers; however, there are instances when such techniques are unsatisfactory. In such instances an appealing alternative is to modulate an acoustic signal in a manner analogous to the way one modulates an electromagnetic signal in a radio system. In spite of the technical feasibility and considerable economic savings that can be realized with such techniques, relatively little has-been done to exploit underwater acoustic telemetry and make simple, efficient and economic systems available to practicing oceanographers and marine technologist. It is the objective of this paper to discuss feasibility and design considerations for such systems and to outline some of the problems which must be overcome in their realization. Considerable emphasis will be placed on utilizing results derived from the solution of similar type problems in the area of radio communications; however, a detailed understanding of these problems is not necessary for the understanding of this paper.

TRANSMISSION CHARACTERISTICS OF THE UNDEIMATER ACOUSTIC CHANNEL

The underwater acoustic channel is a complex randomly time varying media whose exact mathematical description is currently the subject of much active research by scientists and engineers. From a communications theory point of view the effects of prime importance are attenuation, refraction and reverberation. A detailed study of these effects is not possible here; however, an understanding of the approximate nature of these effects is necessary for the evaluation of communications techniques.

Attenuation is a decrease in signal energy with range. In the ocean two types of attenuation manifest themselves. The first is an energy spreading analogous to the spherical spreading encountered in electromagnetic propagation. If for example one places an isotropic source [one which radiates energy equally in all directions] in an infinite homogeneous media, then as is well known the energy distributes itself uniformly about spheres surrounding the source, and there is a decrease in energy with the range, R, proportional to [1/R2 ]. Only in the most idealized sense does an acoustic source located in the ocean exhibit such spherical behavior. In fact, in shallow water, since the air-water and earth-water interfaces are reflecting in nature, the spreading tends to be cylindrical and the energy decreases as-[l/R]. In most locations the actual spreading is observed to be somewhere between the two extremes indicated. A conservative guideline would then be to assume spherical spreading unless some specific information, for a particular location indicated otherwise.