Sonar Techniques

the determination of the position of underwater objects with the aid of sound signals that are emitted by the objects themselves (passive sonar) or occur as a result of the reflection of artificially created sound signals from underwater objects (active sonar). The term “sonar technique” is understood to mean exclusively sound detection, since sound waves are the only type of waves known at present that propagate in a sea medium without considerable attenuation. Sonar is of great value in navigation for detecting unobservable underwater obstacles, in fishing for detecting schools of fish and individual large fish, and in oceanography as a tool for studying the ocean’s physical properties, mapping the sea bottom, searching for sunken vessels, and so forth. It is also used for military purposes to detect and observe submarines, surface ships, and other vessels and to determine the coordinates of targets when using torpedoes and missiles.

In passive sonar (sound direction finding), the direction to a sound source (the bearing of a source) is determined with the aid of a hydrophone by the sonic field created by the source itself. In the process, various methods are used: the rotation of a receiving acoustic antenna with a high directivity response to the position at which the received signal is at its maximum intensity (the so-called maximum bearing method); the measurement of the phase difference between signals at the output of two antennas separated in space (the phase method); the determination of the relative time difference between the receipt of signals by two separated antennas by measuring the mutual correlation (the correlation method); and also a combination of these methods. In passive sonar the distance to an object is determined from two or more bearings obtained by several receiving systems that are separated at distances comparable to the distance to the object being detected (the triangulation method). In this way not only the position of the sound-producing object is determined, but also the trajectory of its motion. Passive sonar systems are used chiefly for the hydroacoustic rigging of submarines and surface ships. Passive sonar is also used in the detection of underwater sound-producing objects with the aid of dispersed coastal and seabed systems of sound receivers, the data from which are transmitted by underwater cables to coastal processing systems, and with the aid of radio buoys, from which information is received on a radio channel by special planes flying in the vicinity of the buoys. In addition, the passive determination of the direction to a sound-producing object is the basis for the operation of acoustic self-guiding torpedoes.

If the sound source emits a short sound pulse, the position of the source may be determined from the differences in the times of arrivals of pulses received by nondirectional receivers in three or more points dispersed in space. This method of localizing sources is used in the coastal system of the long-range detection of vessels in distress in the open sea (the SOFAR system). The sound source in this case is the explosion of a charge that is submerged at a certain depth.

Active sonar systems are based on the phenomenon of the sonic echo and are distinguished by methods of time modulation of the dispatched signal and methods of surveying space. To determine the distance to an object, the pulse modulation, frequency modulation, and noise modulation of the signal are used most often. In pulse modulation the distance R to the target is found from the time of delay t₀ of the reflected pulse: R = c/t₀/2, where c is the velocity of the propagation of sound in the medium. In frequency modulation the frequency ƒ of the emitted signal varies with time t according to the linear lawƒ(t) = ƒ₀ + γt, where ƒ₀ and γ are the constant initial frequency and speed of frequency variation. Therefore, the reflected signal received by the receiver will differ in frequency from the signal emitted at the given moment, since the received signal is a duplicate, delayed by time t₀, of the transmitted signal, and the frequency of the emitted signal has changed in time t₀ according to the above formula. For a stationary target the difference in frequencies will be constant and equal to ƒ_ = γt₀. After determining the difference frequency, the distance to the target R is determined by the formula R = cƒ_/2γ. The operation of sonar with noise emission and correlative processing of the signal is analogous.

The principal feature of sonar systems is the distance of detection, which depends on the power of the signal being emitted, on the level of acoustic noise and on the conditions of propagation of sound in water. The maximum distance of detection is usually determined from the magnitude of the so-called threshold signal, that is, the signal of minimum intensity that is still distinguishable against background noise. If the noise and signal are independent, then the threshold signal is determined by the ratio of the total energy of the useful signal to the noise power in a given frequency interval. Thus, the distance of detection for systems with different types of modulation will be the same if their total emission energy is the same. If the main noise is the chaotic reflection of a signal from the inhomogeneities of the medium (so-called reverberation) then the threshold signal does not depend on the power of the signal being emitted but is determined exclusively by the width of its frequency band. In this case, systems with frequency modulation of the signal and with noise emission are more efficacious.

Besides noise, the distance of detection is influenced by refraction, which occurs under complex hydrologic conditions. Present-day sonar installations are capable of detecting large reflecting objects at distances averaging several kilometers.