Design and Evaluation of Sonars

Long detection ranges require low frequencies because of the attenuation of sound by seawater. Unfortunately, in order to get directionality at low frequencies, large arrays are required. To achieve higher directivity, the product of the size and the frequency must increase. In addition to the choice of frequency and size, the computational requirements of a sonar system need to be considered. The choice of type and duration of active sonar pulses is a classic example of tradeoffs. There are four basic types of pulses: continuous wave (CW), coded pulses (CP), pseudo random pulses (PRN), and explosive or impulsive pulses. Each type has inherent advantages and disadvantages in both detection and localization. Most modern, monostatic active sonars in search mode use omni, or wide-sector transmissions, with a highly directional receiver. These receivers have reduced ambient noise, reduced reverberation, and have the ability to provide accurate bearings for tracking. Controlled Vocabulary Terms impulse noise; noise; pseudonoise codes; sonar