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Diverse Performance Requirements Lead to a Wide Variety of Design Approaches
The forward-looking sonar (FLS), sometimes called an ahead-looking sonar (ALS) or sector-scan sonar, is widely used in a variety of applications including mine hunting and obstacle and terrain detection for underwater vehicles. Many ships and nearly all underwater vehicles are equipped with an FLS.
The same type of sonar, pointed down instead of ahead and with beams formed athwart ship, is used for bathymetric mapping.
Sonar engineers have developed a number of widely divergent designs for a large number of applications. Among these designs are: mechanically scanned single or multiple beams; * acoustic lenses; * CTFM (continuous transmission frequency modulated); * digitally beamformed line arrays; and * cylindrical arrays with analog or digital beamforming.
This article discusses and compares the features of the many approaches available for FLS design.
Alternative Array Designs
Except for submarine sonars, forward-looking sonars operate primarily in the active mode, in which a projector array transmits a sonar pulse that produces echoes detected by a hydrophone array. The sonar engineer must develop designs for both the projector array and the hydrophone array matched to the performance requirements of the application or applications of interest.
In the typical FLS, the projector and hydrophone arrays are oriented in a Mill's Cross configuration, i.e., with the major dimension of the projector arrays) vertical and the major dimension of the hydrophone array horizontal. In an FLS designed for operation in shallow water, the projector is often steerable in the vertical direction and may produce several selectable vertical beam widths along with selectable vertical pointing angles.
Horizontal coverage sector varies with array design type (line, cylindrical, planar, conformal, etc.), while the minimum width of the individual beams depends upon array or aperture dimensions and operating frequency. The forward-looking sonar usually produces beams of moderate width, typically about 2 deg to 6 deg. Effective obstacle detection performance is achieved using beams 10 deg or wider. High-resolution sector scans are sometimes used for classification in mine hunting operations, and sonars used in this application produce very narrow beams, 0.25 deg or finer, over a 20 deg- to 30 deg- wide horizontal sector.
Some applications utilize mechanically steered or indexed single-beam sonars to scan a desired coverage...