Beam steering for a long range acoustic hail andwarning device for maritime applications

A new means for steering the beam of a phased line array, electromechanical steering, is described and applied to a long range Acoustic Hail and Warning Device (AHWD) developed for maritime applications. In electromechanical steering, the array elements pivot individually resulting in elements being off-set and rotated in the desired steered beam/sound projection direction. A digital signal processor is used to align the phase fronts of the elements due to the mechanical offsets. Electromechanical steering has several advantages over the purely electronic steering of the array. The electromechanically steered beams are not subject to the individual element aperture function attenuation that reduces the sound pressure level (SPL) that commonly occurs for electronically only steered beams at large steering angles. Also, the primary lobe distortion and (lower frequency) onset of grating lobes that occur when a beam is steered electronically is absent with electromechanical steering. In short, with electromechanical steering, the SPL and directivity from theAHWD is entirely equivalent to that obtained by rotating the AHWD to the desired angle with a conventional pan and tilt mount but requires much less space and provides for more flexible mounting options including improved system stealth. In addition, a novel type of line array phase shading is presented which allows theAHWD to be electronically switched from directional mode (typical beam steering highly focused mode) to non-directional wide field of regard for applications that require simultaneous ensonification of targets over large lateral angles.