Description of a Navy Holographic Underwater Acoustic Imaging System (AIS)

The Acoustic Imaging System (AIS) was developed by the Naval Ocean Systems Center (NOSC), San Diego, for use aboard deep diving Navy submersibles and also as a research tool. The operational specifications of the AIS were chosen to lie between those of closed circuit television (short range, high resolution) and most sonars (long range, low resolution) to provide moderate range (5–100 feet) and moderate resolution (0.3 degree) (figure 1) for classification of objects. Holography was selected as the most promising technique for this underwater acoustic imaging application over focused or beamformed techniques (figure 2). The primary reasons for selecting holography were for (1) compactness and lack of any cable-snagging protrusions, (2) failure tolerance, and (3) simplicity of hardware. Submersible pilots are generally very particular about external equipment getting entangled in cables. Since holography requires no lens, only the hydrophone array and its immediate processing electronics need to be in the water. Failure tolerance comes from the fact that the most unreliable components in an acoustic imaging system appear to be the hydrophone elements and their associated circuits. In holography, these components are in the Fourier transform domain. Hence, any errors or failures in them contribute only to the overall noise background of the reconstructed image. In focused systems, these components are in the image plane, and failures give rise to holes in the image.