Creating an acoustic synthetic aperture in the ocean

A synthetic aperture experiment was conducted using a ship-towed source and midwater receiving hydrophones buoyed upward 4700 and 8000 ft from bottom moorings in a depth of 17 200 ft. The tows were conducted along straight lines perpendicular to the direction to the hydrophones at ranges of 58, 93, 128, 198, and 268 NM. A cw signal at 400 Hz and a swept FM signal between 350 and 450 Hz were transmitted continuously during the tows. Although major difficulties were encountered in maintaining straight tows in the presence of surface swells, it was possible to construct a number of synthetic apertures for the 400-Hz transmissions on an intermittent basis. The lengths of these apertures extended to 1/2 NM and more, corresponding to coherence time intervals of up to 7 1/2 min in the ocean. Subject Classification: [43]30.20; [43]60.30.