Limitations on towed-array gain imposed by a nonisotropic ocean

This study is concerned with the limitations imposed by the characteristics of the ocean on the gain of hydrophone arrays and on the process of creating a synthetic aperture by using towed-array measurements. The multielement synthetic or physical aperture of a passive array operates in a nonisotropic noise field and in a medium of limited spatial coherence length. In particular, it is assumed that, due to distant shipping, the noise field is partially directive and imposed upon an isotropic noise background, and in the rough-bounded, transmitted medium, many multipaths exist which are closely spaced in arrival time and arrival angle. In the theoretical development of this study, a processing scheme is suggested for the determination of the array gain from the received signal and the directivity pattern of the noise field, which is assumed to be known. Results from simulations of this processing scheme have indicated that a comparison of the experimental estimates of towed-array gains with the above array gain predictions for different values of the spatial coherence length can provide an approximate estimate of the coherence length of the medium. Results from real-data applications of the theoretical development indicate that the coherence properties of the ocean can be sufficient for effective long (80λ) towed-array applications and that the formation of a very long (250λ) synthetic aperture in an anisotropic medium can be successful. The algorithm that is used to create a synthetic aperture is the extended towed-array measurements (ETAM) technique.