Limitations on towed array gain imposed by a non-isotropic ocean

Abstract

This study deals with the limitations imposed by ocean characteristics on hydrophone array gain and on attempts to create a synthetic aperture using towed-array measurements. The multi-element synthetic or physical aperture of a passive array operates in a non-isotropic 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. The coherence properties of the medium are synoptically considered to be characterized by its temporal coherence and the angular uncertainty

which is the inverse of the spatial coherence length. A processing scheme is suggested for the derivation of the array gain from received signals having small values of signal-to-noise ratio and for a given directivity pattern of the non-isotropic noise background. Simulations indicate that

a comparison of predictions against experimental estimates for array gain can provide an approximate estimate of the coherence length of the medium. Real data applications, using the extended towed-array measurements technique, indicate that the physical aperture of an array can be extended successfully by more than one order of magnitude for cases of cw and broadband signals. This study concludes that the coherence properties of the ocean are sufficient for effective long towed-array applications and that a synthetic aperture can be created in an anisotropic medium.