On the gain available from passive acoustic synthetic aperture processing.

Abstract

Since passive synthetic aperture processing (PSAP) has long been understood to involve a trade-off between spatial and temporal gain, it has often been stated that there is no ‘‘new gain’’ to be realized. The argument, based on the model of a plane wave in isotropic noise, is as follows. For a fixed time available for processing, PSAP reduces the amount of integration time available for each sensor while increasing the number of sensors available. Thus, temporal gain is reduced while spatial gain is increased by an equal amount. However, when the noise field is nonisotropic, a different result obtains. It is shown that in the case of fields containing localized noise sources, PSAP can provide an increase in overall gain due to the ability to beamform on a desired source while rejecting undesired sources. A generalized formulation of the spatial-temporal processing gain provided by PSAP in terms of the ratio of coherent noise to incoherent noise power is presented that demonstrates the gains achievable in such a nonisotropic noise field. It is further shown that, even in the case of an isotropic noise field, if sufficient signal to noise is available, an improvement in bearing resolution can be achieved.