Low frequency synthetic aperture sonar: Spatial coherence analyse

Abstract

Spatial coherence is a widely studied notion for HF SAS. It allows the estimation of the navigation of the platform with an accuracy well below the wavelength of the system . It also allows, for high-frequency SAS images, the detection of stable reflectors generally symptomatic of manufactured objects. The theoretical background of spatial coherence is based on a derivation of the Van Cittert Zernike (VCZ) optics theorem adapted to acoustic. However, this theorem uses some assumptions (narrow band signals, Fresnel approximation, Born approximation and homogeneity of the scattering media) that are not necessary met for low frequency system. In this paper effects of wideband systems and presence of a target in the insonified volume are investigated. Do to so, the study is based on a simulation tool whose results are compared with real data from trials conducted by CMRE with a wideband LFSAS. Thanks to the use of a 2-D array, the two dimensional coherence can be analyzed. A sub-band study enables to respect the narrowband VCZ assumption. A statistical study on distribution of degree of coherence estimates allows to point out the presence of strong scatterers symptomatic of manufactured objects.