Low Frequency SAS: Spatial Coherence Study

Abstract

The notion spatial coherence is used by sonar systems such as Correlation Velocity Logs (CVL) and Synthetic Aperture Sonars (SAS). It exploits correlation between acoustic signals sensed in space and time to estimate for example platform displacement. The Van Cittert Zernike (VCZ) theorem, is a classical statistical optics theorem that predicts the spatial coherence of a pressure field backscattered by a random medium. However it appears that intrinsic characteristics of Low Frequency Synthetic Aperture Sonar (LFSAS) can breach VCZ asumptions.In this paper, the notion of spatial coherence applied to a Low Frequency SAS system (LFSAS) is investigated. In a first part, the notion of spatial coherence and the VCZ therorem is presented. Then the High Resolution LFSAS (HRLFSAS) developed by NATO CMRE and used in this work is introduced. In the same time, a simulation tool is presented. In this paper, typical coherence functions obtained on simulation are presented. That allows to point out the link between the shape of the coherence function and the length of the transmit antenna. These functions are compared to the coherence observed on LFSAS data. Differences are investigated in terms of signal to noise ratio.