Abstract

Abstract The performance of real or synthetic aperture sonar (SAS) in shallow waters is degraded by multipath, which leads to ghost targets and reduced image contrast. This loss of image contrast fills in acoustic shadows and is therefore an important limitation for high resolution SAS applications such as minehunting, where target recognition exploits the shape and size of the shadow in addition to the echo structure. Experimental data from a 100 kHz sonar array mounted vertically on a tower in 20 m water depth show the importance of second order multipath. A specific SAS design with optimized transmit and receive beam characteristics in elevation is therefore required to achieve high SNR at large range to waterdepth ratios. Introduction As well known, synthetic aperture sonar (SAS) has the potential to provide very high cross-track resolution at long ranges. In practice, however, multipath interference can be a dominant cause of performance degradation, especially in shallow water. Multipath, besides the well known effect of ghost targets, leads to loss of image contrast (with consequent filling in of shadows) and degrades the quality of bathymetric estimates when interferometry is used. These effects, which are not SASspecific, have nonetheless enhanced relevance in synthetic aperture imaging, since SAS aims naturally to extend the range of a sonar to fully exploit the gain in cross range resolution. In addition, multipath affects specifically SAS performance because of the influence on the datadriven methods, as the Displaced Phase Centre Array (DPCA) micronavigation, used to estimate the platform trajectory. The DPCA technique makes use of the correlation of the sea bottom direct backscatter to estimate the displacement of the SAS between pings, and depends critically [1] on a generalized signal to noise ratio (SNR), where the signal is the seafloor backscatter coming from the direct path, while the noise consists of background noise of the sea, system noise, surface and volume reverberation and, last but not least, multipath interference of various orders. We will adopt the convention of naming a multipath by the a combination of letters 'b' (for bot-