The effect of internal waves on synthetic aperture sonar resolution.

Beamforming techniques for synthetic aperture sonar systems (SAS) operate under the assumption that the water column consists of a constant sound speed or sound speed profile. As has been noted over the past several years, the water column sound speed can sometimes be modified by three-dimensional features such as internal waves or boluses caused by breaking internal waves. Owing to a breaking of the constant sound speed assumption, these types of features may cause a loss of resolution, i.e. defocusing. The defocusing caused by internal waves and boluses can be quantified using estimates of the point spread function (PSF). In this work, estimates of the PSF are made by fitting ellipses to candidate point scatterers in up-sampled SAS images. The ellipse sizes in the along- and across-track directions can be compared across different regions of SAS images that contain refractive artifacts. In this work we describe the ellipse fitting technique and show sample results of PSF estimates obtained from datasets collected by the Norwegian Defence Research Establishment (FFI) using the HISAS system. It is hoped that tying image defocusing effects to water column properties will eventually allow inversions for internal wave and bolus parameters such as temperature and size. [ABSTRACT FROM AUTHOR]