Circular Synthetic Aperture Sonar without a Beacon

Collection of synthetic aperture sonar (SAS) data along a circular track and forming a circular SAS (CSAS) image has several benefits over traditional stripmap SAS: the area of interest is observed from all aspect angles giving a better perception; the resolution in the image increases and shadow zones are avoided. Navigation requirements however, become even more stringent than for rectilinear SAS. Previously, CSAS images have been formed through use of an underwater beacon to alleviate the navigation problem. We present a scheme whereby CSAS may be achieved without use of a beacon. In this paper, we calculate the required accuracy in navigation, bathymetry and sound velocity for successful circular SAS. We present a new processing chain for CSAS with modifications to micronavigation and autofocus. Modifications to micronavigation and autofocus have not previously been discussed in the literature. Finally, we show the results from the new processing chain with circular SAS images of small targets. These data have been collected over an arbitrary seafloor by the HUGIN autonomous underwater vehicle carrying the Kongsberg HISAS 1030. As a result of these investigations we conclude that CSAS without beacon use is feasible for benign imaging geometries. Image interpretation however becomes more challenging. In addition we found that relative height was of considerable importance, with challenging topographic features causing degradation. To combat this we propose a hybrid autofocus / interferometry system to treat the height estimation problem in an iterative framework.