First field results for the HISAS Synthetic Aperture Sonar
Synthetic Aperture Sonar (SAS) combines a number of acoustic pings to form an image that is typically 10 times higher resolution than conventional sonars.
Advanced signal processing
The HISAS sonar is part of the HUGIN system solution for mine counter-measures, which has been ordered recently by the Norwegian Navy. HISAS is a wideband SAS sonar with frequency range of 70-100kHz, capable of producing ultra high resolution acoustic images as well as co-registered bathymetry. The sonar is tightly integrated with the INS navigation and motion sensing platform of the HUGIN AUV, and makes use of modern signal processing such as DPCA (Displaced Phase Centre Analysis) to process the raw data into images.
The sonar prototype has recently been tested with success at sea outside Horten - Norway, with signal processing done by FFI. The image above is from this test and shows a sunken wreck standing upright on the bottom. It has a resolution of about 4 cm both along track and across and each acoustic beam being processed from 40 consecutive pings. Resolution is high enough so that one can observe details such as the wire running from the bow to the top of the mast.
The principle of synthetic aperture sonar is to move a sonar along a line and illuminate the same spot on the seafloor with several pings. This produces a synthetic array equal to the distance travelled. By coherent reorganization of the data from all the pings, a synthetic aperture image is produced with improved along-track resolution. SAS processing have the potential to improve the resolution by one order of magnitude compared to conventional sidescan sonars.