Shadows, synthetic aperture sonar and forward looking gap-filler: different imaging algorithms

SHADOWS is a new sonar system developed by IXSEA SAS. It is a towed fish composed of a synthetic aperture side-scan sonar and a forward-looking sonar. The system produces SAS images with a constant resolution 15 cm from 30 m to 300 m on each side. It also fills the nadir gap from 0 to 30 m with images having 40 cm times 15 cm resolution. The images are provided in real-time and are georeferenced. The synthetic aperture sonar Algorithm uses an accurate and highly synchronized INS combined with the Displaced Phase Center Algorithm (DPC) to calculate the high resolution images. The beam-forming algorithm used is the time-domain fast factorized back projection. This allows to compute a multithreaded algorithm and thus to lower the duration of the calculation. The forward-looking sonar uses a patented "sectorized emission" architecture. Four different signals are transmitted on four sectors of the gap illuminating the sea-bottom from 40 m to 100 m on the front of the fish. The images are side-scan-like, i.e. we see echoes and shadows as on the sides. We have a non-square resolution of 40 cm across track and 15 cm along track. The realtime algorithm can be customized to make some incoherent integration on several pings. A post processing algorithm can also provide an animation on a specific contact on the floor. An additional algorithm on the forward looking sonar signals can be used to increase the precision of the DPC navigation calculated on the sides.