Abstract:

A low-frequency wideband (6--12 kHz) prototype has been developed in the framework of the SAMI (synthetic mapping and imaging) project. The prototype uses a subsurface towed fish. One transmitting array and two parallel receiving arrays 2 m long were installed on the tow-fish. Each array is associated to a synthetic aperture sonar channel. Thanks to the use of suitable transmitted signals (linear period modulated chirps), array synthesis could simply be achieved in the time domain; pulse compression and azimuthal matched filtering could be separated in order to reduce the processing complexity. Synthetic image resolution was measured at sea using reflectors on a frame; the results obtained were comparable to the predicted performance: 0.5x0.15 m. The cross correlation between both synthetic aperture channels output was used for measuring the elevation of the sea bottom reverberation cells. Synthetic aperture maps were then produced. Real-time processing was achieved on board the vessel for a swath of 500 m. Data were stored and post-processing was achieved off-line. Several experimental data obtained at sea in both shallow and deep (2400 m) water are presented. [Work supported by the EC-MAST (Marine Science and Technology) program.]