An MIMO Sonar Array for High-resolution 3D Forward-looking Imaging

The three-dimensional imaging sonar using a planar or volumetric hydrophone array is useful for underwater acoustic imaging, since it can provide the volumetric information of an underwater target. However, the system cost is at a high level due to the large number of hydrophones adopted. To reduce the system cost, the sparse array optimization is often used to remove a number of hydrophones and simultaneously, to keep a desired beampattern performance. Nevertheless, the angle resolution of a sparse optimized hydrophone array is not enough for relatively long range imaging application, such as the small target detection. Different from the sparse array optimization method, using a forward-looking sonar composed of several horizontally paralleled uniform linear arrays (ULAs) can achieve a good 3-D imaging ability. The 3-D forward-looking sonar is based on the slant looking imaging processing, and it combines the conventional horizontal beamforming and the high-resolution vertical beamforming together. Nevertheless, the horizontal angle resolution is restricted by the limited physical size of the sonar platform (e.g., an underwater manned vehicle or an underwater robot). What is worse, the restricted angle resolution in the horizontal direction will degrade the imaging performance of the vertical beamforming. To solve the problem, we design a MIMO sonar array layout for the 3-D forward-looking imaging. The designed MIMO sonar array is composed of two sparsely located transmitting transducers and several parallel ULAs. By doing so, the horizontal angle resolution doubles that of a traditional sonar array (which is composed of one transmitter and several paralleled ULAs), and the vertical imaging ability is substantially improved. We give an example of the MIMO sonar array, and give the imaging performance analysis to validate the effectiveness of the designed array layout.