An Efficient Near-Field Parallel Subarray Beamforming for Portable 3D Imaging Sonar

In complex water environment, the portable 3D imaging sonar is one of the main techniques for flexible underwater tasks, especially in the near field. In the literature, an efficient digital beamforming called parallel subarray (PS) has been introduced for portable 3D sonar working in the far field. This paper proposes an extension of the PS beamforming algorithm that enables to process the multi-frequency signals acquired by the cross array from a scene in the near field. The proposed algorithm achieves two stage parallel beamforming and applies an optimized approximation delay for the near-field beam pattern in the two stage beamforming. Then, the architecture of the algorithm performed on the field programmable gate array (FPGA) is illustrated. The near-field beam pattern of the algorithm is similar to that of the direct method while computational requirement is the lowest compared to other existing methods. To test the validity of the algorithm, a real system was devised and employed in the lake. The results showed that the proposed algorithm could generate satisfactory 3D underwater images.