High-resolution 2D imaging using MIMO sonar with fourier integral method (FIM)

The angle resolution of an imaging sonar is generally determined by the obtainable aperture of the receiving hydrophone array. When mounted onto a small underwater platform (such as an underwater unmanned vehicle, UUV and an underwater robot), the aperture of the imaging sonar is strictly constrained by the limited surface space of the platform. To improve the angle resolution of the imaging sonar without requiring a larger-size underwater platform, we propose a high resolution 2-D imaging method using multiple-input multiple-output (MIMO) sonar with Fourier integral method (FIM). This method is termed as MIMO-FIM for short in this paper. MIMO-FIM shares the same sonar array structure and the same orthogonal transmitting waveforms as the MIMO sonar. Different from the MIMO sonar imaging method where the outputs of matched filters are directly collected for multibeam processing, MIMO-FIM uses FIM to process these matched filters' outputs to extend the aperture of the virtual array. And then, the FIM outputs are imported to beamformers to produce the high resolution 2-D image. Thus, MIMO-FIM has a two-step aperture extending effect. Firstly, the large-aperture virtual array is produced by the MIMO method where the matched filters corresponding to different orthogonal waveforms are used; and secondly an even larger aperture is obtained by FIM where the matched filters outputs are taken as the inputs. Furthermore, we use the time-domain slice window to process outputs of matched filters when using FIM. And the window width and the overlapped ratio of neighboring ones are given. Numerical simulations demonstrate that the proposed MIMO-FIM method shows a better angle resolution than the MIMO sonar and single-input multiple-output (SIMO) sonar, which share the same array physical size and the same working frequency band.