Studies On Beamforming And Beamspace High Resolution Bearing Estimation Techniques In Sonar Systems

Antisubmarine warfare is a crucial underpinning for future navy missions. For a long time,the problem of detecting submarines and estimating their bearings has been remaining a very active research area in underwater acoustics. Modern sonar systems generally use hydrophone arrays and advanced signal processing techniques to improve their detection and localization abilities,whereas beamforming plays an important role. As it is known,the conventional beamforming approach provides limited array gains and low interference suppression abilities,and cannot resolve multi-targets distributed within a small spatial region. All these limitations influence the whole performance available in sonar systems. Supported by the Natural Science Foundation of China,this dissertation systematically studies beamforming techniques and beamspace high resolution bearing estimation algorithms with emphasis on sonar systems. The main contributions are as follows:1. A modified approach for narrowband adaptive beam pattern optimisation is proposed by using a varying step size in the optimisation procedure. With calibrated array manifold vectors replacing their theoretical values,the performance can be further improved and better beam pattern is obtained. Lake-experiment was conducted for a real sonar system and the results showed the effectiveness and superiority of the proposed methods.2. Narrowband beamspace high resolution bearing estimation techniques are studied thoroughly. The construction procedure and performance of beamspace MUSIC are demonstrated for the weak correlated signals. Beamspace MUSIC based on MVDR beamformer outputs is proposed to remove the contribution from interferes outside the beam coverage region,so to ensure the correct estimation of target bearings inside that region. The weighted subspace fitting (WSF) algorithm is extended into beamspace to resolve strongly correlated signals. Computer simulation results show that the beamspace WSF algorithm retains the super performance of its element space counterpart when applied to the beam outputs of some practical acoustic-receiving array.3. An improved form of classical time domain broadband beamforming is proposed by combining digital delay lines and FIR filters. FIR filters with special frequency responses are designed using a modified adaptive design approach for a practical broadband low sidelobe level beamforming problem in time domain. Lake-experiment verified the correctness of this low sidelobe level beamformer.4. The broadband constant beamwidth beamforming vectors for a uniform linear array are designed and its effectiveness when applied to a real array is verified by water tank experiment. A new method of broadband constant beamwidth beamforming for arbitrary geometry arrays is proposed. Lake-experiment for an un-baffled practical circular array verified the effectiveness and error tolerance of this new method.5. The intrinsic mechanism of optimal array gains is studied for the scenario when the element spacing is less than one half-wavelength,with emphasis on un-baffled circular arrays. A broadband high array gain beamforming scheme is developed,based on the narrowband optimal array gain processing procedure. Results of lake-experiment show that the high gain beamforming approach can obtain extra 3dB under the situation of the experiment,compared to that obtained via the conventional method.6. The broadband coherent high resolution processing method based on constant beamwidth beamformers and the incoherent high resolution processing method based on subband beamformers are studied. In the broadband incoherent beamspace high resolution method,the outputs of MVDR beamformers are used to suppress the interference from outside the beam coverage region in each subband. Computer simulation results show good performance at low signal to noise ratio (SNR).