Studies On Wideband Beamforming For Moving Targets' Radiated Noise

Moving targets' radiated noises often cover a wide bandwidth and how to acquire them distortionlessly has attracted many researches in the fields of SONAR, aeroacoustic measurement, and intelligent communication. It can provide more information for post processing techniques, such as target detection, classification and localization and thus is crucial for their performance improvements. Supported by the fund of the Key Laboratory of Military Targets' Acoustic Characteristics, this dissertation systematically and deeply studied on the wideband array beamforming techniques to accurately acquire moving targets' radiated noises. The main contributions are as follows:1. The model of signal received by the array for static sources is extended to the situation in presence of source motion and is further analyzed in acoustic field aspect. A method is then developed to precisely simulate the signal received by the array. Beamformer with constant beamwidth is theoretically demonstrated that it can distortionlessly estimate the moving targets' radiated noise. Besides, it must have great capability of interference suppression to guarantee its performance.2. Two methods to design time domain wideband constant beamwidth beamformer (WCBB) with low sidelobe level are proposed, followed by two design examples to verify their correctness and effectiveness respectively. Through two simulation examples, the drawback of the classical sub-band decomposition based WCBB design method is pointed out. Lake-experiment is used to show the array mismatch errors in practical sonar systems will degrade the performance of WCBB. With measured array manifold, the performance can be improved.3. A time domain optimization method is derived to design time domain WCBB. Different from the sub-band decomposition based method, the derived one can take design results of the whole frequency band into consideration. A two dimensional cost function is introduced. By adjusting the cost function values, the beamformer will satisfy the design requests better at certain frequencies and directions. An iterative algorithm is further proposed to get the optimum cost functions. Via computer simulations, the effectiveness of the presented method is illustrated. 4. A robust adaptive beamforming method with constant beamwidth is presented. It is realized by linear constrained minimum variance adaptive beamforming method. A set of linear constrains are designed to assure the adaptive beampattern to be distortionless and the beamwidth constant over a certain frequency band. The low rank representation of the constraint matrix computed by the singular value decomposition is used to reduce the number of linear constraints. Simulation results show that the proposed method achieves greater interference reduction than data independent methods do, which ensures accurate estimation of the wideband signal.5. A robust generalized sidelobe canceller (GSC) employing frequency invariant beamformer (FIB) is proposed. In this method, the blocking matrix is replaced by a series of FIBs with nulls to block the signal. In order to block the signal reliably, two methods to design broadened nulls for FIBs are proposed. Simulation results show that the proposed method can effectively block the signal and reject interferences. Besides, the signal wave can be exhibited in real time through the proposed method.6. In order to conveniently process signal covering several octave bandwidths, the nested array is introduced and its multirate beamforming method is proposed. The effectiveness of the proposed method is illustrated by numerical examples.7. A tank-experiment and a reservoir-experiment are carried out respectively to verify the performance of the proposed methods in practical situations. Results of the tank experiment show that both WCBB and FIB based GSC can effectively reject interferences and accurately acquire moving targets' radiated noise, the capability of interferences suppression of the former relies on sidelobe level, while the capability of the latter relies on FIB's capability of blocking the signal. From the reservoir experiment, we illustrate frequency characteristics of the moving targets' radiated noises acquired by time domain WCBB gets better than that by conventional delay-and-sum beamformer.