提出了一种迭代算法，在FIR数字滤波器的自适应设计过程中自动改变各个频率指标上代价因子的值，最终实现代价因子的最优取值。利用这种改进方法，可以实现FIR数字滤波器和设计指标之间的最佳拟合。给出了两个设计实例，其中幅度响应每倍频程衰减6dB的滤波器，可以用于模拟水声阵列接收的宽带海洋噪声，而高精度时延的滤波器，则可以实现波束形成中所需要的非整数倍采样间隔的时延。

In this paper, a new method is presented to design low-sidelobe wideband constant beamwidth beamformers (WCBB) in time domain, which alleviates the effect caused by array mismatch errors by applying the measured array models into the design procedure. A combination of FIR filters and digital delay lines is used in the beamformer structure to reduce the order of FIR filters. In FIR filter design, the semi-definite programming is utilized which is able to constrain both the mainlobe shapes and the sidelobe levels within the whole design frequency band. Results from lake experiments with a practical sonar system verified the effectiveness of the proposed method.

A wide-band beamforming scheme for uniform circular array is proposed to improve the detection performance for weak passive signals in this paper. In our processing scheme, the wide-band signals sampled from the underwater acoustic array are firstly decomposed into different narrow-band signals using FFT, and then the narrow-band high gain beamforming process is performed for each subband of the FFT outputs. The subband beamformer outputs are squared nd summed using the Eckart weighting coefficients. The length of FFT is selected to make every subband satisfy the condition of narrow-band assumption. The fifty percent of overlapping ratio and-30 dB sidelobe level chebyshev window are used for FFT processing to obtain better system performance. This approach was employed to process some wide-band experimental data recorded from a 24-element circular array at a lake in China. Compared to the conventional beamforming, enhanced detection performance is obtained without improving the complexity of realization.

To overcome the azimuth sampling constraints imposed in synthetic aperture sonar (SAS), many SAS systems employ a single transmitter and multiple-receiver configuration, which can be regarded as a system containing many bistatic transmit-receive pairs. In traditional reconstruction, the multiple receiver system data is often preprocessed into an equivalent single receiver (monostatic) data by using the phase center approximation (PCA) and along track interpolation, and then followed by standard image reconstruction algorithms. But a blurring will occur at the edge of the target area as the length of the receiver array increasing. In this paper, a novel approach is proposed to overcome the blurring problem caused by PCA based on the bistatic model. In this method, an image is firstly reconstructed with the data of each hydrophone independently, and then all the images are coherently added to give the high quality imagery. Simulation results show the feasibility and availability by applying this algorithm.

The problem of beamforming and related beamspace high resolution direction-of-arrival (DOA) estimation is studied in this paper. All beamspace processing methods are based on the beam outputs and the beampattern design plays an important role in providing high quality beam output data for further processing. Three typical situations which are frequently encountered in practical sonar system working environment and the most widely studied MUSIC algorithm are considered herein. First, when isotropic noise is the dominant noise at sensors, conventional beamforming techniques provide the optimum performance in the sense that DOA estimate is the ML estimate. Good DOA estimates are obtainable by applying MUSIC to the beam outputs directly. Then, uncorrelated interferes with much higher strength than the wanted signals are assumed to be present in the sidelobe region, and low sidelobe Dolph-Chebyshev and adaptive MVDR beampatterns are designed to guarantee the performance of MUSIC. And finally, the robustness of conventional techniques is combined with the adaptivity of MVDR beamforming to deal with the situation when the interfere in the sidelobe region is strongly correlated with one of the wanted sources. Performance in all three situations is studied with numerical examples.