基于超指向性多极子矢量阵的水下低频声源方位估计方法研究

针对水下低频声源的方位估计问题,基于基元紧密排列的二维矢量阵,建立了一种超指向性波束形成方法.根据矢量基元差分运算构建各阶多极子模型,获得了几乎与频率无关的模态函数,并经加权计算可在低频条件下实现超指向性波束,以解决阵列孔径对波束性能的限制.同时,结合输出信噪比最大准则所得波束,分析了超指向性波束形成算法的稳定性与波导的影响程度,探索模态阶数与阵列参数的选取原则.通过阵列性能的仿真计算与实际阵列的测量数据表明,该算法可在小尺寸阵列孔径下获得良好的阵列波束,兼具了水下线型超指向性阵和环形超指向性阵的优点,可有效实现水下低频声源的水平方位估计;且波束性能可通过调节模态阶数与基元间距以达到最佳,并受水下声波导多途与频散效应影响有限.

Direction-of-arrival estimation based on superdirective multi-pole vector sensor array for low-frequency underwater sound sources

With the advances of ship noise reduction technology, the working frequency of the passive sonar must be reduced in order to detect a target. For the conventional array, it requires a large array aperture, comparable to the wavelength, in order to achieve an acceptable angular resolution. Arrays of small physical size with high angular resolution are thus attractive for low-frequency direction-of-arrival estimation of underwater sound source. In this paper, we consider a 3×3 uniform rectangular array which consists of vector sensors with inter-sensor spacing much smaller than the wavelength. A broadband super-directive beamforming method is proposed for this vector sensor array, which extracts multi-pole modes of different orders from the spatial differentials of the sound field. By normalizing the amplitudes of the multi-pole modes, frequency invariant mode functions can be obtained, which are used to build the desired beam pattern, despite the Rayleigh limit on the achievable angular resolution. Vector sensors are used to replace the pressure difference operation, thus to achieve a desirable beam pattern, the order of spatial differential will be reduced. In other words, for the same array configuration, using the vector sensors provides higher directivity than using the pressure sensor. To concentrate on the sources, and to minimize all hindrances from around circumference, a suitable beam pattern is constructed as an example to analysis. To verify the algorithm, a prototype is built and tested in a water tank. Comparisons are carried out between the actually synthesized beam patterns and the theoretical ones. The experimental results show good agreement with the theoretical results, and that the directivity increases with the multi-pole mode order increasing, at the expense of lower robustness. The performances for different values of ka are also investigated, where k is the wave number and a denotes the inter-sensor spacing. Simulation results show that when the inter-sensor spacing is no more than one-sixth of the incident wave length, the error introduced by the approximations for muti-pole mode extraction can be neglected. It should be noted that this result of the inter-sensor spacing still applicable when considering array gain, showing that the array is insensitive to uncorrelated noise while preserving a relatively high array gain. Finally, the influence of the underwater acoustic waveguide on the array performance is analyzed. Simulations and experimental tests show that due to the small array aperture, the waveguide effects on the array performance are limited.