基于多极子展开的超指向性矢量传感器阵列研究

The detection of weak low-freqnecy broadband signals in the ocean is a challenging problem. A sensor array can be applied to enhance the energy the signals. For the low-frequency broadband signals, however, if using a conventional sensor array, 1) it requires a very large array aperture and 2) the array gain varies significantly with the frequency. This is because conventionally the intersensor space is set to be a fraction of the wavelength of the signals. Using the superdirectivity of the moltipoles,one can design a beamformer, which enables obtaining frequency-invariant array gain with limited array aperture. In this research a beamforming algorithm based on multipole expansions will also be proposed, providing the optimum beamformer in the sense of maximizing the array gain. Since a superdirective array is more sensivtive to the array errors, array calibration is the second key problem of the current research. In this context, we will focus on the socalled multiple scattering problem by the array elements which mainly acounts for the errors with the array. Different from the exsiting methods, most of which are overcomplex, we seek for an approximate solution in closed form. This study will also covers the errors due to misalignment of the array elements and that the sensors are not identical for fast array calibration. The effects of the ambient noise and the self-noise of the sensor will be studied as well. To verify the efficency of these algorithms, a 3-by-3 superdirective vector sensor array will be developed and tested at lake. Based on the expriments, the algorithms can be furhter improved.

检测水下低频宽带弱信号从工程的角度来讲仍存在许多困难。利用阵列可以增强信号。但对于低频宽带信号而言，应用传统的阵列所面临的问题有：1）阵列孔径过大；2）阵列增益随信号频率显著变化。一个主要原因是传统的阵列依据信号的波长来设定阵元间距。本项目拟在小孔径矢量传感器阵列的基础上，利用多极子的超指向性提出一个波束成形算法，使信号在阵列孔径小于其波长的条件下获得一定的阵列增益且该增益与频率无关。本项目还将研究阵列误差以及海洋背景噪声和传感器自噪声对超指向性阵列的影响。鉴于阵元之间的互散射问题是造成阵列误差的主要因素之一且现有的求解方法大多过于复杂，设法简化该问题并以解析形式给出其近似解是研究的关键之一。本项目还拟就阵列校准问题提出一个快速算法。以上波束成形算法的性能将通过湖上实验来验证。为此，本项目将试制一套3×3的超指向性矢量传感器阵列，并根据实验结果优化完善算法。

利用传统的阵列处理技术检测水下低频弱目标在实际应用中存在许多困难。本项目研究了低频矢量传感器小孔径差分阵列的设计方法，针对3×3矢量传感器小孔径均匀方阵提出了一个基于多极子展开的超指向性波束形成算法，抑制海洋背景噪声，从而获得一定的阵列增益。研究了阵列校准方法。与哈尔滨工程大学合作试制了一套具有超指向性的3×3矢量传感器小孔径均匀方阵。利用该系统在完成了湖上实验，验证了波束形成算法的性能。还研究了传感器自噪声对超指向性矢量传感器阵列性能的影响，给出了设定其指向性指数上界的依据。鉴于实验室内难以获得阵列校准所需要的低频自由场环境，而外场作业的代价过高，为避免阵列校准，本项目还提出了一个基于概率约束的稳健的自适应波束形成算法，可以使小孔径阵列获得一定的超指向性。本项目的研究成果可以为设计低频超指向性小孔径阵列提供一定的参考。另外，本项目还分析了海洋背景噪声的动力学特性，研究了利用舰船辐射噪声的动力学特性从海洋背景噪声中提取弱目标的可行性，为水下弱目标检测提供了新思路。