Tao Hailiang, Krolik Jeffrey L
Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA.
J Acoust Soc Am. 2008 Mar;123(3):1338-46. doi: 10.1121/1.2832335.
The performance of broadband sonar array processing can degrade significantly in shallow-water environments when interference becomes angularly spread due to multipath propagation. Particularly for towed line arrays near endfire, elevation angle spreading of multipath interference often results in masking of weaker sources of interest. While adaptive beamforming in a series of narrow frequency bands can suppress coherent multipath interference, this approach requires long observation times to estimate the required narrowband covariance matrices. To form wideband covariance matrices which can be estimated with less observation time, plane-wave focusing methods have been used to avoid interference covariance matrix rank inflation. This paper extends wideband focusing to the case of coherent multipath interference. The approach presented here, called waveguide invariant focusing (WIF), exploits a robust relationship for the frequency dependence of horizontal wave number differences. Unlike matched-field methods, WIF does not model multipath wave fronts but rather makes the interference appear to occupy the same rank-one subspace across frequency. This permits formation of wideband covariance matrices without interference rank inflation. Simulation experiments in a realistic ocean environment indicate that adaptive beamforming using WIF covariance matrices can provide a significant array gain improvement over conventional adaptive methods with limited observation time.
在浅水环境中，当干扰由于多径传播而在角度上扩散时，宽带声纳阵列处理的性能会显著下降。特别是对于靠近端射的拖曳线列阵，多径干扰的仰角扩展常常导致较弱的感兴趣源被掩盖。虽然在一系列窄频带中进行自适应波束形成可以抑制相干多径干扰，但这种方法需要较长的观测时间来估计所需的窄带协方差矩阵。为了形成可以用较少观测时间估计的宽带协方差矩阵，平面波聚焦方法已被用于避免干扰协方差矩阵秩的膨胀。本文将宽带聚焦扩展到相干多径干扰的情况。这里提出的方法，称为波导不变聚焦（WIF），利用了水平波数差频率依赖性的稳健关系。与匹配场方法不同，WIF不对多径波前进行建模，而是使干扰在整个频率上看起来占据相同的秩一子空间。这允许形成宽带协方差矩阵而不会出现干扰秩膨胀。在实际海洋环境中的仿真实验表明，使用WIF协方差矩阵的自适应波束形成在有限观测时间下比传统自适应方法能显著提高阵列增益。
