Multi-rate adaptive beamforming (MRABF)

An important trend in underwater acoustics is the use of large arrays with many elements and small resolution cells in an effort to detect weak signals. Frequently, the background noise is dominated by multiple moving surface ships that act like discrete interferers. These interferers may move through many spatial resolution cells in the time required to obtain sufficient "snapshots" for covariance matrix estimation. The non-stationarity of the background limits the performance of adaptive algorithms. Reduced degree-of-freedom approaches that can track and rapidly null interferers before they move too far are of great interest. The paper presents a new class of algorithms in which a low-rank, dynamically changing interference sub-space is estimated and tracked using a short-term average. An orthogonal projection is then applied to the data to null components in the interference sub-space. A second stage of adaptive beamforming that uses a longer averaging time is then applied to the projected data. Unlike most sub-space approaches that work for strong signals, there is no requirement that the signal be contained in the interference sub-space. It is shown how multiple interferers moving rapidly across beams can be tracked and nulled to reveal the presence of a weak target not visible with conventional, adaptive, or usual sub-space methods. The effectiveness of multi-rate adaptive beamforming is illustrated by examples. It is shown that moving interferers are effectively nulled to allow detection of weak signals when other methods including MVDR, conventional, and signal subspace fail.