Detection of Slow Targets by a Moving Active Sonar

In this paper detection of slowly moving targets in the presence of reverberations, i.e. targets whose Doppler response falls into the reverbs Doppler band is of primary interest. While stationary targets are characterized by an equivalence between angular direction and Doppler frequency these parameters are independent of each other for moving targets. Therefore, target echoes and reverbs can be characterised by a direction-Doppler (φ-ω) power spectrum, where stationary objects appear on the diagonal while moving objects may appear anywhere in the φ-ω-plane. Therefore, moving targets can be discriminated by use of two-dimensional φ-ω-filters. Such filters operate on space-time data fields as obtained from a sensor array. It is shown that these filters compensate for the platform motion so that no degradation in target detection due to platform motion will occur. A linear array with its axis in motion direction (e.g. a towed array) is best suited for this kind of adaptive motion compensated reverbs suppression technique. Some considerations related to sound propagation effects on the performance of such filters are given.