I. Introduction
The low-frequency high-power active sonar (generally, composed of a low-frequency high-power source and a towed hydrophone array) provides a useful way for underwater target detection. However, the received echoes reflected back from multiple targets used to be considered highly correlated. Under the circumstances, that kind of adaptive beamformers like MVDR could not be applied straightforwardly for direction of arrival (DOA) estimation. To solve this problem, a kind of preprocessing algorithm was proposed by Evans, which is well known as spatial smoothing technique [1]–[4]. However, there is an obvious drawback that it would result in the reduction of effective array aperture and lead to a lower angle resolution.