A major problem in moving platform active sonar systems is the detection of targets in reverberation. Reverberation is caused mainly by the multiple reflections, diffusions, or diffractions of the transmitted signal by the surface and bottom interfaces. When the target is close to one interface, the target echo is hidden in the reverberation resulting in a low signal-to-reverberation ratio (SRR). Moreover, since the reverberation is strongly correlated with the signal, classical detection methods like matched filtering (MF) are inefficient. In order to improve detection, the researchers use a model of reverberation, but a correct model is difficult to find because reverberation contains both diffuse components (which look like noise) and also more discrete components (which look like signal).

In order to solve the problem better, Li We and Zhang Qinyu of Harbin Institute of Technology in collaboration with Ma Xiaochuan and Hou Chaohuan and Institute of Acoustics, Chinese Academy of Sciences carried out a series of studies and developed a new algorithm called Signal Subspace Extraction (SSE) for detecting and estimating target echoes in reverberation.

The new algorithm can be taken as an extension of the Principal Component Inverse (PCI) and maintains the benefit of PCI algorithm and moreover shows better performance due to a more reasonable reverberation model. In the SSE approach, a best low-rank estimate of a target echo is extracted by decomposing the returns into short duration subintervals and by invoking the Eckart-Young theorem twice. It was assumed that continuous wave (CW) is less efficiency in lower Doppler than broadband waveforms in spectrum methods; however, the subspace methods show good performance in detection whatever the respective Doppler is. Hence, the signal emitted by active sonar is CW in the new algorithm which performs well in detection and estimation even when low Doppler is low. Further, a block forward matrix is proposed to extend the algorithm to the sensor array problem. The comparison among the block forward matrix, the conventional matrix, and the three-mode array is researched. Echo separation is also provided by the new algorithm. Examples show the availability of the algorithm.