Because of the environment uncertainty and mode coupling, source location in shallow water is a very difficult problem, and the study concentrates in vertical array by matched fields and matched model. But it is not easy to be placed or moved, it is difficult for the vertical array to be used in practice. To overcome this problem, horizontal array could be used in source location, while it requires a large number of hydrophones. As s result, the need of large aperture is the main difficulty of horizontal array.

In order to solve the problem, researchers of Institute of Acoustics, Chinese Academy of Sciences conducted a series of studies. Their precious studies have indicated that mode-filtering technique based on moving line array is suitable for source localization on towed array. But, it required the knowledge of mode wave-number. They carried out practical experiments and found the waveguide parameters were usually not known and were difficult to be measured accurately. Hence, before mode filter, it was necessary to estimate the modal wave-number of the different normal modes propagating in the waveguide.  

So recently, they furthered their researches and provided a moving synthetic aperture processing with high resolution spectral analysis method to estimate the mode wave-number of acoustics in shallow water waveguide, and to overcome the problem of short aperture.

In the research, they first create large aperture synthetic array by a moving of short horizontal array, and then apply a MUSIC processing to the estimation of normal mode wave-number. They carried out simulation and sea trial and both results indicate that the problem of short aperture can be resolve by using of synthetic aperture processing, and the accuracy of estimation in normal mode wave-number could be improved by varying the array depth when it is being towed array from the source.