Sonar technology in entering 21 century

The requirement of ocean exploring and anti-submarine warfare is the main motivation of sonar technology development. The advances in the field of underwater acoustic physics, underwater acoustic signal processing and other related sciences and technology considerably improve the sonar design. The development opportunities and challenges, which sonar technology faced in enter 21 century are described in this paper. Some recent hot topics in underwater signal processing and technical innovation problem in sonar design are presented.