Abstract: Waveform invariance' is the basic principle of correlation sonar A ray theory based method is adopted to explain the principle Two approaches,temporal correlation and spatial correlation,to the design of correlation sonar are analyzed Criterion for designing a prototype based on spatial correlation is proposed,involving transducer geometry,signal design and software process The prototype correlation sonar has undergone two tests at the west Pacific Ocean and the South China Sea respectively Correlation function of the test data is used to confirm the' waveform invariance' By comparing the velocity results obtained by the correlation sonar with those by acoustic Doppler current profiler (ADCP) and differential GPS (DGPS) respectively,the feasibility of the correlation sonar for measuring current velocity and ship velocity,the design principles,and signal processing techniques used are verified.