A multistation coastal acoustic tomography system with coherent operation

Abstract

A multistation coastal ocean acoustic tomography (CAT) system with coherent operation has been developed and applied to field experiments over ten times in the last 8 years. The system is designed to measure both temperature and current velocity fields in the coastal ocean. As is well known, an ocean acoustic tomography is generally based on travel time measurements. In the CAT system, the clock accuracy for travel time measurement is most important to realize coherent transmission from multistations and to measure especially current velocity. For highly accurate coherent transmission from multistations, each station is equipped with a timing module synchronized by the clock signals of global positioning system (GPS). As a result, time error is maintained less than 0.1 μs. In the sound transmission from multistations, the code division multiple access (CDMA) and spread spectrum technology are applied. The coded signals also improved remarkably the signal‐to‐noise ratio of the CAT system. With detecting not only the amplitude, but also the phase of received signals, the accuracy of travel time measurement is improved within a sampling interval of data. This multistation CAT system served to map 2‐D time‐varying tidal current structures in Tokyo Bay (Kaneko et al., 2005).