広帯域音源を用いた海中での高速ディジタルデータ伝送に関する研究

摘要

To advance basic and applied research in the deep sea where human access isdifficult, studies were made on reliable techniques for underwater transmission ofdigital data by means of wideband ultrasound transducers. Several transducers withmultiple resonance frequencies in the wideband were developed to transmit acousticdata at high speeds. In particular, a tilted toroidal-beam wideband transducer wasdesigned on the basis of the free-flooded ring method. This transducer has a 40 kHzbandwidth that corresponds to 50% of its central frequency. Linear and nonlinearadaptive equalizers were designed to reduce multipath effects, and these were used toprocess acoustic waves propagated through the sea. The transducer and adaptiveequalizers were successfully incorporated in a transmitter-and-receiver system. Finally,experiments carried out at 20 kHz with a 16-quadrature amplitude modulation schemeshowed that it is possible to transmit data from a vehicle and from moored sensors in thedeep sea to a surface ship at a rate of 32 kbps. Almost identical transmissionexperiments were performed over a range of about 500 m using a quadrature phase-shiftkeying modulation scheme with a transmission rate of 80 kbps and a transducer ofcentral frequency 80 kHz to control a remotely operated unmanned vehicle. Theseshort-range transmission experiments showed that error-free communication is possibleover a range of 570 m in seas approximately 1,000 m deep. Furthermore, a practicalacoustic data transmission system was developed and fitted to the autonomousunderwater vehicle Urashima; this system permits color images to be transmitted to amother vessel at a rate of 32 kbps over a range of more than 4,000 m.