Yasutaka Amitani

Experiments for Acoustic Digital Data Communication Using Frequency Shift Keying Modulation

In recent years, there has been a growing demand for acoustic communication equipment for collecting data from observation devices installed underwater and controlling remotely operated vehicles. With the objective of collecting data from these devices, we developed a packet transmission system based on frequency shift keying (FSK) modulation. This equipment is a half-duplex acoustic communication device with a frequency of 20 kHz±1.25 kHz, transmission rate of 1,250 bps and a maximum acoustic power of 20 W. Using this equipment, a trial was carried out in the sea at the depth of 3,500 m in October 1998. At a transmission distance of approximately 4,000 m, it successfully transmitted error-free 15-kbytes data.

Sea Trial Results of a Cross Fan Beam Type Sub-Bottom Profiler

Based on a joint development with SEABEAM Instruments of the U.S., we were the first in the world to develop a cross fan beam type high-resolution narrow beam sub-bottom profiler(SBP) that consisted of a transducer array fitted to a ship bottom. After completion, the resolving power was investigated by targeting a sunken ship, and as a result, the horizontal resolution was estimated to be approximately 3.5 degrees, while the vertical resolution was estimated to be about 2 m. Furthermore, comparison with the parametric high-resolution SBP revealed that the performance of the cross fan beam type SBP was by no means inferior.

Development of an oceanographic observation buoy for rough and icy sea conditions

Performing oceanographic measurements from the sea surface to the sea bottom is technically challenging under rough or icy sea conditions. As a measure to deal with rough seas, a new dynamic simulation program was developed and its validity was verified by comparison with the results of actual sea tests. This program can therefore be used as a design tool. To develop countermeasures against icing, we carried out basic tests in a large-scale snow and ice laboratory. Some of these countermeasures were then implemented in real sea tests and were found to be effective under icy conditions. These results indicate that deployment of moored surface buoys in regions of rough and icy sea conditions will soon be practical.

Deployment of an ice buoy at 60 °S in the Southern Ocean

Performing oceanographic measurements from the sea surface to the sea bottom is technically challenging under rough or icy sea conditions. In 2011, we tested measures for protecting a buoy under such severe conditions, by incorporating anti-icing and tolerance to high winds and rough seas. Based on the results, we carried out detailed designs of a suitable buoy system. In January 2012, we succeeded in deploying the buoy in the Southern Ocean off the Adelie Coast at 60°S and 140°E. Some of the meteorological and oceanographic data being gathered by the buoy can now be monitored in real time. In this paper, we describe some details concerning the design of the sensor pole and mooring line, in addition to the data already received from the buoy.

Basic Study of Synthetic Aperture Sonar and Experimental Results at Acoustic Tank

Synthetic Aperture Sonar has a great advantage of high azimuth resolution compared with legacy side scan sonar. Calculating theoretical resolution of Synthetic Aperture Sonar, it needs approximations and limitations to express that as elementary mathematics. In this study, we discuss an influence on resolution by these mathematical approximations and limitations. We had made a trial Synthetic Aperture Sonar to study basic characters both of theory and experiment. As a result of our experiment at acoustic tank, it is turn out that resolutions on experiment, simulation and theory are same. And we discuss about conditions to achieve theoretical resolutions at sea area.

Fundamental experiments on motion analysis of deep-water risers for OD21 drilling vessel

The OD21 scientific drilling vessel of JAMSTEC, which is scheduled to be internationally operated from 2006, is planned to equip the 2,500 meters long riser string in the initial stage, and the 4000 meters class in the future. The operations of such deep-water riser strings as more than several thousands meters are still far from enough experiences, and the dynamic motions of the riser suspended from the surface vessel are not well examined. The authors carried out the 1/100 scale model experiments with using the visualization method to analyze the dynamic response of risers, and they obtained basic, but such important characteristics against the actual designs of deep-water risers as the longitudinal dynamic elastic response, the lateral motion induced by the parametric oscillation, and so on.

Development of the acoustic packet data relay communication system and the results of its sea trial

In order to recover the data obtained by various sensors deployed in the ocean, a study was made concerning the acoustic digital data communication system [Ochi et al., Proc. Meeting Marine Acoust. Soc. Jpn., pp. 85–86 (1996) (in Japanese)]. The data communication system was constructed by up to 99 of the same type of equipment in the area, and its specifications are: a half‐duplex communication, FSK modulation, a 2500bps data rate, HDLC (high‐level data link control procedure) protocol packet transmission, CRC (cyclic redundancy check) code, and frame retransmission for error detection/correction. The sea trial of this system was done at about a 4000‐m depth area. Three sets of equipment were deployed in the ocean, and one of them hangs from the ship. Then, relay data transmission, among the three sets of equipment, was carried out. The detail of that sea trial will be shown at this presentation.

Investigation of sound absorption in the deep sea using a submergence vehicle

For the design of underwater acoustic equipment, it is very important to estimate the sound absorption, especially since the longer the propagation range, the greater the effect of sound absorption. Generally, the equations of Thorp, Schlkin, and Marsh and Francois and Garrison are often used for the determination of sound absorption, but these equations yield values that are different from each other. So, the signal level from the data transmitter (center frequency: 20 kHz) of the deep submergence vehicle ‘‘Shinkai 6500’’ were measured in detail on the mother ship. The dives for this investigation were carried out at a maximum depth of 6500 m in the northwest Pacific area. It is found that Francois and Garrison’s equation was comparatively suitable for the measured data.

Change of sound ray by various equations of the sound velocity in the 621‐km propagation experiment

Generally, sound velocity used to be calculated according to equations by Wilson (1960), DelGrosso (1974), Chen and Millero (1977), and others, which were obtained by direct measurement in laboratories based on water depth, temperature, and salinity data collected by the XBT (expendable Bathey‐tharmograph) or CTD (conductivity temperature depth) system. However, in observations such as ocean acoustic tomography (OAT) where it is necessary to make accurate measurement of the sound wave propagation time over the distance of hundreds to thousands of kilometers, even the slightest difference of the sound velocity can produce significant errors in the observatory accuracy. Thus the effect of various equations of the sound velocity on the formation of the specific sound ray in the SOFAR channel was examined by using the simulation method by the sound ray theory. The simulation results were compared with the specific sound ray data obtained in the 621‐km propagation experiment in the northeast Pacific Ocean, and...

Digital acoustic video image transmission system for deep‐sea research submergence vehicle ‘‘SHINKAI 6500.’’

High‐speed video image transmission from a research submersible to the mother ship is desirable to facilitate effective deep‐sea research by a submersible. Therefore, a digital acoustic image transmission system was developed, and was installed in the manned deep‐sea research submergence vehicle ‘‘SHINKAI 6500’’ of the Japan Marine Science and Technology Center. This system which adopts 4‐DPSK modulation with a carrier frequency of 20 kHz, has a communication capability of 16 kbit/s and is able to transmit a color‐still image of 256×240 pixels in 8 s. Its communication range is more than 6800 m. In order to transfer a high‐quality color image in a short time, (a) a wide band transducer, that doubles the transmission band, (b) a DCT‐ (discrete cosine transform) based image coding technique that reduces transmission data volume, and (c) an adaptive equalizer and phase controller, that improve communication quality and Doppler compensation, are employed. Results of a performance test (various video images) w...