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The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) operates several unmanned and manned underwater vehicles for scientific research in deep water. Recently, we have developed a new high-rate underwater acoustic communication system, or acoustic modem, for vertical uplink data transmission from these vehicles. This acoustic modem uses single-carrier modulation with a decision feedback equalizer (DFE), and it has been demonstrated that the modem's performance is very stable under the Doppler effect and much better than that of orthogonal frequency division multiplexing (OFDM). In 2019, a successor acoustic modem was developed for a lander operable to a depth of 11,000 m. In an at-sea experiment conducted at a depth of 6,180 m, an effective data rate of 98.9 kbps was achieved, using the $\mathbf{18\pm 6.25{kHz}}$ frequency band with 256 quadrature amplitude modulation (QAM). In an experiment at a depth of 9,230 m, an effective data rate of 69.24 kbps was achieved, using the $\mathbf{18\pm 5{kHz}}$ frequency band with 128QAM. Thus, it was demonstrated that the performance of this acoustic modem exceeds 600 kbps $\times$ km with the index of communication data rate $\times$ range.

The interference method is one of the most sensitive methods for measuring the refractive index of seawater. We developed a state-of-the-art density sensor for seawater measurements based on measuring the refractive index by the interference method. The resolution of the density sensor is 0.00006 kg/m3 for changing temperature at constant salinity and pressure, 0.00012 kg/m3 for changing salinity at constant temperature and pressure, and 0.00010 kg/m3 for changing pressure at constant temperature and salinity. These resolution values are the best in the history of seawater density measurements. The ultra high-resolution density sensor will contribute notably to climate research at full ocean depth and measurement of seawater sampled from the deep ocean, to research on metrology to establish the traceability of salinity measurements, and to submarine resource exploration to detect spatial changes in the absolute salinity anomaly by combining with conventional conductance-based salinity measurements.

We are developing an underwater glider for virtual mooring. It can remain in a designated area for several years, moving between the sea surface and the seafloor up to 2,100 m deep, monitoring the sea environment. It can sleep on the seafloor or while drifting in water to extend the monitoring duration. If drifting far away from a designated area, it can glide back to the target point. If it is just at the target point, then one method to maintain its position is to glide spirally. As described herein, we present the spiral gliding tests results. We also evaluate some elements that affect the turning characteristics by simulation and demonstrate that simulation results agree well with experimentally obtained results. Results confirmed that spiral gliding is useful to maintain a position. Moreover, a small misalignment of wings can affect the rotation characteristics.

Most estuaries and inland waters are significant source for atmospheric CO2 because of input of terrestrial inorganic carbon and mineralization of terrestrially supplied organic carbon. In contrast to most coastal waters, some estuaries with small freshwater discharge are weak source or sometimes sink for CO2. Extensive surveys of pCO2 in Tokyo Bay showed that the overall bay acts as a strong net sink for atmospheric CO2. Although small area was a consistent source for CO2, active photosynthesis driven by nutrient loading from the land overwhelmed the CO2 budget in the bay. Here we show a comprehensive scheme with a border where air-sea CO2 flux was ±0 between nearshore waters emitting CO2 and offshore waters absorbing CO2. The border in Tokyo Bay was extremely shifted toward the land-side. The shift is characteristic of highly urbanized coastal waters with an extensive sewage treatment system in the catchment area. Because highly urbanized coastal areas worldwide are expected to quadruple by 2050, coastal waters such as Tokyo Bay are expected to increase as well. Through extrapolation of Tokyo Bay data, CO2 emission from global estuaries would be expected to decrease roughly from the current 0.074 PgC year−1 to 0.014 PgC year−1 in 2050.

We developed 6,000 m class cylindrical pressure-tight housings using ceramic cylinders and metal caps. The housings have a simple structure, comprising ceramic cylinders, aluminum alloy caps, and couplings between the ceramic cylinders. They are considerably lighter than metal housings. We applied nonlinear finite element analysis (FEA) to evaluate the stress distribution. Results show no stress exceeding the strength of the ceramics in the ceramic cylinder. Also, the plastic deformation of caps reduced stress concentration at the ceramic cylinder edge. Pressure tests confirmed its reliability against hydraulic pressure and water sealing.

We have developed a new system for real-time observation of tsunamis and crustal deformation using a seafloor pressure sensor, an array of seafloor transponders and a Precise Point Positioning (PPP ) system on a buoy. The seafloor pressure sensor and the PPP system detect tsunamis, and the pressure sensor and the transponder array measure crustal deformation. The system is designed to be capable of detecting tsunami and vertical crustal deformation of ±8 m with a resolution of less than 5 mm. A noteworthy innovation in our system is its resistance to disturbance by strong ocean currents. Seismogenic zones near Japan lie in areas of strong currents like the Kuroshio, which reaches speeds of approximately 5.5 kt (2.8 m/s) around the Nankai Trough. Our techniques include slack mooring and new acoustic transmission methods using double pulses for sending tsunami data. The slack ratio can be specified for the environment of the deployment location. We can adjust slack ratios, rope lengths, anchor weights and buoy sizes to control the ability of the buoy system to maintain freeboard. The measured pressure data is converted to time difference of a double pulse and this simple method is effective to save battery to transmit data. The time difference of the double pulse has error due to move of the buoy and fluctuation of the seawater environment. We set a wire-end station 1,000 m beneath the buoy to minimize the error. The crustal deformation data is measured by acoustic ranging between the buoy and six transponders on the seafloor. All pressure and crustal deformation data are sent to land station in real-time using iridium communication.

For long-term monitoring, the authors are developing a prototype underwater vehicle that can sleep on the seafloor or sleep while drifting underwater. Using these functions, it can carry out long-term monitoring of ocean environments while remaining in a designated area. Recent landing-sleep and drifting-sleep experiments yielded good results. Results of these experiments reported herein demonstrate that the landing motion showed good coincidence with the simulation result.

We are now developing a prototype underwater glider for long-term virtual mooring. It can land on the seafloor and remain inactive (sleep) there for a predetermined time to extend its monitoring period. If the seafloor is too deep, then it can sleep while drifting underwater. If it drifts because of the sea current, it will glide back to the designated area and remain there for more than one year. We have conducted a series of sea tests in the initial stage and confirmed its basic functions including landing-sleep, drifting-sleep, and heading control. To extend the maximum descent depth, we are also developing ceramic pressure-tight housings. Because of the high compressive strength of ceramics, we expect to be able to extend the maximum depth of the underwater glider to 6,000 m, which covers more than 98% of the world oceans. As described herein, an outline of the prototype underwater glider and sea test results is presented. An outline of the ceramic pressure-tight housings is also presented.

We propose a simple design method of a ceramic pressure-tight housing having an extended length of cylinder and metal caps. Ceramics have higher compressive strength and lower specific gravity than typical metals. Moreover, they are free from erosion by seawater. For that reason, we can produce light pressure-tight ceramic housings that have good durability for deep-water applications. The proposed ceramic housings have greater buoyancy than metal housings. We also propose a simpler design method of metal caps. We confirmed its validity through Finite Element Method (FEM) analysis and hydraulic pressure tests using small-sized ceramic housings with metal caps.