A Sea Floor ElectroMagnetic Station (SFEMS) has been newly developed in the hope of long-term electromagnetic (EM) observations at the seafloor. As the first attepmt, it was equipped with an Overhauser magnetic sensor and deployed at the Choshi spur (35°24.85'N, 141°34.96'E, 1593 m). In future, it can be easily integrated to SFEMS, which measures 3 components of the geomagnetic field, 2 horizontal components of the geoelectric field and 2 components of tilts in addition to the total force. SFEMS is planned to measure these signals at the seafloor continuously for as long as 2 yrs. Realtime data telemetry of the in-situ geomagnetic total force is achieved by an Acoustic Telemetry Modem (ATM) attached to SFEMS. In the present experiment, the acoustic connection was tested while the apparatus was wire-suspended, and it was at the seafloor. In both experiments, the signal was successfully transferred to the surface at a rate of 300 baud. For the seafloor measurement, an averaged value of 45582.0 nT with a standard deviation of 0.10 nT was obtained by a half-an-hour measurement with a 30 sec interval. SFEMS has been originally developed for long-term seafloor EM observations in search for detecting deeper structures via seafloor magnetotellurics and/or geomagnotic secular variational signals. However, it can be applied to tectonomagnetism since it enables repetitive absolute geomagnetic measurements which have been logistically very difficult so far.
This paper presents the outline of an autonomous underwater free swimming vehicle called "R-oneRobot", which equips with an air-independent power (AIP) system of a Closed Cycle Diesel Engine (CCDE). In August 1996, sea trails of the robot were successfully conducted in the Pacific Ocean, about 30 km off Tanabe City, Wakayama. In the 9th trail, the robot did a diving about 20 km range in 4 hours passing through 6 waypoints in turn of different depth. Through the sea trials, it is confirmed that the CCDE is a suitable and reliable AIP system for long-range autonomous underwater vehicles, and the results indicate that AUVs will be not only used for the survey of the seabed but also used as a stable platform for the various and continuous observation of the properties of middle layer sea water.
We have successfully developed application technologies of GPS in the oceans. The progress of technologies in our company is based on our continuous investigations with some manufactures. First, The Real Time Kinematic (RTK) system was installed in hydrographic survey system. Second, it was then installed on each work ship. GPS was also used in the reconstruction of Hyogoken-Nanbu Earthquake for damage investigation and further reconstruction work, and proved its effectiveness. By comparing large-scale Static (STA) survey to RTK survey, which uses 70 MHz 3 W exclusive radio wave installed by the Ministry of Transportation, we gained more correct RTK accuracy data. This report explains the comprehension of these experiments, results, and further RTK application to marine construction.
The MSA (Maritime Safety Agency) is promoting the development of a DGPS (Differential Global Positioning System) service network, which will contribute to improvement of GPS positioning accuracy. Back in December 1995, the MSA established experimental stations in Tsurugizaki in Kanagawa Prefecture and Daiozaki in Mie Prefecture. The MSA's DGPS evaluates errors in the GPS at specified points on a coordinate system on which the DGPS is based. The DGPS then broadcasts information on corrections to the distances between the reference points and all the GPS satellites and the rates of change in them. The resolution of the corrected values and that of the rates of change are ±0.1 m and ±0.001 m/s respectively. The information is updated at two to five seconds intervals. This information was received at points remote from the sending stations, and GPS positioning was performed. As a result of one day experiment standard deviation of positioning was around ±1 m. This value is a little too large compared with the resolution of corrected values. In order to investigate the cause, these errors were arranged on the time base and the way the errors were produced was studied. As a result it was found that the errors were not produced at random but exhibited one-day periodicity in sidereal-time. Though the cause of the periodical errors is unknown, it is possible to estimate errors. So, the error at a time of a day was estimated based on measured values obtained at the same time of the previous day, and correction was made. As a result, the standard deviation was improved to within ±0.5 m. Thus it was found that the use of the DGPS developed by the MSA would achieve a standard deviation of around ±0.5 m for mobile positioning of navigating ships and so on, and of a root mean square of around ±1 cm on average for long-hour positioning of stationary pionts.