Performance of a new shipboard three-component magnetometer system utilizing a GPS (Global Positioning System) attitude sensor was compared with that of a conventional system using a gyrocompass for ship's heading and a pitch/roll sensor constituted by accelerometer and fibre-optics gyro. Response of the GPS attitude sensor is quick enough for shipboard magnetic measurement. During "figure-8 turns" for measuring ship's magnetization, difference in heading between the two systems varied about 1° systematically with heading. Differences in pitch and roll changed at about 1° depending on the turning directions. This is most likely caused by error in the gyros due to acceleration during the turns. Previously it was known that significant bias often occurred in each component of magnetic vector anomalies after the correction of ship's magnetization. The significant error in attitude determination during "figure-8 turns" as well as viscous remnant magnetization are responsible for the bias. Significant error of about 0.5° at maximum remains in gyrocompass for 30 to 60 minutes after a turn, and this causes strange fluctuation in Y (east) component of magnetic vector anomalies at the beginning of each survey line. GPS attitude sensor can overcome these problems, and increase the accuracy of shipboard magnetic vector measurement.
Tokuda and Nagamatsu (1999) proposed an automatic extraction method of surface current information from high-frequency radar echoes. Accuracy of this method was proved to be stable without the aid of an expert. We used the method, called "regression line method", to analyze Doppler spectra contaminated with high levels of external noise, and found it was unable to extract the surface current. Because power of the noise exceeded that of the so-called "first-order" echo which contains informations of the surface current. From this reason, we have developed a new algorithm on the basis of the Bragg resonant mechanism that can extract the first-order echo from the spectra contaminated above. The algolism was verified by the following facts. Surface currents extracted by this algorithm were in good agreement with those measured by a current meter, which were measured at the same point simultaneously. The eddy pattern appeared on the surface current map showed the remarkable similarity to the one appeared on the image data of MESSER of MOS-I, though each observation was carried out at different times. Consequently we can conclude that the regression line method used in combination with this algorithm can be a more reliable method for automatically extracting surface currents.
The development of technology in electronics, communications and measurements with digital equipment have spawned a lot of new measuring techniques in the fields of researches and thus the conventional research techniques will be replaced by the new ones. Concerning the fields of seafloor topography survey, narrow-multibeam echosounders designed for shallow water have been used in Japan since 5 years ago. This equipment enabled us to obtain bathymetric information in large features at one time more precisely compared with the conventional equipment which obtains seafloor topography as a collection of depths data measured at numerous points. In addition, GPS enabled us to obtain real-time information of its position anywhere on the globe with great precision in three dimensions. Consequently more discoveries in the fields of marine science will be made from the information of the more detailed seafloor topography. This paper will describe some ways to improve the narrow multibeam echosounder system designed for shallow water (SEABAT9001S, Reson) which we, Kokusai Kogyo Co., Ltd., have been working on, and its future prospects.
A new quality-controlled bathymetry mesh dataset (mesh size=500m) around Japan was created based on enormous sounding data. The new dataset includes recent multi-beam survey results and old survey results were also edited and compiled together. The dataset is now provided for academic users from JODC (Japan Oceanographic Data Center). In order to create and to edit mesh data efficiently, we developed a visual edit program. The program helps us to delineate erroneous data and to make adequate interpolation, which was very difficult in previous processing software.