The data available on this web site is 1-sec mean CTD data integrated with "URASHIMA" (hereafter, the submersible vehicle) positioning data in latitude and longitude. The SSBL (Super Short Base Line) method is used to measure the submersible vehicle's position, which requires transponder mounted on the submersible vehicle and an array of transducers equipped on the bottom of the mother ship. The position is measured by both phase lag measured from angles of received sound waves and distance calculated from travelling period of them. Because the baseline length (i.e., a distance between transducers and the transponder) is short, a horizontal error is about 2.5% of slant range (i.e., a distance between the submersible vehicle and the mother ship). The SSBL method has a characteristic that it is a little less accurate but easier to operate than the LBL (Long Base Line) method because it doesn't need to deploy sea-bed mounted transponder(s). Vertical profile of sound velocity is needed to calculate accurate distance from the travelling period. Therefore, the temperature measurement using XBT etc. of each sea area is executed.
The submersible vehicle positioning data was calculated by adding the relative distance to the mother ship's position. The simplified equation with the area-dependent coefficients every 30 degrees in latitude and longitude was applied to the distance (XY) to Lon/Lat conversion, which provided by Japan Coast Guard. Here, the original time interval of position data is more than 10 seconds. The noises remaining in the position data are manually eliminated and linearly interpolated when the speed calculated from adjacent two position data is greater than 4 knot which is the maximum operation speed of the submersible vehicle. Moreover, noises remained in the depth, temperature, and salinity data are visually checked and replaced to missing values only when the data seemed to be obviously abnormal.
The CTD system was installed as one of the navigation equipment to monitor the ambient environmental conditions of the submersible vehicle, of which sensor calibration is normally executed about every two years.
After considering the accuracy of the sensors, the significant digit of data was changed as in the following list.
||On this web site