Department of Ocean Science, Tokyo University of Marine Science and Technology, 4-5-7 Minato, Tokyo 108-8477, Japan
We developed a 1.1 MHz split-beam echo sounder system as a prototype to measure the target strength (TS) of zooplankton and made a comprehensive evaluation of this sounder. The calibration of the sounder was performed by the standard sphere calibration method. We derived an appropriate size of the standard sphere, “2 mmφ” for the 1.1 MHz sounder. The directivity pattern of the sounder was also measured by the sphere. It was ascertained that the standard sphere calibration method is useful to calibrate such a high-frequency sounder in a water tank. The detection range of the sounder was examined to evaluate the range for a reliable TS measurements and to consider the complementary use with a multi frequency plankton measurement system (TAPS-6). In order to examine the performance of TS measurements, three tungsten carbide spheres of different sizes were used as dummy scatterers of zooplankton. The TS of the dummy scatterers at five different positions in the beam width were measured and agreed well with the theoretical TS values within 1 dB. In conclusion, we were able to verify the capability of the ultra high frequency split beam system for practical applications.