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1IntroductionConcerning the usage of undersea radio waves, the needs such as the radar for resource exploration under the seabed and the several Mbps undersea wireless communi-cations come to light in association with the progress of recent exploratory techniques targeted to the deep sea and ocean oor[1][2]. Figure 1 shows the radio usage example in waters to which JAMSTEC points. ere are high expec-tations for electromagnetic wave utilization for exploratory radar for seabed minerals and seabed exploration robot control with an aim of resource exploration under the seabed.To design and develop the undersea radio wave usage system based on the said usage examples, it is essential to reveal the characteristics of the radio propagation in the waters. To measure the radio propagation characteristics, it is possible to utilize an analytical method in which the channel sounder is built on land and its measuring results are taken, but the building of a new measuring system is required because many restrictions occur undersea due to eects such as hydraulic pressure. For this reason, this paper describes the development of the underseawater FiF1　Example of undersea electromagnetic wave utilization [1][2]Seafloor exploration‣Exploration radar‣Exploration robot controlEM profilerEM radar(conductivity)(permittivity)Measurement on EM field fluctuation caused by seafloor structure.Measurement on EM wave reflection caused by seafloor structure.FiF2　Appearance of underseawater channel sounderOptical fiberCameraTxantennaRx antennasA switching device for Rx antennas is inside the vessel.A vector network analyzer to measure S parameters is inside the vessel.Batteries2-8 Under Seawater Radio CommunicationsHiroshi YOSHIDA, Ryotaro SUGA, Takashi MATSUDA, Kenichi TAKIZAWA, and Fumihide KOJIMACurrently, wireless communications under seawater has been one of attractive research fields of radio frequency (RF) communications. NICT and JAMSTEC have started a joint research on this research field, and have developed a under-seawater channel sounder for conducting measurement on radio propagation characteristics available in the depth of 500 meters. This report shows an overview of the channel sounder and a couple of measurement results.532 Terrestrial Communication Technology Research and Development