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sounder to elongate the distance between the sending an-tenna and the receiving antenna to more than 2 m.Figure 5 shows a part of the measuring result. It is the measuring result of comparing the relative power level, which denes the received signal power as 0 dB at a 0.1 m distance, to the distance between the sending antenna and the receiving antenna. e measurements are the cases where the depths of the measuring antenna are 1 m (blue solid line) and 1.9 m (red solid line).From the measuring result, we conrmed that the measured power level decayed in accordance with theo-retical values (green dashed line) of the power attenuation in fresh water up to a 2 m distance. e gure tells that a oor is made at a distance of 2 m or more. To consider the causes, we evaluated the delay characteristic of the signal strength from measured signal waveforms for each depth. As the result, we found that for the synchronization point — the timing for which the correlation value between the sending waveform and the receiving wave form peaks — the delay of one sampling was created if the distance is 3 m or more at a depth of 1 m. For a depth of 1.9 m, the said synchronizing delay is not created for the distance between transceivers. It is believed that the oor creation at a depth of 1 m is due to the slow-wave activity from the water surface direction because of the eect of the slow wave activity.In addition, we implemented the test for the direction of arrival (DoA) estimation underwater at the same time. Consequently, Fig. 6 shows the direction of arrival estima-tion by the MUSIC algorithm. As 10 MHz is used as a frequency, the wavelength for 0.6 m distance between re-ceiving antennas is around 3.3 m (underwater), and for normal signal processing (creation of a covariance matrix from measured values), a sharp MUSIC spectrum is not created due to high correlation between antennas (red dashed line). In contrast, we conrmed that applying the space smoothing method — the processing that assigns the single dimension to the smoothing processing — provided the sharp spectrum (blue solid line) as shown in the gure.3Future prospectsAs the underseawater channel sounder developed this time can measure up to a depth of 500 m, we will measure the radio propagation characteristics in various environ-ments (water depth, depth, ocean area, etc.) and clarify the undersea radio propagation characteristics.ReferenceR1H. Yoshida, et al, “EM Field Telemetry and a Robot for Under-Sea-Floor Explorations,” Proceedings of the SEGJ Conference 133, pp.159–161, Sept. 2015. (in Japanese)2H. Yoshida, et al, “Applications of Underwater Electromagnetic Waves,” IEICE Society Conference, Sept. 2015.3H. Yoshida, et al, “Estimation of Radio Propagation Characteristics Under Seawater through Underwater Channel Sounding,” Advanced Marine Science and Technology Society Conference, May 2015.4N. Iwakiri, et al, “Underwater Channel Sounding Using PSK Modulated Signals,” IEICE Society Conference, Sept. 2015.5M. Hirose, et al, “Measurements of Underwater Ultra Wideband Propagation Channels in a HF Band,” IEICE Society Conference, Sept. 2015.6M. Hirose, et al, “Underwater Wideband Propagation Channels in a HF Band,” IEICE General Conference, March 2016.7N. Iwakiri, et al, “Underwater Channel Sounding Using SIMO-OFDM Signaling,” IEICE General Conference, March 2016.FiF5Delay characteristic of received signal power and signal strength by distances(Left: 1 m depth; right: 1.9 m depth)FiF6Test result of direction of arrival estimation (at the arrival from zero degrees direction)552-8 Under Seawater Radio Communications