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Nowadays, robots are used for many types of underwater work, and knowing their position accurately is very important. Conventional positioning systems are based on the time difference or phase lag of sound waves. However, such systems are complex and expensive, and real-time positioning is difficult and not very accurate. We propose a new system using the loss of sound propagation and a sensor network. In a previous study, we separated directly arrived waves from received waves and measured their amplitude or power. Using this data, we calculated the distance. However, the signal included many types of reflected wave, and we were not able to obtain a correct value over long distances where the direct waves cannot be separated from the influence of the reflected waves. In this study, we propose a new method based on sounds at multiple frequencies. This method uses sounds of several different frequencies, and sends them sequentially at constant intervals. The distance can be calculated by taking the average power of the signals received. The advantage of this method is that it is not necessary to use any separating process, and longer distances can be measured.
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