The construction and maintenance of port facilities are carried out by human labor of divers. Mechanization of underwater work is necessary for safer and more efficient operation of port construction work.
To satisfy the underwater construction standards by machine work, high accuracy positioning technology that has accuracy of ±10 cm or less is required for port construction work. However, conventional ultrasonic positioning devices have accuracy of 50-60 cm at least. The main factors of such accuracy of the conventional systems are caused by the measurement error of the sound propagation time from responders to receivers and the incorrectness of the average sound velocity in the propagation path.
Therefore, to achieve the positioning accuracy less than ±10 cm in the arrangement that receivers can be equipped on a work vessel, we have developed the new type underwater positioning system. The 4 receivers system can give information about the mean sound velocity between responder and receivers. To improve the measurement accuracy of propagation time, the system employs the ultrasonic signal modulated by M-sequence.
In the simulation, accuracy of the system and the required length of the base line have been calculated. These results have been verified with the small-scale tank tests.

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In our experiments of the ocean acoustic tomography, M-sequence signal is used in order to detect a signal in the noisy environment in the ocean and improve the resolution on the time axis after correlation of signals. The frequency of the sound source for our tomography transceiver is 200 Hz to ensure the propagation distance of 1000 km. The acoustic characteristics of the source were evaluated using M-sequence signal, and compared with the results of the conventional method by using sinusoidal waves. In this report, the results of the simulation of the waveform variation of M-sequence signal with various wave numbers per digit are shown, and then the experimental results in the ocean using M-sequence and sinusoidal waves at the depth of 800, 1000 and 1200 m are shown. The relationship between the wave number per digit of M-sequences and the frequency response using sinusoidal waves is discussed. Finally, the experimental results for a long-range propagation in the Western Equatorial Pacific are introduced.

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