Using ultrasonic sound, the authors have been investigating a high accuracy indoor positioning system with an error of just a few cm. Since ultrasonic transmitters are not in common use, this represents a barrier to diffusion. This paper presents a method using widely dispersed smartphones which satisfies the twin demands in practical use for high accuracy and the accommodation of multiple sounds. The spread spectrum for sound transmitted from multiple smartphones and correlation processing make it possible to accurately detect when a sound is received, as well as accommodation and discrimination of multiple sounds. We verified that the positioning error was about 13 mm in the horizontal plane and that each sound could be clearly discriminated by conducting experiments using the positioning system according to the proposed method. It is also shown that the positioning accuracy was maintained when an experiment using a moving model railway train was carried out in noisy environment.

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