Accuracy of distance measurements using signal tracking of spread-spectrum ultrasonic waves with CDMA

Autonomous robots moving indoors require information about their position with high accuracy in order to reach their destination safely and correctly. To realize self-localization for such autonomous robots, we aim to measure the indoor position of a moving object by using signal tracking with spread-spectrum (SS) ultrasonic signals. In this paper we investigate the accuracy of distance measurements using signal tracking of SS ultrasonic signals using Code Division Multiple Access (CDMA). To detect the SS ultrasonic signals, we perform correlation calculations between a range of received waves and a similar range of replica signals identical to the transmitted SS signals. Conventional positioning systems using SS signals employ signal acquisition to calculate the coordinates of objects from correlation values and signal tracking to measure the relative shift of distances of moving objects. After investigating the signal acquisition method, we found that 3-D positioning with an error of 50 mm was possible, even when transmitting simulcast ultrasonic signals with CDMA. Other studies using our proposed signal tracking method for moving targets without CDMA have maintained correlation values using a limited correlation range for correlation calculations, though they have yielded reduced correlation values for unmoving targets. In this paper, we develop a signal tracking method using the simulcasting of multiple signals with CDMA and discuss its effectiveness. We conduct an experiment on distance measurements to determine the method's measurable range and measurement error. We perform ten measurements of distances from 1000 to 6000 mm between a transmitter and a receiver, in 1000 mm intervals, using 50% of the correlation range. The results indicate an average measurement error of 11 mm, which is less than that of conventional signal acquisition methods.