PURPOSE: To improve the time resolution of the system at the time of measuring delay time by using such a Manchester-coded PN signal that the main lobe of its auto-correlation function is 2/3 of one chip rate.

CONSTITUTION: A Manchester-coded PN signal can be obtained by EX-ORing a PN signal from a PN signal generator 3 and clock signal having a frequency which is twice of the chip rate of the PN signal and subjecting the EX-ORed signals to level conversion at a level converter 5. The signal thus obtained is transmitted on a carrier. In order to take out the same signal as the Manchester-coded PN signal generated from a transmitter on a receiver side, the receiving time is decided by detecting correlation or synchronization and the distance L between the transmitter and receiver is calculated by measuring the time lag τ from the transmitting time by means of a time lag measuring instrument 15. The distance L is calculated from L=cτ, where (c) represents the acoustic velocity.