The ultrasonic pulse-echo method is an essential technique for underwater target ranging, and a pulse compression method with a modulated transmitting signal is usually employed to enhance both the signal-to-noise ratio and the resolution. Moreover, a time reversal (TR) method has been proposed to compensate for the phase distortion of echo signals caused by multi-path fading. By introducing the TR method, we have proposed an amplitude- and frequency-modulated sensitivity-compensated (AFM-SC) signal whose efficiency was verified with a single-channel transmitting experiment. Considering the difference between the multi-paths of propagation during TR wave acquiring and distance measurement, this paper studies the affection of multi-paths on the accuracy of distance measurement under single transmission and multi transmission, respectively. A target was set up in different positions to examine differences in multi-paths. The results of our comparison experiments show that, when the position of the target is changed, direct reflection waves can be focused, and the accuracy of distance measurement is improved by multi transmission. This suggests the possibility of target ranging with higher accuracy for a broader range by using a multi-channel transmission AFM-SC signal and TR method.
