Evaluation of ultrasonic target detection by alternate transmission of different codes in M-sequence pulse compression

M-sequence pulse compression is one of the typical techniques of distance measurement by the time of flight (TOF) of ultrasound. In the cross-correlation function (CCF) between the received signal and the transmitted M-sequence code, a sharp peak occurs at the time corresponds to the TOF. The limit of the measurable TOF corresponds to the maximum measurable distance is determined by the length of the M-sequence coded ultrasound. Furthermore, there is a trade-off between the measurable distance and the temporal resolution of the measurement. The alternate transmission of two different codes in M-sequence pulse compression has been proposed to extend the measurable distance to double with keeping the temporal resolution in the one-code transmission of the M-sequence. However, truncation and truncated interference noises are generated around sharp peaks and make it difficult to identify sharp peaks. Amplitudes or shapes of these noises are varied by the combination of M-sequences. Therefore, the selection of suitable combination of M-sequences can suppress the effect of these noises.