If there is no wind, there must be no phase difference
between two ultrasonic signals that are received by two
receivers that measure the phase difference to determine
the wind velocity in a CWUA. The signals that are actually
received by the two receivers have an offset as that shown
in Fig. 1a, though. If there is an offset phase difference, the
actually measurable maximum phase difference is the
half-cycle of the used ultrasonic waves minus the offset
phase difference.
The offset phase difference is generated by the minute
differences in the sensor module structures and the difference
in the response characteristics of the IC devices that
are used in the receiving circuit. If there is a 1 mm error in
the distance between the transmitter and the two receivers
due to a minute difference between the structures, a
difference of 2.91 ls occurs in the time required for the