divides
the transmitting unit and the receiving unit. The
transmitting unit performs only continuous transmitting
of ultrasonic waves with the same frequency. And the
receiving unit determines the wind velocity by measuring
the phase difference between two ultrasonic signals that
are received through two receivers that are located at the
same distance and angle from the transmitter. These two
receivers exist within the directivity range of the ultrasonic
waves that are transmitted from the transmitter. The measured
phase difference can be expressed as in Eq. (1) [9], in
which L is the straight distance between the ultrasonic
transmitter and receivers, c is the propagation velocity of
the ultrasonic waves, v is the wind velocity, and u is the
angle between the movement path of the ultrasonic waves
and the wind. The measured phase difference is converted
to wind velocity through Eq. (2), which is a rearrangement
of Eq. (1) for the wind velocity.