As a method of measuring a velocity of a mobile body, a Doppler method that transmits an electromagnetic wave or an ultrasonic wave as a transmission wave and calculates the velocity from a frequency variation between the transmission wave and a reflected wave from the mobile body is widely known.
Techniques that use the above Doppler method to measure both the movement direction and the movement velocity of a mobile body have been proposed (see for example Japanese Unexamined Patent Application Publication Nos. 2002-071802 and 2012-063151).
Japanese Unexamined Patent Application Publication No. 2002-071802 describes transmitting ultrasonic waves from a single transmitter toward a mobile body (golf club head) and receiving reflected waves reflected from the mobile body with three receivers provided at different locations. The relative velocities of the mobile body in the direction of the received waves is calculated based on Doppler signal components included in the reflected waves detected by the receivers, a velocity vector of the mobile body is calculated on the basis of the relative velocities, and the movement direction (angle) of the mobile body is calculated on the basis of the orientation of the velocity vector.
Japanese Unexamined Patent Application Publication No. 2012-063151 describes previously deriving a correlation between velocities measured by using a plurality of antennas disposed away from each other and actual measurement values of a movement direction and a movement velocity of a mobile body. During the measurement, the movement direction and the movement velocity of the mobile body are calculated by using the velocities measured with the plurality of antennas and the previously derived correlation.
However, there is a need to install a plurality of receivers (antennas) in order to measure the movement direction and the movement velocity of the mobile body in the aforementioned conventional techniques. As a result, there is a problem that installation costs of the measuring equipment increases and the size of the measuring equipment also increases.