Patent ID: 8564474

Claim:
A method of setting a threshold according to a level of an echo signal of an unused component, the echo signals being generated by transmitting and receiving a radio wave with an antenna while the antenna revolves, the method comprising: calculating a difference value between a level of the echo signal at an observing position and a level of the echo signal at a second position located between the observing position and the antenna and closer to the observing position than to the antenna; selecting a process for setting a threshold from either one of a first threshold setting process and a second threshold setting process according to the calculated difference value; and updating the threshold set for the observing position by using the selected threshold setting processing wherein the first threshold setting process includes decreasing the threshold set for the observing position when the level of the echo signal at the observing position is below the threshold set for the observing position; increasing the threshold set for the observing position when the level of the echo signal at the observing position is above the threshold set for the observing position and the threshold set for the second position is above the threshold set for the observing position; and decreasing the threshold set for the observing position when the level of the echo signal at the observing position is above the threshold set for the observing position and the threshold set for the second position is below the threshold set for the observing position.