Patent ID: 8830117

Claim:
A method of setting a threshold according to a level of an echo signal containing an unused component, the echo signal being generated by transmitting and receiving a radio wave with an antenna while the antenna revolves, the method comprising: acquiring levels of the echo signals at every predetermined distance interval; and updating, with a processor, a threshold set for an observing position based on the level of the echo signal at the observing position, the threshold set for the observing position, and a threshold set for a position closer to the antenna than the observing position by the predetermined distance on the same sweep, wherein the updating the threshold includes updating the threshold to be lower when the level of the echo signal at the observing position is below the threshold set for the observing position, and updating the threshold to be higher 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 observing position is below the threshold set for the position closer to the antenna than the observing position by the predetermined distance on the same sweep.