A plurality of antennas may be arranged at positions non-linear to each other. Processing circuitry may set an initial value of one of an attitude angle and an azimuth angle of an azimuth angle calculating device. An integer value bias of a carrier phase difference between at least two groups of antennas may be determined by using the initial value. A base-line vector between the at least two groups of antennas may be calculated by using the integer value bias corresponding to the group of antennas. A multiple base-line verification may be performed, in which validity of the initial value is verified by using each of the base-line vectors calculated using the integer value bias. An azimuth angle may be calculated by using the integer value bias when the multiple base-line verification is successful.