A tracking error covariance matrix updating unit 6 that updates a tracking error covariance matrix Pk (−) before update at a sampling time k by using a nominal distance difference error parameter σΔrnom and that outputs the tracking error covariance matrix Pk (+) after update is disposed, and a TrackDOP calculating unit 7 calculates an evaluation index TrackDOP for tracking accuracy for a target by using both the tracking error covariance matrix Pk (+) after update, and the nominal observation error parameter σΔrnom.