#### Abstract

PURPOSE: To improve the precision of tracking of a circling target by using a target distance variation rate for calculating the respective reliability of a plurality of motion models and by using the respective reliability of the motion models in a plurality for calculating the elements of the motion of the target.
CONSTITUTION: A position observation error evaluator 2 for each motion model evaluates a target position error for each of motion models in a plurality which are composed of a plurality of constant vectors of the same dimension. A distance variation rate observation error evaluator 14 for each motion model evaluates a target distance variation rate observation error for each motion model. A gain matrix calculator 5 calculates a gain matrix from an observation noise covariance matrix obtained from an observation model set beforehand and an estimation error covariance matrix for each motion model calculated one sample before the present time point. A reliability calculator 3 for each motion model calculates reliability, using the target position observation error for each motion model calculated by the evaluator 2 and the target distance variation rate observation error calculated by the evaluator 14. An estimated value calculator 7 calculates estimated values of the elements of the motion of the target, using the reliability calculated by the calculator 3.
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