Use of Global Positioning System (GPS) for space applications, especially in low-earth orbit (LEOs) formation flying missions, has recently attracted more and more attentions. This paper proposes an Extended Kalman Filter (EKF) to increase the accuracy of relative position of LEO formation flying missions. The EKF uses satellite orbital dynamic model, with combination of GPS and carrier phase measurements upon the ionosphere-free model. The carrier phase fuzziness and the position of spacecraft

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... form the state vector, which are predicted in the EKF. Meanwhile, influence of uncertainty of systematic errors is taken into account. The optimal estimation and its modified covariance matrix are derived upon the principle of minimum error covariance. The proposed EKF method is compared with the traditional double difference method in the simulated experiments. The results demonstrate the validity and improvement of the proposed method especially in the long baseline scenarios.