1. Lecturer, School of Astronautics, Beihang Univ., Beijing 100191, P.R. China (corresponding author).
2. Ph.D. Student, School of Astronautics, Beihang Univ., Beijing 100191, P.R. China.
3. Associate Professor, School of Automation and Electrical Engineering, Univ. of Science and Technology Beijing, Beijing 100083, P.R. China.