1. State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China. E-mail: sdni@whigg.ac.cn
2. School of Geophysics and Information Technology, China University of Geosciences, Beijing 100083, China
3. Department of Geophysics, School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China