SHEN Wenbin, Dept. of Geophysics, School of Geodesy and Geomatics, Wuhan University, 129 Luoyu Road, Wuhan 430079, China; The Key Laboratory of Geospace Environment and Geodesy, Wuhan University, 129 Luoyu Road, Wuhan 430079, China. E-mail: wbshen@sgg.whu.edu.cn