1. School of Geodesy and Geomatics, Wuhan University, No. 129, Luoyu Road, Wuhan 430079, Hubei, China, ymwen@sgg.whu.edu.cn
2. Also at Collaborative Innovation Center of Geospatial Technology, Wuhan University, No. 129, Luoyu Road, Wuhan 430079, Hubei, China; and Key Laboratory of Geospace Environment and Geodesy, Ministry of Education, Wuhan University, No. 129, Luoyu Road, Wuhan 430079, Hubei, China.