jgg     2015 35 (1):  72-76    ISSN: 0412-1961  CN: 21-1139/TG Research on Three-Dimensional Water Vapor Tomography Using  GPS/GLONASS PPP Method 1 Key Lab of Mine Spatial Information Technology, NASMG,Henan Polytechnic University,  2001 Shiji Road, Jiaozuo 454000, China 2 Research Center of GNSS, Wuhan University, 129 Luoyu Road, Wuhan  430079, China Received: 2014-01-16  Revised: null  Online: 2015-02-06 Supporting info