jgg     2015 35 (1):  100-105    ISSN: 0412-1961  CN: 21-1139/TG Stochastic Modeling for GNSS Pseudo-range Observations Using  Undifferenced Geo-Ion-Free Combinations Key Laboratory for Land Environment and Disaster Monitoring of NASMG, CUMT, 1 Daxue Road, Xuzhou 221116, China Received: 2014-02-28  Revised: null  Online: 2015-02-06 Supporting info