jgg     2015 35 (3):  379-382    ISSN: 0412-1961  CN: 21-1139/TG Research on Multi-Frequency Observation Linear Combination of GNSS 1 Key Laboratory for Land Environment and Disaster Monitoring of NASMG, China University of Mining & Technology， 1 Daxue Road, Xuzhou 221008,China Received: 2014-05-10  Revised: null  Online: 2015-06-02 Supporting info