jgg     2015 35 (4):  649-652    ISSN: 0412-1961  CN: 21-1139/TG The Method for Determining the Stochastic Model of Single  Epoch Relative Positioning of Combined GPS and BDS 1 School of Civil Engineering, Hefei University of Technology, 193 Tunxi Road，Hefei 230009，China Received: 2014-07-14  Revised: null  Online: 2015-07-29 Supporting info