jgg     2014 34 (4):  161-164    ISSN: 0412-1961  CN: 21-1139/TG A METHOD FOR DETERMINING RELATIVE POSITIONING RANDOM  MODEL OF THE COMBINATION OF GPS AND BEIDOU Hefei University of Technology，Hefei 230009 Received: 2013-11-18  Revised: null  Online: 2014-07-22 Supporting info