Uncombined precise orbit and clock determination of GPS and BDS-3

There is increasing concern about the uncombined(UC)observation model in the field of global navigation satellite system(GNSS).Based on the global positioning system(GPS)and the third-generation BeiDou navigation satellite system(BDS-3),this study processed the UC precision orbit determination(POD)for single and dual systems.First,a UC observation model suitable for multi-GNSS POD was derived,and the ionospheric-free(IF)combination observation model was presented.Although the ambiguity parameters of UC and IF strategies were different after reparameterization,the difference could be removed when processing ambiguity resolution,and the equivalence was proved theoretically.To demonstrate the accuracy of BDS-3 orbits fully,the observation data of approximately 1 month were selected for determining the precise orbit for global positioning system(GPS)only,BDS-3 only,and GPS/BDS-3 systems based on the UC and IF models.The orbit precision of BDS-3 satellites was validated by using metrics,including comparison with precision products released by Wuhan University,orbit boundary discontinuity,and satellite laser ranging(SLR)residuals.The results show that the orbit accuracies of the IF and UC models are almost the same,the difference in orbits is approximately several millimeters,and the clock difference is within 0.01 ns.The GPS/BDS-3 combined solution shows better accuracy compared to other solutions.The average accuracies in the R and 3D directions are approximately 4 and 15 cm,and the clock standard deviation is approximately 0.2 ns compared to external orbit product.The root mean square of SLR residuals is approximately 4 cm.