Characteristic analysis and calibration of CDMA and FDMA intra-system and inter-system biases in multi-GNSS precise point positioning

Inter-system bias (ISB) parameters are usually introduced in multi-GNSS processing. However, the differences in correlation processing within GNSS receivers can introduce signal distortion bias (SDB) into pseudorange observations. SDB differences of these satellites from the same constellation, similar to the GLONASS inter-frequency bias (IFB) related to individual frequencies, can cause intra-system biases. Based on multi-GNSS observations that consider intra-system bias, the relationship between intra-system and inter-system biases and their impacts on the multi-GNSS undifferenced (UD) model are analyzed in this article. Then, empirical calibration strategies are proposed. Ten receiver types from four manufacturers are selected to analyze the intra-system bias characteristics of code division multiple access (CDMA) and frequency division multiple access (FDMA), and the calibrations and improvements of the multi-GNSS solution are also analyzed. The results show that the intra-system biases of GPS and Galileo remain at the centimeter–decimeter level with 10-cm stability, while bias of BDS can reach several meters with worse stability. The intra-system biases of CDMA signals have stronger consistency among the same types of receivers than those of FDMA. One month of data verified the centimeter-level differences between ISB parameters that were independently estimated based on multi-GNSS precise products with unified datum and inter-constellation differences in intra-system biases. The pre-estimated intra-system bias can be used to calibrate pseudorange residuals and further promote the convergence speed of single or multi-GNSS precise point positioning (PPP), especially for BDS and GLONASS. Comparative experiments show that intra-system bias calibration can decrease the residual RMS of BDS by 22 cm on average and that of GLONASS by 41 cm, with 15% and 23% improvement rates, respectively. The convergence time of BDS PPP in the horizontal and vertical directions can be reduced by 10 min to half an hour with approximately 20–60% improvement rates. For GLONASS PPP, the convergence time can also be reduced by more than 40 min with a 50–90% improvement.