## 4 Experimental Validations
### Assessment of Positioning Performance with the Proposed Quality Control Method  
In order to assess the positioning performance of the multi-GNSS combined PPP based on the proposed quality control method, the observation data collected at 32 stations from March 5 to 11 (DOY 064-070) 2017 were used to calculate the positioning results of the seven combination schemes. The performance was analyzed based on daily solutions, for which the average positioning accuracy and the convergence time statistic are depicted in Figures 9-11. The bottom panel in Figure 9 denotes the result of the GPS system and the top panel is the GRCE combined result. It shows that the positioning RMS of the GRCE combination is better than the single GPS result. The positioning RMS of the GPS system and the GRCE combination in the east component is 3.2 cm and 1.2 cm, respectively, and it is 2.7 cm and 1.0 cm in the north component, while the RMS is 6.7 cm and 3.0 cm in the upward component. The corresponding positioning accuracy is improved by 62.5%, 63.0%, and 55.2% in the east, north, and upward components, respectively. The single GPS PPP takes a much longer time to converge. For example, only 35.7% and 42.9% of stations converge in 20 min in the horizontal and vertical components, respectively. With the integration of the other three systems, the percentage is improved to 75.5% and 77.7% correspondingly and the average time is  
Figure 8: The number of visible satellites (**left**) and the position dilution of precision (PDOP) value (**right**) for each combination scheme at the station BRST. Figure 9: The positioning RMS of the GRCE combined PPP and the GPS PPP in east, north, and upward coordinate components (**left**), and the convergence time statistic in the horizontal and vertical components (**right**). 12.8 min and 12.2 min. Due to the significantly increased number of satellites, the GRCE combined PPP can still provide the reliable positioning service, although there are some observations with low elevation angle. Figure 10 shows the results of the GRC, GRE, and GCE PPP. The positioning RMS values of the GRC and GRE PPP are comparable to the GRCE combined PPP and they are better than the accuracy of the GCE combined PPP. The further integration with another system for the triple-system combination, however, does not make large improvement in the positioning accuracy and the convergence time. Compared to the GRC combined PPP, the GRE combined PPP achieves slightly better positioning accuracy. For the horizontal and vertical components, 71.0% and 77.2% of stations for the GRE combination converge in 20 min, respectively, while 69.7% and 76.3% are obtained for the GRC combination. The average convergence time is about 13 min for both the GRE and GRC PPP. The GCE combined PPP takes a much longer time to converge than the GRE and GRC PPP and the average time is about 17 min. Figure 11: The positioning RMS of the dual-system combined PPP in the east, north, and upward coordinate components (**left**), and the convergence time statistic of the dual-system combined PPP in the horizontal and vertical components (**right**). Figure 10: The positioning RMS of the triple-system combined PPP in east, north, and upward coordinate components (**left**), and the convergence time statistic of the triple-system combined PPP in the horizontal and vertical components (**right**). The positioning RMS and the distribution of the convergence time of the GR, GC, and GE PPP are shown in Figure 11. It can also be seen that the combination of two systems not only improves the positioning accuracy, but also shortens the convergence time compared to the single GPS PPP. The improvement of the positioning RMS in the east component for the GR, GE, and GC combination is 53.1%, 43.8%, and 40.6%, respectively, while for the north component, it is 55.6%, 48.1%, and 40.7%. It is 47.8%, 40.3%, and 34.3% in the vertical component. The GE and GC PPP accuracy is slightly poorer than the GR PPP because the number of satellites is lower.