## 3 Results
### Performance of the Five-System PPP Time Transfer under Open Sky  
To assess the remote PPP time transfer performance of the five-system combination under open sky, the elevation cut-off angle is set to 10\({}^{\circ}\). For comparison, the results of BDS-only, GPS-only, GLONASS-only, and Galileo-only PPP time transfer are also provided. The IGS final products, which are generated by combining the GPS-only or GPS/GLONASS network solutions of all analysis centers, are used as the reference. Figure 4 shows the difference in the receiver clock offset between PPP estimates and IGS final products for the five-system combination case and four GNSS single-system cases at stations BRUX and SPT0 on DOY 252, 2021. The effects of different time datrums are eliminated by subtracting the average value. The results in the first two hours are excluded to ensure that only the fully converged results can be involved in the analysis. It is seen that the receiver clock difference usually varies within a range of 1 ns. In addition, the receiver clock difference exhibits some consistent trends at the two stations, which may be attributed to the commonly employed precise satellite products. Figure 5 further displays the corresponding difference of the time link BRUX-SPT0. The five-system combination case has the smallest time link difference, followed by the GPS-only and Galileo-only cases, while the varying range of the time link difference can be 0.25 ns for the BDS-only case and 0.5 ns for the GLONASS-only case. Benefiting from the strong satellite geometry and the considerable number of visible satellites under an open-sky environment, as well as the high-accuracy satellite orbit and clock products, the observation information is sufficient even for GPS-only and Galileo-only cases. Thus, the improvement on the time link difference for the five-system combination case over the GPS-only and Galileo-only cases is not significant. Usually, the standard deviation (STD) statistics of the time link differences between PPP estimates and IGS final products can be taken as an important index to evaluate the PPP time transfer accuracy. We first calculated the STD statistics of the daily time link differences, and then obtained the average STD statistics over a week. Table 3 illustrates the derived results of four different time links for the five-system combination case and the four GNSS single-system cases. The time transfer accuracy of the five-system integrated PPP is better than that of the GNSS single-system PPP for most of the cases. According to the average results over the four time links, the time transfer accuracy is 0.074 ns for  
Figure 3: Number of visible satellites and TDOP values at stations BRUX, ROAG, and SPT0 on DOY 254, 2021.  
the GPS-only PPP, 0.359 ns for the GLONASS-only PPP, 0.068 ns for the Galileo-only PPP, and 0.346 ns for the BDS-only PPP. The five-system integrated PPP can improve the time transfer accuracy by 18% over the GPS-only case, 83% over the GLONASS-only case, 10% over the Galileo-only case, and 82% over the BDS-only case, to 0.061 ns. All the time links involved in this contribution take the station BRUX as the center node, but the two stations that constitute the time link have different time and frequency references. As such, it is difficult to directly compare the PPP time transfer performance  
Figure 4: Difference in the receiver clock offset between PPP estimates and IGS final products for the five-system combination case and the four GNSS single-system cases at stations BRUX and SPT0 on DOY 252, 2021. Figure 5: Difference in the time link BRUX-SPT0 between PPP estimates and IGS final products for the five-system combination case and the four GNSS single-system cases on DOY 252, 2021.