## 3 Results
### Performance of Five-System PPP Time Transfer at Locations with Limited Sky View
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Elevation Cut-Off Angle** & **GPS** & **GLO** & **GAL** & **BDS** & **GRECJ** \\ \hline
20\({}^{\circ}\) & 100.0 & 70.6 & 79.3 & 91.3 & 100.0 \\
30\({}^{\circ}\) & 88.9 & 30.0 & 37.4 & 40.5 & 100.0 \\
40\({}^{\circ}\) & 35.7 & 4.5 & 5.2 & 5.8 & 100.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Service rates of each PPP time transfer case using the cut-off elevation angle of 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\) (unit: %). Figure 13 shows the difference in the receiver clock offset between PPP estimates and IGS final products for five PPP time transfer cases at stations BRUX and SPT0 under an elevation cut-off angle of 20\({}^{\circ}\) on DOY 252, 2021, and Figure 14 displays the corresponding difference of the time link BRUX-SPT0. Obviously, when the elevation cut-off angle is increased to 20\({}^{\circ}\), the performance of the receiver clock offset estimates and time link estimates of each single-GNSS case is worse than that under an elevation cut-off angle of 10\({}^{\circ}\). The fluctuation range gets larger for the errors of receiver clock offset estimates and time link estimates, and the results at some epochs are absent. By contrast, the receiver clock offset difference and time link difference of the five-system combination case are similar to the results at a mask elevation of 10\({}^{\circ}\), and show good performance in terms of data integrity. Tables 7 and 8 provide the average STD statistics of time link differences over a week under an elevation cut-off angle of 20\({}^{\circ}\) and 30\({}^{\circ}\), respectively. According to the average results over the four time links, the time transfer accuracy is 0.234, 0.849, 0.254, and 0.571 ns for the GPS-only case, GLONASS-only case, Galileo-only case, and BDS-only case when the elevation cut-off angle is set to 20\({}^{\circ}\). The corresponding accuracy of the five-system combination case is 0.227 ns. In the case of the elevation cut-off angle of 30\({}^{\circ}\), the time transfer accuracy is 0.368 ns for the GPS-only case, 1.029 ns for the GLONASS-only case, 0.318 ns for the Galileo-only case, and 0.725 ns for the BDS-only case. As for the five-system integrated PPP time transfer, the corresponding accuracy is 0.264 ns. With the increment of the elevation cut-off angle, the PPP time transfer accuracy of each single-GNSS case significantly degrades. Although the PPP time transfer accuracy of the five-system combination case also declines with the increasing elevation masks, it is much more reliable than that of the single-GNSS cases. When the elevation mask is further increased to 40\({}^{\circ}\), a time transfer accuracy of 0.376 ns can still be achieved for the five-system integrated PPP. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Elevation Cut-Off Angle** & **GPS** & **GLO** & **GAL** & **BDS** & **GRECJ** \\ \hline
20\({}^{\circ}\) & 100.0 & 91.7 & 96.9 & 98.8 & 100.0 \\
30\({}^{\circ}\) & 99.0 & 66.6 & 95.9 & 96.4 & 100.0 \\
40\({}^{\circ}\) & 97.7 & 37.6 & 97.5 & 90.3 & 100.0 \\ \hline \end{tabular}
\end{table} 
Table 6: Success rates of each PPP time transfer case using the cut-off elevation angle of 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\) (unit: %). Figure 13: Difference of receiver clock offset between PPP estimates and IGS final products for five PPP time transfer cases at stations BRUX and SPT0 under an elevation cut-off angle of 20\({}^{\circ}\) on DOY 252, 2021. To analyze the frequency stability of PPP time transfer under an elevation cut-off angle of 20\({}^{\circ}\), the MDEV statistics of three time links, BRUX-MGUE, BRUX-PTBB, and BRUX-SPT0, on DOY 252, 2021, are displayed in Figure 15. The MDEV value of the GLONASS-only case has a significant increase after the cut-off elevation is set to 20\({}^{\circ}\), and thus the GLONASS-only PPP cannot provide the reliable time transfer service for the three time links.