## 3 Results
### Performance of Five-System PPP Time Transfer at Locations with Limited Sky View
The other three single-GNSS PPP time transfer cases also show an increase in the MDEV values compared with those at an elevation cut-off angle of 10\({}^{\circ}\) (see Figure 6). For the five-system combined PPP time transfer, the increased cut-off elevation of 20\({}^{\circ}\) has less effect on the frequency stability, and its time transfer results are considered to be more reliable than the single-GNSS cases. When the elevation cut-off angle is further increased to 30\({}^{\circ}\), the service rate of single-GNSS cases is reduced to a low level (see Table 5), and the time transfer results of single-GNSS cases are incomplete. Thus, they cannot provide a reliable time transfer service. Based on the MDEV statistics derived from the incomplete time links, it is difficult to analyze the frequency stability of single-GNSS PPP time transfer. By contrast, the service rate of the five-system combined PPP time transfer can still maintain 100.0%, and the MDEV values can still keep within a reasonable range. \begin{table}
\begin{tabular}{l c c c c c} \hline
**Time Links** & **GPS** & **GLO** & **GAL** & **BDS** & **GRECJ** \\ \hline BRUX-CRO1 & 0.325 & 1.061 & 0.363 & 0.735 & 0.320 \\ BRUX-PTBB & 0.119 & 0.633 & 0.138 & 0.369 & 0.106 \\ BRUX-SPT0 & 0.060 & 0.568 & 0.115 & 0.319 & 0.056 \\ BRUX-USN7 & 0.431 & 1.133 & 0.398 & 0.862 & 0.427 \\ \hline \end{tabular}
\end{table} 
Table 7: Average STD statistics of time link differences over a week under an elevation cut-off angle of 20\({}^{\circ}\) (unit: ns). Figure 14: Difference of the time link BRUX-SPT0 between PPP estimates and IGS final products for five PPP time transfer cases under an elevation cut-off angle of 20\({}^{\circ}\) on DOY 252, 2021. \begin{table}
\begin{tabular}{l c c c c c} \hline
**Time Links** & **GPS** & **GLO** & **GAL** & **BDS** & **GRECJ** \\ \hline BRUX-CRO1 & 0.440 & 1.239 & 0.404 & 0.940 & 0.382 \\ BRUX-PTBB & 0.232 & 0.862 & 0.172 & 0.582 & 0.135 \\ BRUX-SPT0 & 0.249 & 0.695 & 0.137 & 0.501 & 0.081 \\ BRUX-USN7 & 0.552 & 1.319 & 0.560 & 0.878 & 0.457 \\ \hline \end{tabular}
\end{table} 
Table 8: Average STD statistics of time link differences over a week under an elevation cut-off angle of 30\({}^{\circ}\) (unit: ns). In view of the superior performance of the five-system integrated PPP time transfer under high cut-off elevations, more related results under the elevation cut-off angle of 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\) are provided. Figure 16 shows the epoch-wise average number of visible satellites and TDOP value over all the employed days and stations under the elevation cut-off angle of 20\({}^{\circ}\), 30\({}^{\circ}\). and 40\({}^{\circ}\). The visible satellite numbers at the common epoch of all stations (13 stations) from all the days (7 days) are used to compute the epoch-wise average number of visible satellites, and it is also the case for the epoch-wise average TDOP value. According to the average results of epoch-wise statistics over 24 h, the visible satellite number is 22.3 for the cut-off angle 20\({}^{\circ}\), 18.7 for the cut-off angle 30\({}^{\circ}\), and 11.6 for the cut-off angle 40\({}^{\circ}\). Regarding the TDOP, the numerical value is 0.82 for the cut-off angle 20\({}^{\circ}\), 2.44 for the cut-off angle 30\({}^{\circ}\), and 3.23 for the cut-off angle 40\({}^{\circ}\). It is clear that both the visible satellite number and TDOP value get worse with the increasing elevation mask angles. Despite this, the five-system combination case can still guarantee the sufficient number of visible satellites and acceptable TDOP values even under a cut-off elevation angle of 40\({}^{\circ}\). Figure 17 shows the MDEV value for three time links (BRUX-MGUE, BRUX-PTBB, and BRUX-SPT0) derived from the five-system integrated PPP when the elevation cut-off angle is set to 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\) on DOY 252, 2021. It is found that the MDEV value increases with the increment of the cut-off elevation.