## 4 Discussion  
PPP is one of the satellite-based positioning technologies. Due to the strong satellite geometry and the increased number of available satellites, both the accuracy and frequency stability of PPP time transfer can be improved by including more satellite systems, especially for the frequency stability. Since the users also have the requirement of high-accuracy time transfer at locations with limited sky view in the practical applications, the considerable service rates should be ensured. The service rates of single-GNSS PPP time transfer cases with high cut-off elevation angles are low, while those of the five-system integrated PPP time transfer remain at 100.0%. The satisfactory time transfer performance of the five-system integrated PPP can facilitate the applications of PPP-based time transfer.  
There is a strong correlation among the estimated parameters in the PPP. The day-boundary jumps of ISB estimates can be mitigated by considering the difference in the satellite clock datums between two adjacent days. The adjusted ISBs can be taken as the a priori constraints to reduce the correlation among the estimated parameters, so that the time transfer performance can be enhanced.  
With the rise of large-scale low earth orbit (LEO) constellations, the LEO-based performance enhancement for GNSS applications has become a research focus. The PPP time transfer technology can also benefit from the integrated data processing with the satellites at high, medium, and low orbits. On the one hand, the inclusion of tens of LEO satellites can further increase the number of available satellites and improve the satellite geometry, and thus the accuracy, frequency stability, and service rates of PPP time transfer can be enhanced. On the other hand, the separation of various estimated parameters will be easier, as the LEO satellites travel faster over stations and show rapid changes in spatial  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Time Links** & **Average Time (s)** & \
\begin{tabular}{c} **Without ISB** \\ **Constraints** \\ \end{tabular} &
\begin{tabular}{c} **With ISB** \\ **Constraints** \\ \end{tabular} \\ \hline \multirow{3}{*}{BRUX-PTBB} & \(10^{2}\) & \(1.03\times 10^{-13}\) & \(8.78\times 10^{-14}\) \\  & \(10^{3}\) & \(2.45\times 10^{-14}\) & \(2.30\times 10^{-14}\) \\  & \(10^{4}\) & \(5.70\times 10^{-15}\) & \(4.01\times 10^{-15}\) \\ \hline \multirow{3}{*}{BRUX-SPT0} & \(10^{2}\) & \(8.42\times 10^{-14}\) & \(6.60\times 10^{-14}\) \\  & \(10^{3}\) & \(2.08\times 10^{-14}\) & \(1.97\times 10^{-14}\) \\  & \(10^{4}\) & \(4.86\times 10^{-15}\) & \(3.89\times 10^{-15}\) \\ \hline \multirow{3}{*}{BRUX-USN7} & \(10^{2}\) & \(1.27\times 10^{-13}\) & \(1.08\times 10^{-13}\) \\  & \(10^{3}\) & \(3.27\times 10^{-14}\) & \(3.02\times 10^{-14}\) \\ \cline{1-1}  & \(10^{4}\) & \(1.49\times 10^{-14}\) & \(1.05\times 10^{-14}\) \\ \hline \hline \end{tabular}
\end{table}
Table 10: Average MDEV values over six days for three time links derived from the five-system integrated PPP with and without ISB constraints at an average time of \(10^{2}\), \(10^{3}\) and \(10^{4}\) s.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Time Links** & **Without ISB Constraints** & **With ISB Constraints** \\ \hline BRUX-PTBB & 0.064 & 0.031 \\ BRUX-SPT0 & 0.029 & 0.018 \\ BRUX-USN7 & 0.064 & 0.035 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Average STD statistics of daily time link differences between PPP estimates and IGS final products over six days for the five-system combination case with and without ISB constraints for three time links (unit: ns).  
geometry. In the future, with the deployment of LEO constellations, a better performance of GNSS/RNSS/LEO integrated PPP time transfer can be expected.