## 5 Precise Point Positioning (PPP)
### Static PPP Solutions  
For static PPP, considering data integrity and quality, four stations located in Australia were selected, namely, TOMP, ALBY, HOB2, and STR1. Their 24-hour solutions from DOY 052 to DOY 068 were calculated. We have adopted the following strategy, that is: when TOMP is used for PPP, the remaining stations are used for orbit and clock calculation without TOMP, the same as ALBY, HOB2 and STR1. Figure 8 shows the stations used for static and kinematic PPP. Figure 9 shows the satellite number and variations in position dilution of precision (PDOP) of GPS, GB114, GB134, B114 and B134 solutions for station STR1 on DOY 057, 2017. About eight BeiDou-2 satellites were tracked and the PDOP values ranged from 2 to 6, which presents the worst performance of space geometry in the selecting four cases. As depicted by Figure 9, GPS involved solutions (GPS/GB114/GB134) showed more stable overall tendency and better PDOP than BeiDou solutions (B114/B134). However, when one or more BeiDou-3e satellites were visible from the station, the PDOP performance could be improved. On average, PDOP of B134 (3.6) could be decreased by nearly 20%, compared with that of B114 (2.9). In addition, when about 20 GPS and BeiDou satellites could be tracked, the variations in the PDOP values of GB114 and GB134, which fluctuated around 1, were much smoother and steadier than that for the GPS-only solution. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**BeiDou-2 GEO** & **BeiDou-2 IGSO** & **BeiDou-2 MEO** & **BeiDou-3e IGSO** & **Beidou-3e MEO** & **ALL** \\ \hline STD & 0.22 & 0.17 & 0.15 & 0.18 & 0.17 & 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Average STD values for BeiDou-2 and BeiDou-3e clock offset in overlapping arcs with the middle day in the first arc as a reference (units: ns). \begin{table}
\begin{tabular}{c c} \hline \hline
**Solution** & **Description** \\ \hline GPS & GPS only \\ GB114 & GPS and BeiDou PRN 1-14 \\ GB134 & GPS and BeiDou PRN 1-14, 31-34 \\ B114 & BeiDou PRN 1-14 \\ B134 & BeiDou PRN 1-14, 31-34 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: GPS and BeiDou solutions utilized to achieve precise point positioning (PPP). We assessed the positioning precision using RMS values of the East (E), North (N), and Up (U) components. When compared with the corresponding ground truth values, the average coordinates calculated by IGS final orbit and clock products were used. The results are shown in Table 10.  
Figure 8: Stations used for static and kinematic PPP. Black points represent four stations, TOMP, ALBY, HOB2, and STR1, which are located in Australia. Figure 9: Variations in number of satellites used and position dilution of precision (PDOP) values of GPS, GB114, GB134, B114 and B134 solutions for station STR1 on DOY 057, 2017. Figure 10 shows the average RMS values for the four stations. The N components provided the best performance for all solutions, followed by the E and U components. The precision of solutions GPS, GB114 and GB134 mostly remained at the same level, with values of about 0.5 cm for the horizontal components and 0.8 cm for the vertical components. As depicted by Figure 10, for precisions in U components, GB134 performed little worse than GB114. The reasons maybe include as follows: PCO and PCV for BeiDou-2 series are corrected parameters supplied by Wuhan University [7], while PCO for BeiDou-3e satellites adopted in our analysis is initial values provided by OCC without PCV, as mentioned in Table 4 and Section 4.1, which may contain errors. Furthermore, precisions in U components are greatly influenced by corrected precision of \(Z\) direction in PCO. It is noteworthy that corrected value in Z direction for PCO is much larger than corrected values in X and Y, which results in relatively worse vertical precision for PPP. For the BeiDou-only solutions, a precision of 1.0 cm for the horizontal components and 1.7 cm for the vertical component were achieved.