## 2 Methods
### Hourly Updated Ultrarapid POD Strategy  
Nowadays, most of the IGS/MGEX/iGMAS analysis centers update their ultrarapid orbit products every 6 h or ever 3 h, few of which contain BDS, Galileo and QZSS. In order to enhance the solution accuracy and stability, an arc length of 24 h is selected, and then recent adjacent normal equations are combined up to a length of 72 h for the orbit fitting and prediction. The procedure of ultrarapid orbit determination and prediction is illuminated in Figure 2. The sliding window size is 24 h, and the arc length for each solution is 24 h shifted by 1 h. For the current solution, 24 h arc observations are processed and then combined with two existing adjacent 24 h arcs at a normal equation level to generate a 72 h solution. Finally, a 24 h orbit prediction is conducted after orbit fitting, and the predicted part of the orbit is further used by satellite clock error estimation program. Meanwhile, we proposed a station-clustered and parallel processing strategy using two identical Linux cloud servers (Operating system: CentOS Linux release 7.8, CPU 3.30 GHz, CPU cores: 8, total memory: 148,688,684 kB) to improve the computation efficiency and to ensure the accuracy and reliability of the predicted orbit. Ground tracing stations are classified into six clusters according to geographical distribution and then run in parallel at the parameter estimation stage with idle CPU cores automatically selected. Then, the six clusters are combined at the normal equation level to generate the current solution. For the 72 h combination and 24 h  
Figure 1: Tracking stations used in multi-GNSS real-time POD and SLR validation. The stations are remarked by blue circles, red five-pointed stars and green diamonds for IGS, iGMAS and SLR tracking stations, respectively. prediction processing, the five-constellation POD can be conducted within one hour, and thus, hourly updated orbit is achievable. The post-processing software platform, which has been developed by the analysis center of iGMAS at APM, has been adapted for ultrarapid POD processing in this contribution. Observations from five constellations have been processed simultaneously while the observational geometry for Galileo and BDS has been significantly improved. The main aspects of real-time POD processing strategy are listed in Table 3, and the procedure is summarized in Figure 3. \begin{table}
\begin{tabular}{c c} \hline \hline
**Aspect** & **Summary** \\ \hline Observations & Double-differenced ionosphere-free phase. GPS/GLONASS/QZSS: L1, L2, BDS: B1I, B3I; Galileo: E1, E5a \\ Elevation angle cut-off & 7° \\ Stations & 21 iGMAS stations and 100 IGS/MGEX stations \\ Sampling rate & 300 s \\ Data coverage & A length of 72 h arc combined through three adjacent 24 h arc normal equations \\ Orbits & Initial positions and velocities from broadcast ephemeris \\ Weighting & 6 mm for phase observationswith elevation-dependent weighting \\ Satellite antenna PCOS and PCVs of GPS/GLONASS/QZSS L1/L2, \\ Galileo E1/E5a and BDS B1I/B3I from igs14_2196.atx. For BDS and Galileo receiver antenna, the values of GPS L1/L2 have been used for the BDS B1I/B3I and Galileo E1/E5a, as their calibrations are unavailable in the igs14.atx [30] \\ Attitude model & GPS/GLONASS/Galileo/BDS-3: Yaw steering \\ Troposphere & GMF mapping functions; ZTDs estimated at intervals of 2 h \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Outline of the ultrarapid POD strategy for multi-GNSS. Figure 2: Flowchart of ultrarapid orbit determination and prediction. Figure 3: Flowchart of the procedure for real-time multi-GNSS POD.