## 3 Results  
### Availability of Five-System Integration  
Based on the number of theoretically visible satellites, we assessed the availability of the BDS/GPS/GLONASS/Galileo/QZSS five-system combination. Following [PERSON] et al. (2018), the whole world can be evenly divided into 72 \(\times\) 72 grids; each of them extends for 2.5\({}^{\circ}\) and 5\({}^{\circ}\) in terms of latitude and longitude, respectively, and it is assumed that there is a virtual receiver at an altitude of 25 m in the center of each grid. The receiver sampling interval is set to 30 s, and the elevation cut-off angle is set to 10\({}^{\circ}\). With the use of the satellite coordinates provided by GFZ precise products, the theoretical availability of five-system combination can be assessed by analyzing the relative geometry of each satellite and the virtual receiver. The theoretical distribution of the average number of visible satellites (over a day) under the five-system combination case on DOY 254, 2021, is shown in Figure 2. Obviously, a diamond-shaped area exists in the figure. It centers around the point at 120\({}^{\circ}\)E longitude at the equator, and extends from 60\({}^{\circ}\)E to 180\({}^{\circ}\) in the east-west direction and from 60\({}^{\circ}\)N to 60\({}^{\circ}\)S in north-south direction. The number of visible satellites in the diamond-shaped \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Station** & **Receiver** & **Antenna** & **Clock** & **Latitude (\({}^{\circ}\))** & **Longitude (\({}^{\circ}\))** \\ \hline BRUX & SEPT POLARX4 TR & JAVRINGANT\_DM & H-MASER & 50.798 & 4.359 \\ CEBR & SEPT POLARX5 TR & SEFCHOKE\_B3E6 & H-MASER & 40.453 & \(-\)4.368 \\ CRO1 & JAVD TRE 3 DELTA & JAVRINGANT\_DM & H-MASER & 17.757 & \(-\)64.584 \\ KOKB & SEPT POLARXSTR & ASHT01945G\_M & H-MASER & 22.126 & \(-\)159.665 \\ MATE & LEICA GR30 & LEIAR20 & H-MASER & 40.649 & 16.704 \\ MQUE & SEPT POLARX5 TR & LEIAR25.R4 & H-MASER & \(-\)35.777 & \(-\)69.398 \\ PTBB & SEPT POLARX5 TR & LEIAR25.R4 & H-MASER & 52.296 & 10.460 \\ ROAG & SEPT POLARX5 TR & LEIAR25.R4 & H-MASER & 36.463 & \(-\)6.206 \\ SPT0 & SEPT POLARX5 TR & JNSCR\_C146-22-1 & H-MASER & 57.715 & 12.891 \\ TIDI & SEPT POLARX5 & AOAD/M\_T & H-MASER & \(-\)35.399 & 148.980 \\ USN7 & SEPT POLARX5 TR & TPSCR.G5 & H-MASER & 38.921 & \(-\)77.066 \\ WTZR & LEICA GR50 & LEIAR25.R3 & H-MASER & 49.144 & 12.879 \\ ZIM3 & TRIMBLE NETR9 & TRM59800.00 & H-MASER & 46.877 & 7.465 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detailed information of 13 MGEX stations. \begin{table}
\begin{tabular}{l l} \hline \hline
**Item** & **Strategies** \\ \hline Observable & GPS: L1/L2, GLONASS: G1/G2, Galileo: E1/E5A, QZSS: L1/L2, BDS (BDS-2 and BDS-3): B1/B3 \\ Sampling rates & 30 s \\ Elevation cut-off angle & 10\({}^{\circ}\), 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\) \\ Weighting scheme & Elevation-dependent weighting \\ Estimator & Kalman filter \\ Precise orbit and clock & Using the products provided by GFZ \\ products & Earth rotation parameters & Fixed with the products provided by GFZ \\ Relativistic effects & Applied \\ Phase wind-up effects & Corrected \\ Station displacement & Ocean tide loading, pole tide, Solid Earth tide, IERS Convention 2010 \\ Differential code bias (DCB) & Corrected with DCB products provided by CODE \\ Ionospheric delays & Eliminated by forming IF combination \\ Phase center offset and & Corrected by IGS14 atx file \\ Tropospheric delays & Dry part: corrected with a priori modelWet part: estimated as random-walk (RW) process \\ Receiver clock offset & Estimated as white noise process \\ Station coordinate & Estimated as constants \\ Phase ambiguity & Estimated as float constants \\ ISB & Estimated as RW process \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Processing strategy of five-system integrated PPP time transfer. regions far exceeds other regions, with an average of 48.7.