## 3 Broadcast Ephemeris Performance
### Broadcast Ephemeris Orbit Performance  
The broadcast ephemeris orbit accuracy of GPS, BDS-2, BDS-3, GLONASS, Galileo, and QZSS satellites from DOY 283 to 289 in 2021 are presented in Figure 7, and the mean orbit accuracy is listed in Table 4. For GPS satellites, the radial accuracy of G04, G14, G18, and G23 satellite orbit is worse than that of other satellites, which may be that these four satellites belong to Block III, and the operation period is shorter, and their broadcast ephemeris orbit accuracy still needs to be improved; the orbit accuracy in the radial direction of other satellites is better than 0.25 m. For the along and cross components, the orbit accuracy is between 0.25 m and 1.5 m, and 0.25 m and 1 m, respectively. The mean 3D RMS is 0.60 m. The orbit accuracy is significantly poor for BDS-2 satellites, especially for GEO satellites, which mainly be since the GEO satellites are static relative to the ground stations, resulting in a strong correlation among the observations at different epochs. Compared to BDS-2 satellites, the broadcast ephemeris orbit accuracy of BDS-3 satellites is considerably improved, the satellite orbit accuracy in radial, along, cross, and 3D RMS is 0.11 m, 0.25 m, 0.25 m, and 0.21 m, respectively, and the improvements are 94.24%, 81.06%, 90.23%, 89.29%, respectively. The reasons can be attributed to the following: Firstly, the inter-satellites link technology is employed to determine the satellite orbit. Secondly, the number of BDS-3 satellites is much more than that of BDS-2, and the satellite type is mainly MEO satellites, the redundancy of observation data has been improved. The satellite orbit accuracy after the PRN 38 is slightly inferior, and the operation period of these satellites is shorter, the stations that can receive the signal of these satellites are few. With the increase of stations, satellite orbit accuracy can be improved in the near future.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**System** & **Radial** & **Along-Track** & **Cross-Track** & **3D RMS** \\ \hline GPS & 0.24 & 0.84 & 0.57 & 0.60 \\ BDS-2 & 1.91 & 1.32 & 2.56 & 1.99 \\ BDS-3 & 0.11 & 0.25 & 0.25 & 0.21 \\ GLONASS & 0.46 & 1.92 & 0.99 & 1.27 \\ Galileo & 0.69 & 0.68 & 0.46 & 0.62 \\ QZSS & 0.56 & 0.80 & 1.06 & 0.83 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Average accuracy of DOY 283–289 broadcast orbit for Multi-GNSS satellites (unit: m).  
Figure 7: Broadcast orbit accuracy of GNSS five systems.  
The satellite orbit accuracy in radial and cross is small for GLONASS satellites, most satellites are better than 1.5 m, whereas the broadcast ephemeris orbit accuracy in along component shows poorer performance, the mean orbit accuracy is 0.46 m, 0.99 m, 1.92 m, and 1.27 m for radial, cross, along, and 3D direction, respectively. The orbit difference of each Galileo satellite is small, and the orbit accuracy for most satellites is better than 1.0 m in along, cross, and radial directions, and the mean 3D RMS is 0.62 m, which is comparable to that of GPS. The orbit accuracy of QZSS satellites is similar to that of BDS-2, the orbit accuracy of three IGSO satellites is tremendously better than that of GEO satellites. The mean 3D RMS is 0.83 m, which is poorer than that of BDS-3, GPS, and Galileo satellites.