## 4 Result and Analysis
### SISRE and Orbit-Only SISRE  
To comprehensively assess the accuracy of BDS-2 and BDS-3 broadcast ephemeris, the SISRE and orbit-only SISRE between BDS-2 and BDS-3 from three ACs (GFZ, WHU and SHA) are presented and analyzed in this section. Figures 11 and 12 present the SISRE and orbit-only SISRE of BDS-2 and BDS-3 broadcast ephemeris, respectively. In order to show SISRE and orbit-only SISRE more clearly, the SISRE and orbit-only SISRE values are shifted by the same amount (3 m) to avoid overlapping. From Figures 11 and 12, the consistency of GFZ, WHU and SHA performs better in SISRE and orbit-only SISRE and the three ACs' consistency of BDS-3 is significantly better than that of BDS-2. BDS-2 GEO satellites perform poorly in SISRE and orbit-only SISRE due to its special orbit and the immaturity of orbit determination strategy. As mentioned in Figure 2, the precise products of some BDS-3 satellites are missing for three ACs, the corresponding SISRE and orbit-only SISRE are also missing. Combining Figures 11 and 12, we can find that the BDS-3 is significantly better than BDS-2 in terms of SISRE and orbit-only SISRE. As shown in Figure 13, the STD and RMS of SISRE are better than 0.2 m and 1.5 m for all BDS-3 satellites, respectively. However, the variations between each BDS-2 satellite are relatively serious compared with BDS-3. The STD of SISRE ranges from 0.2 m to 0.6 m, and RMS ranges from 0.8 m to 3.5 m. In fact, different ACs adopt different POD solution strategies, systematic error will inevitably be absorbed into SISRE and orbit-only SISRE. Table 6 shows the final statistical results of each ACs and the average values of three ACs. For BDS-2 signal-in-space accuracy, the mean RMS and STD of three ACs are (1.78, 0.40) m for SISRE, and (1.72, 0.34) m for orbit-only SISRE, which is consistent with [PERSON] and [PERSON] (2016). About the hot topic BDS-3, the mean RMS and STD of three ACs are (0.50, 0.14) m for SISRE, and (0.17, 0.04) m for orbit-only SISRE, respectively. The orbit-only SISRE accuracy of BDS-3 is roughly improved by one order magnitude compared to BDS-2, which is the same as the corresponding orbit accuracy, respectively. Because the improvement of BDS-3 clock \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Agency** & **Items** & **Track Direction** & **BDS-2** & **BDS-3** & **Improvement Rate** \\ \hline \multirow{6}{*}{GFZ} & \multirow{3}{*}{RMS} & Radial & 1.79 m & 0.20 m & 88.83\% \\  & & Along & 3.28 m & 0.33 m & 89.94\% \\  & & Cross & 1.39 m & 0.29 m & 79.14\% \\ \cline{2-6}  & \multirow{3}{*}{STD} & Radial & 0.32 m & 0.07 m & 78.13\% \\  & & Along & 0.99 m & 0.25 m & 74.75\% \\  & & Cross & 1.04 m & 0.24 m & 76.92\% \\ \hline \multirow{6}{*}{WHU} & \multirow{3}{*}{RMS} & Radial & 1.78 m & 0.16 m & 91.01\% \\  & & Along & 3.28 m & 0.30 m & 90.85\% \\  & & Cross & 1.23 m & 0.27 m & 78.05\% \\ \cline{2-6}  & \multirow{3}{*}{STD} & Radial & 0.34 m & 0.05 m & 85.29\% \\  & & Along & 1.03 m & 0.18 m & 82.52\% \\  & & Cross & 0.98 m & 0.21 m & 78.57\% \\ \hline \multirow{6}{*}{SHA} & \multirow{3}{*}{RMS} & Radial & 0.81 m & 0.23 m & 71.60\% \\  & & Along & 2.56 m & 0.35 m & 86.33\% \\  & & Cross & 1.02 m & 0.30 m & 70.59\% \\ \cline{2-6}  & \multirow{3}{*}{STD} & Radial & 0.37 m & 0.09 m & 75.68\% \\  & & Along & 0.99 m & 0.28 m & 71.72\% \\  & & Cross & 0.95 m & 0.25 m & 73.68\% \\ \hline \multirow{6}{*}{Average} & \multirow{3}{*}{RMS} & Radial & 1.46 m & 0.20 m & 86.30\% \\  & & Along & 3.04 m & 0.32 m & 89.47\% \\ \cline{1-1}  & & Cross & 1.21 m & 0.28 m & 76.86\% \\ \cline{1-1} \cline{2-6}  & \multirow{3}{*}{STD} & Radial & 0.34 m & 0.07 m & 79.41\% \\ \cline{1-1}  & & Along & 1.00 m & 0.23 m & 77.00\% \\ \cline{1-1}  & & Cross & 0.99 m & 0.23 m & 76.78\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The mean RMS and STD statistical results of BDS-2 and BDS-3 orbit errors and the improvement rate of BDS-3 compared to BDS-2 in terms of RMS and STD (DOY 089-123, 2020).