## 4 Experiment and Analysis
### Orbit Accuracy Analysis in Radial Direction Based on the SLR Data  
The orbit accuracy in radial direction for satellites C01, C02, C03, and C05 was validated with SLR observation data, and the SLR residual details of satellites C01 and C03 are shown in Figures 11 and 12, respectively. The orbital accuracies in radial direction for satellites C01, C02, C03, and C05 based on RDSS data only are 0.547, 0.569, 0.510, and 0.617 m, respectively, with a mean value of 0.561 m. The accuracies are slightly inferior to the orbit accuracy of BeiDou GEO satellites based on the global network data. The large deviation of SLR residuals comes from the orbital error in the radial direction of GEO satellites. In this paper, the observation stations were deployed in regions in China, and there are only seven stations with a limited observation accuracy of RDSS data of 1 to 2 m. Due to the geostationary characteristics of GEO satellites, the constraints of RDSS data in the radial direction for GEO satellites are limited. Moreover, the constraints in the normal and tangential directions are very small by the satellite to ground measurement data for GEO satellites. Therefore, the orbit errors in the radial, normal, and tangential directions cause a large deviation in the SLR residual error in the radial direction.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Satellites** & **R** & **T** & **N** & **Positions** & **SISRE** \\  & & & & & **(Orbit)** \\ \hline C01 & 1.95 & 8.76 & 5.33 & 10.44 & 2.16 \\ C02 & 2.21 & 9.78 & 6.05 & 11.71 & 2.44 \\ C03 & 2.14 & 9.32 & 6.14 & 11.29 & 2.36 \\ C04 & 2.80 & 10.14 & 7.60 & 12.98 & 3.02 \\ C05 & 2.78 & 10.06 & 7.55 & 12.88 & 3.00 \\ C59 & 1.90 & 8.65 & 5.13 & 10.23 & 2.10 \\ C60 & 1.78 & 8.14 & 4.89 & 9.66 & 1.97 \\ C61 & 1.67 & 8.13 & 5.03 & 9.71 & 1.88 \\ Mean values & 2.15 & 9.12 & 5.97 & 11.11 & 2.37 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Orbit accuracy in Short-arc orbit determination for GEO satellites based on RDSS data (unit: m).  
The SLR measurements for C01 to C05 satellites are carried out intermittently lasting 2 months from 1 June to 31 July 2021 in Beijing. The experiments are performed to verify the orbit accuracy analysis in a radial direction over multiple days. Table 5 gives the SLR residuals of the orbit for the C01-C05 satellites based on RDSS data. Figure 13 gives the SLR residuals for the C01 and C02 satellites.  
Figure 11: SLR residuals of the orbit for C01 satellite.  
Figure 12: SLR residuals of the orbit for C03 satellite.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Satellites** & **2 July 2019** & **1 June to 31 July 2021** \\ \hline C01 & 0.55 & 0.56 \\ C02 & 0.57 & 0.54 \\ C03 & 0.51 & 0.50 \\ C04 & - & 0.55 \\ C05 & 0.62 & 0.57 \\ Mean values & 0.56 & 0.54 \\ \hline \hline \end{tabular}
\end{table}
Table 5: SLR residuals of the orbit for C01-C05 satellite based on RDSS data (unit m).  
The tables and figures show that the SLR residuals are greater than \(-1\) m but less than 1 m. The RMSs of the SLR residuals for C01 and C02 satellites are 0.56 m and 0.54 m, respectively. The RMS of the SLR residuals for GEO satellites in BDS-2 system is 0.54 m. By contrast, the accuracy of the RNSS phase data of the global network is about 40 cm. There are a large number of observation stations. With the development of the BDS, the improvement of observation accuracy of RDSS data, and the deployment of RDSS calibration stations on a wide scale, the orbit accuracy based on the RDSS data only for GEO satellites is expected to improve by a large margin.