# Performance of BDS-3: Measurement Quality Analysis, Precise Orbit and Clock Determination
## 3 Results
### Clock Performance  
The onboard atomic frequency standard is a key component of a GNSS satellite. The stability of the satellite clock affects the accuracy of the clock offset modeling and prediction of broadcast \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Satellite** & **\# NP** & **Mean (cm)** & **STD (cm)** & **RMS (cm)** \\ \hline C31 & 4 & \(-\)19.6 & 0.4 & 19.6 \\ C32 & 20 & 25.0 & 10.7 & 27.1 \\ C33 & 21 & 9.8 & 18.3 & 20.7 \\ C34 & 6 & \(-\)3.3 & 10.4 & 10.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: SLR residuals for BDS-3 satellites C31, C32, C33, and C34. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **Satellite** & **Along** & **Cross** & **Radial** & **3D** \\ \hline \multirow{2}{*}{IGSO} & C31 & 36.1 & 18.6 & 5.9 & 41.1 \\  & C32 & 21.9 & 11.8 & 5.9 & 25.6 \\ \hline \multirow{2}{*}{MEO} & C33 & 54.3 & 28.1 & 14.2 & 62.8 \\  & C34 & 40.4 & 23.1 & 12.6 & 48.2 \\ \hline BDS-2 GEO & & 224.0 & 8.1 & 7.1 & 224.2 \\ BDS-2 ISO & & 17.9 & 11.2 & 4.6 & 21.6 \\ BDS-2 MEO & & 10.3 & 6.2 & 2.8 & 12.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Averaged RMS values of 48-h orbit overlap errors for BDS-3 and BDS-2 satellites (unit: cm). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Satellite** & **Along** & **Cross** & **Radial** & **3D** \\ \hline \multirow{2}{*}{IGSO} & C31 & 46.9 & 38.1 & 8.3 & 61.0 \\  & C32 & 38.7 & 34.5 & 7.5 & 52.4 \\ \hline \multirow{2}{*}{MEO} & C33 & 53.9 & 27.2 & 15.1 & 62.2 \\  & C34 & 50.4 & 37.0 & 14.1 & 64.1 \\ \hline BDS-2 ISO & & 41.7 & 29.4 & 7.2 & 51.5 \\ BDS-2 MEO & & 41.4 & 29.7 & 9.3 & 51.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Averaged RMS values of 48-h orbit overlap errors for BDS-2 satellites using only the reference stations with BDS-3 and BDS-2 simultaneous tracking capability (unit: cm). information and, thus, directly limits the user positioning and timing accuracy. Therefore, a high predictability of the onboard frequency standards is required to overcome this limitation and enable an extended validity of clock correction parameters. Since the early days of BDS, an improved accuracy of the onboard clocks has been striven for with every new generation of satellites. Compared to BDS-2, the primary frequency standard of BDS-3 satellites is based on the new RAFS and PHM made by Chinese technology. During the experiment period, the new rubidium clocks are active as the primary frequency standards in the C31, C33, and C34 satellites, and a PHM is active in satellite C32. To assess the performance of the BDS-3 onboard clocks in terms of frequency stability, modified Allan deviations (MADEV) computed using the 300 s clock estimates obtained from the above-mentioned precise orbit and clock determination are shown in Figure 8. Before the computation of MADEV, the outliers, clock jumps and frequency steps in the satellite clock time series were detected and preprocessed [34]. In all cases, the hydrogen maser at site XATT located in Xi'an (China) has been selected as a reference for the clock estimation. For BDS-2 satellites, at an integration time of 1000 s, the values of MADEV for all satellites are about 2-\(4\times 10^{-13}\). At an integration time of 10,000 s, the values are about 6-\(10\times 10^{-14}\). These results are consistent with those given in [PERSON], et al. [10] on the magnitude and relative size of each satellite, except for the C02 satellite, which is known to suffer clock adjustments. Note that the results of BDS-2 shown here are slightly worse than those of [PERSON], et al. [7] and [PERSON], et al. [11], likely due to the quality of the reference clock, the orbit and clock quality. The degraded clock performance of C06 shown in Figure 8 compared to the other IGSO satellites is also mentioned by [PERSON], et al. [35]. For BDS-3 satellites, the frequency stability of onboard clocks is better than that of BDS-2 on the whole, particularly for the two MEO satellites C33 and C34.