# Performance of BDS-3: Measurement Quality Analysis, Precise Orbit and Clock Determination
## 4 Conclusions  
The transmitted signals of BDS-3 satellites on B1 and B3 frequencies have been tracked by several GNSS monitoring stations from GA and iGMAS network, enabling a meaningful characteristic analysis and performance assessment of this newer generation of the GNSS constellation. We analyzed the observed C/N0 values of BDS-2 and BDS-3 B1 and B3 signals at two sites equipped with different receivers and antennas. The results indicate that the signal strength of BDS-3 exceeds BDS-2. Based on MP combinations, we investigated the multipath effect and receiver noise structure, as well as bias variations between pseudo-range and carrier-phase measurements. Different from other GNSS, the BDS-2 code measurements are affected by satellite-induced biases larger than 1 m from horizon to zenith, which will inevitably affect the BDS-2 included precise applications which involve the code measurements, e.g., ambiguity resolution based on the Hatch-Melbourne-Wubbena (HMW) combination [36; 37; 38]. Fortunately, these biases do not seem to appear in BDS-3 B1 and B3 code measurements.  
The BDS-3 orbit and clock parameters were estimated with a smaller amount of tracking stations, showing that an orbit overlap precision of 2-6 dm in 3D RMS and 6-14 cm in the radial component for four BDS-3 satellites is achievable. With a better spatially-distributed tracking network and less data loss, the accuracy of orbit estimates is expected to be considerably improved, reaching a comparable accuracy with BDS-2 IGSO and MEO satellites on the level of 1-2 dm. The new-generation RAFS and PHM are currently active as primary frequency standards for BDS-3 satellites. We evaluated the performance of the BDS-3 clocks using a modified Allan deviation. The results indicate that the frequency stability of BDS-3 clocks is improved by about 20-50% compared to BDS-2.  
The results obtained in this contribution are rather promising. A larger tracking network, in particular a network providing global coverage for the MEO satellites is an important step for the improvement of the BDS-3 orbit and clock solutions. However, there are still many related issues requiring further investigations to achieve better accuracy. First, the PCO and PCV of the BDS-3 satellites should be estimated and refined in the future. Second, the attitude control mechanism and solar radiation pressure model for BDS-3 should be tested and confirmed. In addition, the comparison of BDS-3 clocks performance for both long and short-term stability with the latest generation of RAFS and PHM of other GNSS, such as GPS and Galileo, is considered as an on-going research effort.  
The authors are very grateful to iGMAS, GA, ICS MGEX, and CMONOC for providing BDS-3 and BDS-2 tracking data. This work is supported by the National Nature Science Foundation of China (NOs. 41404032, 41674004, 41574030), and Fundamental Research Funds for the Central Universities (NO. 2042016 kf0185). [PERSON] provided the initial idea for this study. [PERSON] and [PERSON] designed and performed the data analysis and wrote the article. [PERSON] and [PERSON] supervised the experiments. [PERSON] helped with the data analysis.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**MADEV** & **BDS-2 GEO** & **BDS-2 IGSO** & **BDS-2 MEO** & **BDS-3 IGSO** & **BDS-3 MEO** \\ \hline
1000 s & \(2.80\times 10^{-13}\) & \(3.02\times 10^{-13}\) & \(2.99\times 10^{-13}\) & \(2.44\times 10^{-13}\) & \(1.89\times 10^{-13}\) \\
10,000 s & \(8.43\times 10^{-14}\) & \(9.65\times 10^{-14}\) & \(6.65\times 10^{-14}\) & \(5.91\times 10^{-14}\) & \(4.61\times 10^{-14}\) \\
1 day & \(8.94\times 10^{-15}\) & \(1.02\times 10^{-14}\) & \(7.98\times 10^{-15}\) & \(6.80\times 10^{-15}\) & \(3.59\times 10^{-15}\) \\ \hline \end{tabular}
\end{table}
Table 7: Statistics of frequency stability of BDS-2 and BDS-3 satellite clocks.