# _remote sensing_
## 4 Conclusions  
Based on the precise ephemeris products provided by COD, we studied the annual evolution of the orbit error, clock error, and SISRE of 24 MEO satellites of BDS-3 in 2022. At the same time, the orbit accuracy of D1 and B-CNAV navigation messages and the clock accuracy of the two new atomic clocks of BDS-3 MEO were compared and analyzed. Within this work, the performance quality of the BDS-3 MEO satellite space server was assessed, the difference between the D1 and B-CNAV navigation messages was obtained, and the annual evolutions of the accuracy of the orbit and the new atomic clocks was presented. It can be concluded that (1) the SISRE is mainly affected by the clock error, and it exhibits different evolution trends for different satellites, which is consistent with the evolution trends of clock errors. The annual RMS of SISRE was 0.556 m, and the annual RMS of 95%SISRE was 0.920 m. The SISRE_ORB of BDS-3 MEO is mainly affected by the orbit radial accuracy; it shows an evolution trend with high precision and high stability throughout the year, and its annual accuracy was 0.092 m. (2) For the orbit of the BDS-3 MEO broadcast ephemeris, its annual accuracy was the highest and most stable in the radial direction, and it had a certain degree of volatility in the cross and along directions. The orbit accuracies with respect to the three directions were 0.067 m, 0.273 m, and 0.297 m, respectively. Compared with the cross and along directions, the accuracy of the satellite orbit in the radial direction was an order of magnitude higher, and the 3D accuracy of the orbit was 0.450 m. The B-CNAV navigation message of BDS-3 MEO had obvious advantages with respect to orbit radial accuracy over D1, and its accuracy improvement ratio can reach 19.06%. This also produced obvious improvements with respect to the accuracy of SISRE_ORB, and its improvement ratio was 8.40%. (3) The annual clock error of the BDS-3 broadcast ephemeris had good performance, and its RMS was 0.541 m. For the new two types of atomic clocks, the RMS and standard deviation of hydrogen clocks were 0.563 m and 0.231 m, and the values of the rubidium clock were 0.519 m and 0.281 m, respectively. Obviously, the hydrogen clock is more stable than the rubidium clock. Due to the systematic error term in satellite clock errors, the accuracy of the hydrogen clock was lower than that of the rubidium clock.  
Overall, the SISRE of BDS-3 MEO is better than the reference standards. Compared with the D1 navigation message, B-CNAV has obvious advantages in the radial accuracy of the satellite orbit. The two new types of atomic clocks have good accuracy, and the systematic deviations in satellite clock errors need to be further studied and eliminated.  
Conceptualization, Z.D. and S.Z.; methodology, Z.D.; software, Z.D.; validation, Z.D.; formal analysis, Z.D.; investigation, Z.D.; resources, S.Z.; data curation, Z.D.; writing--original draft preparation, Z.D.; writing--review and editing, Z.D.; visualization, Z.D.; supervision, S.Z.; project administration, S.Z.; funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript.  
Figure 9: The improvement ratios of SISRE_ORB of each satellite of the B-CNAV navigation message. SISRE (orbit, clock, and bias errors) improvement ratio (purple) and orbit-only SISRE improvement ratio (green).  
**Funding:** This research was supported by the National Key R&D Program of China, grant number 2017 YFB0502700, and the National Science Foundation of Shanghai, grant number 19 ZR1459700.  
**Data Availability Statement:** Data are contained within the article.  
**Acknowledgments:** The ICS and TARC are greatly acknowledged for providing BDS-3 broadcast navigation and precise products.  
**Conflicts of Interest:** The authors declare no conflicts of interest.