# Impacts of Arc Length and ECOM Solar Radiation Pressure Models on BDS-3 Orbit Prediction
## 5 Conclusions  
Three-month data from 1 July 2021 to 30 September 2021 were used to analyze orbit-prediction accuracies under different conditions, including precise ephemerides and SLR data, and to study the effect of arc lengths on orbit-prediction accuracy. We also explore the orbit-prediction accuracy of different ECOM SRP models during and outside of the eclipse seasons. Based on the above results, we can reach the following conclusions. The final precise ephemerides and SLR validations show that the best-fitting arc length for the BDS-3 satellites is 42-48 h. In contrast to the BDS-3 MEO satellites, the BDS-3 IGSO satellites have diminished orbit-prediction accuracies when the fitting arc length is shorter than 36 h. The result shows that the orbital accuracy of the BDS-3 MEO satellites maintains a high level after 24 h of satellite orbit predictions. The UREs of BDS-3 MEO- and IGSO-predicted orbits are 0.051 m and 0.231 m, respectively. SLR validation shows  
Figure 9: Average RMS and URE of BDS-3 predicted orbits with different ECOM SRP models in eclipse seasons (24 h prediction interval). that the BDS-3 satellites have the best orbit prediction accuracy for the arc length of 42 h; thus, the average SLR residual is approximately 5.59 cm with a prediction interval of 24 h.  
Regardless of whether the satellites are within or outside of the eclipse seasons, the best fitting arc length is 42-48 h. Compared with the BDS-2 satellite's attitude control mode, the BDS-3 satellites adopt the CYS mode, which does not significantly reduce orbit-prediction accuracies during the eclipse seasons. When the arc length is 42 h and the orbit is predicted for 24 h, the UREs of the BDS-3 MEO satellite are 0.077 m and 0.059 m during and outside of the eclipse seasons, respectively. The experimental results show that the ECOM9 SRP model has the best orbit prediction accuracy during and outside of eclipse seasons. The UREs of the 6 h predicted orbit are 0.036 m and 0.030 m, respectively. Compared with ECOM7 and ECOM9 SRP models, the ECOM5 SRP model has worse orbit prediction accuracies during the eclipse seasons. Methodology, R.L. and C.Z. ; software, H.M. and J.W. ; validation, Y.Z. and G.Y. ; investigation, J.W. ; writing--original draft preparation, R.L. ; writing--review and editing, J.W. and H.Y. ; visualization, H.M.; supervision, J.W., Y.Z., G.Y. and H.Z. All authors have read and agreed to the published version of the manuscript. This work was supported by State Key Laboratory of Geo-Information Engineering and Key Laboratory of Surveying and Mapping Science and Geospatial Information Technology of MNR, CASM (No. * _[PERSON] et al. (2018)_ [PERSON]; [PERSON]; [PERSON]; [PERSON] Comparison of ultra-rapid orbit prediction strategies for GPS, GLONASS, Galileo and BeiDou. _Sensors_**2018**, _18_, 477. [CrossRef] [PubMed]
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