# Impacts of Arc Length and ECOM Solar Radiation Pressure Models on BDS-3 Orbit Prediction
## 3 Results and Analysis
### Orbit Prediction Accuracy When Satellites Are during and Outside of Eclipse Season  
For the BDS-2 satellite, studies have shown that its orbit determination accuracy will decrease significantly during the eclipse period [20], but there are few studies analyzing the orbit prediction accuracy of the BDS-3 satellite during the eclipse season. Therefore,  
Figure 4: SLR residuals for BDS-3 satellites with prediction interval of 24 h (red circles represent SLR residuals).  
Figure 3: SLR residuals for BDS-3 MEO satellites with different arc lengths.  
in the following section, this study explores the orbit prediction accuracy of the BDS-3 satellite during the eclipse season. The BDS-3 IGSO and MEO satellites enter the eclipse season when the \(1\beta\) 1 angle (the elevation angle of the sun above the orbital plane) is less than 8.7\({}^{\circ}\) and 12.97\({}^{\circ}\), respectively [33]. Figure 5 shows the eclipse seasons for the BDS-3 satellites. The period of the BDS-3 satellites during the eclipse season is from 1 July 2021 to 30 September 2021.  
As shown in Figure 5, 18 BDS-3 satellites experienced the eclipse seasons during the selected three months. Day of year (DOY) 202 to 230, 2021, was selected, during which eight satellites (C19, C20, C21, C22, C32, C33, C41, and C42) were within the eclipse season. Meanwhile, DOY 243 to 273, 2021 was selected, during which the same eight satellites were outside if the eclipse season. Taking the selected eight BDS-3 satellites as an example, we analyzed the orbit prediction accuracy of BDS-3 satellites using different arc lengths when satellites move in and out of eclipses seasons. It should be pointed out that the selected eight BDS-3 satellites are all MEO satellites. Figure 6 shows the orbit prediction accuracy of different arc lengths when the satellites move in and out of eclipses seasons.  
It can be seen from Figure 6 that the arcs of 42-48 h length are the best fitting arc lengths for both satellites moving out of eclipse seasons and those into eclipse seasons. Taking the 48 h fitting arc length as an example, when the prediction intervals are 6 h, the URE increases from 0.0344 m out of eclipse seasons to 0.0419 m during eclipse seasons, which corresponds to an increase of 22%; when the prediction intervals are 24 h, the URE increases from 0.0592 m out of eclipse seasons to 0.0772 m in eclipse seasons, which corresponds to increases of 30%. Compared with the significant decrease in the orbital accuracy of the BDS-2 satellite moving in eclipse seasons, the BDS-3 satellites do not experience a significant decrease in their orbit prediction accuracy. The reason may be due to the advanced attitude control mode of the BDS-3 satellites. In addition to the yaw-steering (YS) mode, the BDS-3 MEO/IGSO satellites adopt the continuous yaw steering (CYS) attitude mode in eclipse seasons [20]. The CYS mode not only meets the requirements of the power supply and thermal control of the BDS-3 MEO/IGSO satellites but also avoids rapid yaw slews near the sun-spacecraft-Earth geometries. The results indicate that the proper attitude mode improved BDS-3's orbit prediction accuracy.  
Figure 5: Eclipse seasons for the BDS-3 satellites (1 July 2021 to 30 September 2021, red dots represent eclipse seasons).