# Impacts of Arc Length and ECOM Solar Radiation Pressure Models on BDS-3 Orbit Prediction
###### Abstract  
The BeiDou global navigation satellite system (BDS-3) has already provided worldwide navigation and positioning services for which the high-precision BDS-3-predicting orbit is the foundation. The arc length of the observed orbits and the solar radiation pressure (SRP) are two important factors for producing precise orbit predictions. The contribution studies the influences of these factors on BDS-3 orbit prediction. Three-month data from 1 July 2021 to 30 September 2021 are used to analyze optimal arc lengths and different ECOM SRP models for obtaining precise BDS-3 orbit predictions. The results show that the best-fitting arc length for the BDS-3 MEO/IGSO satellite is 42-48 h by comparing the final precise ephemeris and SLR validation. Furthermore, the ECOM9 SRP model shows improved orbit-prediction accuracy than that of the ECOM5 SRP model when the satellites move in and out of the eclipse season. As for the ECOM9 SRP model, the user range error (URE) accuracy of 6 h orbit predictions when satellites are in and outside of the eclipse season is 0.036 m and 0.030 m, respectively. In addition, the orbit prediction accuracy of the BDS-3 satellites does not decrease significantly since BDS-3 satellites apply the continuous yaw-steering (CYS) attitude mode during the eclipse season. BDS-3; precise orbit prediction; satellite laser ranging; eclipse season; ECOM solar radiation pressure model 2022143999  
the next 24 h. However, only certain parts of the predicted orbits are useful for real-time users since the IGU orbits are updated every 6 h with a latency of 3 h. Therefore, it is essential to study BDS-3-predicted orbits. The accuracy of the predicted orbits depends on the accuracy of the initial satellite conditions (satellite position and velocity at the initial epoch) and the accuracy of the solar radiation pressure (SRP) models [11]. The initial conditions of the satellite are affected by the fitted arc length of the observed orbit, and the satellite orbits are then predicted based on the initial conditions. [PERSON] et al. (2013) studied Global Positioning System (GPS) ultra-rapid orbit-prediction strategies and found that the optimal arc length of the observed orbits is around 40-45 h [11]. [PERSON] et al. (2015) assessed the impact of the arc length on GPS precise point position (PPP) solutions [12], and the results showed that the highest PPP ambiguity fix rates can be achieved when using predicted orbits with an arc length of 42 h. [PERSON] et al. (2018) analyzed the effect of the arc length of the observed orbits on multi-GNSS orbit prediction performances [13], and they found that the optimal arc length is 42-45 h. The optimal arc lengths for predicting orbits of GPS, GLONASS, BDS-2, and GALILEO satellites have been studied for many years; however, few existing manuscripts involve the same research effort for the BDS-3 satellites. How the accuracy of the BDS-3 predicted orbits varies with the arc length of the observed orbit needs to be uncovered. Therefore, it is necessary to study the optimal fitting arc length for BDS-3-predicted orbits. As one of the major error sources of satellite orbit determination, solar radiation pressure model errors significantly affect the precision of precise satellite orbit determination and prediction [14,15]. Numerous studies have conducted research on the SRP model of GNSS satellites [16, 17, 18]. However, the research on the SRP model is limited to precise orbit determination, and few are related to precise orbit prediction. The accuracy of GPS satellites could decrease to 300 m in 3 days because of the existence of solar radiation pressure perturbations [19]. Therefore, investigating different solar radiation pressure models in orbit prediction is necessary. Although the ECOM SRP model is a widely used SRP model in precise GNSS orbit determination [18], the ECOM SRP model is designed for GPS satellites, and its applicability to BDS-3 satellite orbit prediction needs to be further verified.