# Precise Orbit Determination of BDS-2 and BDS-3 Using SLR
## 3 Processed Data and SLR Validation
### SLR Validation  
As a unique geodetic survey technology without ambiguity, the SLR technology has always been used to validate the GNSS microwave-based orbits externally and to investigate the deficiency of the error model in the microwave-based orbit determination [20; 25; 26; 27; 28; 29; 30]. In this contribution, the motivation of the SLR validation is to flag and eliminate the worse SLR observations, which can improve the precision and stability of SLR-only orbit determination. In Table 3, the percentage of qualified SLR observations is over 95.5% for all BDS satellites, with the number of available SLR observations of 237, 554-972, 1712, and 808-1037 for BDS-2 GEO C01, BDS-2 IGSO, BDS-2 MEO C11, and BDS-3 MEO satellites respectively. The RMS of SLR residuals is 19.0 cm, and the mean offset is -7.5 cm for the unique BDS GEO C01. As the only BDS-2 MEO satellites, the RMS of C11 is 3.4 cm with the mean offset of 0.9 cm. The SLR residuals pattern are no significant variation for BDS MEO satellites in the selected period, including in the YM period. While the orbit accuracy becomes worse during eclipse period for the three IGSO satellites, especially in the YM period due to the inaccuracy attitude modelling. But they still have the credible RMS of 5.2-7.3 cm, with the mean offset of -4.2--1.8 cm. Notably, the accuracy of the new microwave-based precise orbit derived from WUM has a significant improvement by almost 30 cm for GEO C01 comparing with the previous SLR validation results before March 2018 [20]. Moreover, the SLR residuals do not decrease significantly during the eclipse period for MEO C11, even during YM period. Figure 3: The contribution of each SLR site to the daily SLR observations of all BDS satellites. Similarly, the time series of SLR residual patterns do not have significant deterioration for BDS-3 during the eclipse period, as well as during the YM period, as shown in Figures 4 and 5. And their overall mean offset and RMS of BDS-3 MEO satellites is -1.4-0.3 cm and 4.4-5.7 cm respectively. The excellent SLR validation results of the WUM's microwave-based orbits are attributed to the improved microwave-based orbit determination algorithm, especially the Yaw attitude modelling, and some systematic error compensation for BDS satellites, particularly for GEO C01 [31; 32]. In addition, no orbit accuracy decreases for BDS-3 occurs in the eclipse period, especially in the YM period. An important factor is caused by the different attitude control modes between BDS-2 and BDS-3 satellites in the YM period. The empirical Normal Yaw Steering (NYS) mode of BDS-3 replaces the Orbit Fixed Yaw (OFY) mode of BDS-2 in this period. The experienced manoeuvring only occurs in noon- and midnight-point, as in an approximately range of \(\beta\in[-3^{\circ},\ 3^{\circ}]\), for BDS-3 satellites [31]. Figure 4: The SLR validation time series for BDS-2 satellites derived from WUM in the half-year time series since 2019. The blue scatters, green scatters, magenta scatters, and red line refer to the Sun elevation angle of \(\beta\), the SLR residuals during the non-eclipse period, the Normal Yaw Steering (NYS) of the eclipse period, and the Yaw Manoeuver (YM) period respectively. The definitions of these concepts for BDS-2 satellites were summarized and described in [PERSON], [PERSON], [PERSON], Gao and Guan [20], and we apply exactly the same SLR validation strategy in this contribution.