# Performance of BDS-3: Measurement Quality Analysis, Precise Orbit and Clock Determination
## 3 Results
### Orbit Validation  
We assess BDS orbit quality through both internal consistency and external validation. For internal consistency, the direct comparison of satellite positions in the overlapped time span from different orbit solutions is utilized in this study. For any two adjacent three-day solutions shifted by one day, there are 48-h orbit overlap errors. Figure 7 shows the RMS values of 48-h orbit overlap errors of every orbital arc during the experiment period for BDS-3 satellites C31, C32, C33, and C34 in along-track, across-track, and radial directions. As an unfortunate exception, the C32 satellite was in a test orbit from DOY 210 to 214 in 2016 and, therefore, no results of orbit validation can be shown. The RMS differences between the days are quite large (Figure 7), which is because of serious loss of data at some stations, especially for site THAT (Table 2) having only two days of tracking data available. In addition, the mean RMS values of orbit overlap comparison for each BDS-3 satellite and each type of BDS-2 IGSO and MEO satellite are listed in Table 4. From the results of orbit overlap comparison in Figure 7 and Table 4, the RMS values of the along-track component are the largest for all satellites and the radial component has the smallest RMS values due to the observation geometry. The averaged 3D RMS values are 33.4 cm and 55.5 cm, and the averaged RMS values in the radial component reach 5.9 cm and 13.4 cm for BDS-3 IGSO and MEO satellites, respectively. BDS-3 IGSO satellite shows smaller orbit overlap errors than MEO satellite, simply because there are better spatially-distributed BDS-3-capable tracking stations used in the POD and ground traces of satellites (refer to Figure 1).  
of the tracking stations are the main reasons why BDS-3 POD precision is relatively poor, as listed in Table 4. However, the averaged RMS values for BDS-2 MEO are still smaller than the BDS-3 MEO as listed in Table 5. This might be due to a lesser number of available observations for C33 and C34 satellites.  
For external validation, satellite laser ranging (SLR) residuals provide the opportunity to assess estimated orbit accuracy as they are based on an independent observation technique. BDS-3 satellites C31, C32, C33, and C34 are equipped with laser retro-reflectors and tracked by several SLR stations coordinated by the International Laser Ranging Service (ILRS) [33]. Since the length of a POD arc is three days, only the orbital solutions of the middle day are used for validation. For the time period considered in this study, only a very limited number of normal points (NPs) for BDS-3 satellites are available: one SLR station tracked C31 (Mt Stromlo: four NPs), two SLR station tracked C32 (Shanghai, Changchun: 20 NPs), three stations for C33 (Herstmonceux, Matera, Monument Peak: 21 NPs), and two stations for C34 (Shanghai, Yarragadee: six NPs). One has to keep in mind that the SLR validation primarily assesses the radial component of satellite orbit. The mean, standard deviation (STD), and RMS of the SLR residuals for the four BDS-3 satellites are listed in Table 6. The overall RMS values for C31, C32, C33, and C34 are 19.6 cm, 27.1 cm, 20.7 cm, and 10.9 cm, respectively, which are slightly larger than that of overlap comparison in the radial component.