## 4 POD and the Atomic Clock's Frequency Stability Performance  
In order to analyze the POD performance of the BD3 satellites, about 100 stations in the iGMAS and IGS backbone network are used, which can track BD3 as well as the GPS and Galileo satellites. The sites distribution is shown in Figure 1. Note that, the dual-frequency IF combination of B1I and B3I is employed and a 72-h arc length is adopted in batch processing. The \"One-step\" method is utilized in the study and the POD strategy is similar to the method listed in Table 3 using data from DOY 030 to 090. Figure 8 shows the RMS values of two-day overlap orbit in the along-track, cross-track and radial direction. As few stations can track PRN (Pseudo Random Noise code), C36 (M17), and C37 (M18) signals now, the statistical data only contains the sixteen BD3 MEO satellites. We divide these satellites into two categories according to the manufacturers, that is, C19/C20/C21/C22/C23/C24/C32/C33 made by CAST and the remaining satellites made by SECM. Figure 7: The geometry-free ionosphere-free phase (GFIFP) combinations as a function of the epoch for the BD3 C20 satellite (B1C/B2a/B3I and B1I/B2a/B3I) as well as the GPS G06 and Galileo E01 satellites. The legends 1/2/5/6 in satellite C20 stand for B1C/B1I/B2a/B3I respectively, and 1/2/5 in G06 represent L1/L2/L5, and 1/5/7 in E01 stand for E1/E5a/E5b, respectively. Seen from Figure 8, the two-day overlap results are almost at the same magnitude, which are about 5.0 cm in the along-track, 2.2 cm in the cross-track, 1.5 cm in the radial and 5.7 cm in the 3D direction. Besides, the SECM satellites are a little better than the CAST satellites in overlapping accuracy. Among all the sixteen satellites, C25, C26 and C35 present the worse accuracies, compared with the other satellites. This may be related to the observation count used in POD processing. It should be noted that these three satellites were launched in the second half of 2018, later than other satellites. At present, only a few receivers can track these satellites due to the old firmware version of receivers. The detailed statistical results are listed in Table 6. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Satellite** & **PRN** & **Along-track** & **Cross-track** & **Radial** & **3D** \\ \hline \multirow{6}{*}{CAST} & C19 & 4.8 & 1.8 & 1.7 & 5.4 \\  & C20 & 5.7 & 2.0 & 1.9 & 6.3 \\  & C21 & 5.2 & 1.7 & 1.9 & 5.8 \\ \cline{1-1}  & C22 & 5.0 & 1.9 & 1.9 & 5.7 \\ \cline{1-1}  & C23 & 5.5 & 3.2 & 1.4 & 6.5 \\ \cline{1-1}  & C24 & 5.2 & 2.7 & 1.4 & 6.0 \\ \cline{1-1}  & C32 & 5.4 & 2.2 & 1.6 & 6.1 \\ \cline{1-1}  & C33 & 6.3 & 2.2 & 2.1 & 7.0 \\ \hline \multirow{6}{*}{SECM} & C25 & 10.9 & 6.1 & 2.5 & 12.8 \\  & C26 & 6.6 & 3.5 & 1.8 & 7.7 \\ \cline{1-1}  & C27 & 4.8 & 2.5 & 1.3 & 5.6 \\ \cline{1-1}  & C28 & 4.0 & 1.7 & 1.1 & 4.5 \\ \cline{1-1}  & C29 & 4.5 & 2.4 & 1.3 & 5.2 \\ \cline{1-1}  & C30 & 4.9 & 2.2 & 1.3 & 5.5 \\ \cline{1-1}  & C34 & 3.6 & 1.9 & 1.1 & 4.2 \\ \cline{1-1}  & C35 & 14.1 & 10.4 & 4.3 & 18.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The results of the two-day overlap comparison of BD3 medium Earth orbit (MEO) satellites in the along-track, cross-track, radial and 3D directions (units: cm). Figure 8: The RMS values of the BD3 MEO satellites’ two-day (2 days) overlap comparison in the along-track, cross-track and radial directions. The left panel presents the results of the satellites made by CAST and the right panel is the results of satellites made by SECM. Satellite Laser Ranging (SLR) is an optical system and widely used in orbit determination and orbit validation, which can be regarded as an external measurement method to evaluate the range between satellites and laser stations. It is a two-way ranging instrument. In general, the laser is transmitted by the SLR stations and reflected by laser retroreflector array (LRA) which was installed on the satellite. Then, the range can be calculated through multiplying the time interval between laser transmitting and receiving by the vacuum speed of light.