# An Analysis of Satellite Multichannel Differential Code Bias for BeiDou SPP and PPP
## 4 Time-Varying Characteristics of DCB and OSB on BDS Satellites
### Stability of DCB and OSB Obtained from the CAS
The monthly stability of C2I-C7I DCBs is higher than that of C2I-C6I DCBs for BDS-2 in Figure 7. The stability of the DCB of C2I-C6I for the C07, C08, C09, C10, C13, and C16 satellites is worse in July 2020, January 2021, and February 2021. For the DCBs of BDS-3, the mean monthly STDs of every signal pair of the MEO satellites are all smaller than those of the IGSO satellites, which indicates the better monthly stability of MEO satellites. Overall, the DCBs of C1X-C5X and C1P-C5P for BDS-3 have the best and worst stability, with mean monthly STDs of 0.12 ns and 0.63 ns, respectively. The monthly stabilities of the C1P-C5P and C1P-C6I DCBs are similar. The overall stabilities of the C1X-C5X and C1X-C6I DCBs are consistent and good in Figure 8. The sum of STDs of C1X-C5X and C1X-C6I DCBs are less than 2 ns for all the displayed satellites except C36, C38, C39, C40, and C46. The maximum values of the monthly STDs of the C1X-C7Z and C1X-C8X DCBs are found for satellite C33, with values of 14.26 ns and 14.42 ns, respectively. The stability of the DCBs for satellite C33 is very poor for C1X-C7Z, C1P-C5P, C1X-C6I, and C1X-C8X, and the STD varies greatly from month to month. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**System** & **Signal** & **Mean (ns)** & **STD (ns)** & **Excluded Satellites** \\ \hline \multirow{3}{*}{BDS-2} & C2I & \(-\)15.02 & 2.54 & No \\  & C6I & \(-\)22.75 & 3.84 & No \\  & C7I & \(-\)15.02 & 2.57 & No \\ \hline \multirow{6}{*}{BDS-3} & C1P & 12.36 & 2.54 & C35, C41, C42, C45, C46 \\  & C1X & 12.94 & 2.36 & C35, C41, C42 \\  & C2I & 8.34 & 1.15 & No \\ \cline{1-1}  & C5P & 12.66 & 2.47 & C35, C41, C42, C45, C46 \\ \cline{1-1}  & C5X & 12.94 & 2.38 & C35, C41, C42 \\ \cline{1-1}  & C6I & 12.94 & 2.11 & C35, C41, C42 \\ \cline{1-1}  & C7Z & 12.92 & 2.85 & C35, C41, C42 \\ \cline{1-1}  & C8X & 12.93 & 3.11 & C35, C41, C42 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Mean values and STDs of the BDS OSBs. Figure 6: Mean values and STDs of the BDS OSBs. The monthly STDs of the raw BDS OSB values are shown in Figures 10-12. The monthly stabilities of the C6I, C2I, and C7I OSBs of BDS-2 remain highly consistent in Figure 10. The mean monthly STDs of the C6I, C2I, and C7I OSBs are 0.21, 0.33, and 1.69 ns  
Figure 8: Monthly STDs of C1X–C5X, C1X–C6I, and C1X–C7Z DCBs. Figure 7: Monthly STDs of C2I–C6I, C2I–C7I (BDS-2), and C2I–C6I (BDS-2) DCBs. Figure 9: Monthly STDs of C1P–C5P, C1P–C6I, and C1X–C8X DCBs. for the GEO satellites, 5.95, 9.01, and 6.75 ns for the MEO satellites, and 18.22, 27.59, and 18.63 ns for the IKSO satellites, respectively. The OSBs of the GEO satellites have the best monthly stability. The OSBs of the IKSO satellites of BDS-2 were less stable during July, January, and February. For the OSBs of BDS-3, the mean monthly STDs of the IKSO satellites are less than 1.33 ns and those of the MEO satellites are less than 3.45 ns. Overall, the OSBs of C2I and C8X for BDS-3 have the best and worst stability, with mean monthly STDs of 0.53 ns and 3.18 ns, respectively. Satellites C38-C46 started broadcasting the OSBs of C1P and C5P in 2021, as shown in Figure 11. Perhaps because the number of satellites participating in OSB calculations in January and February was not stable, the STDs of the OSBs of all satellites in January and February were higher than those of the previous month. The OSBs of the BDS-3 pilot components of the C1P signals are as stable as those of the C5P signals. The CIP and C5P OSBs for satellites C32, C33, C38, C39, C40, C41, C42, C45, and C46 fluctuate considerably during October, November, and January. The OSBs of mixed pilot and data components of C1X are as stable as the OSBs of C5X. The C1X and C5X OSBs of satellites C26, C32, C35, C36, C41, C42, and C46 were not stable between August and October. The OSBs of C6I and C2I for satellites C32, C41, C42, and C46 were not stable in August and October, as shown in Figure 12. In addition, the satellites with the larger monthly STDs of C7Z OSBs are basically the same as those of C8X OSBs.