# 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 temporal variations in DCBs for the signals of BDS-2 and BDS-3 from the CAS are shown in Figures 1 and 2, respectively. Each satellite is represented by a different color in the sequence diagram, and the satellites in bold font are IGSO satellites (i.e., C07, C08, C09, C10, C13, C16, C38, C39, and C40). As can be seen from Figure 1, there is no obvious fluctuation in the DCBs of C2I-C7I. In January 2021, the DCBs of C2I-C6I for the IGSO satellites jump up about 4 ns to 13 ns, while those of MEO and GEO satellites drop by about 2 ns. We learned through consultation that it was due to a change in the carrier-to-noise ratio (C/N0) of B3I that corresponded to several IGSO satellites of BDS-2 for a period of time in early 2021. There will be a maximum variation of about 10 dB from station to station, and anomalous changes in the C/N0 of these satellites (possibly from on-board adjustments or tests) lead to changes in the corresponding B3I pseudo-range observations, which in turn cause changes in the C2I-C6I DCB. The reasons for the inconsistent jumps between different satellites are mainly related to the different observation times, geometries, and orbit repetition periods of different types of satellites. As shown in Figure 2, the DCBs of C1P-C5P and C1P-C6I are more stable in 2020 than in 2021. The DCBs of C1X-C5X, C1X-C6I, C1X-C7Z, and C1X-C8X have good stability. The DCBs of C1P-C5P and C1P-C6I for every satellite show obvious jumps on Day of Year (DOY) 8 and DOY 90. This is because the DCBs of C1P-C5P and C1P-C6I for satellite C41-C46 were observed from DOY 8. The DCBs of C1P-C5P and C1P-C6I for C38, C39, and C40 were just obtained from DOY 21 to DOY 90 in 2021. Instead of daily DCB values, the DCB of each signal pair of each satellite stays the same for every ten days from DOY 101 in 2021 onwards. Figure 3 shows the temporal variations in OSBs for the three frequencies of BDS-2. The OSBs of C2I, C6I, and C7I remain stable from July to December 2020. In February 2021, the OSBs of C2I, C6I, and C7I for the six IGSO satellites (in bold), i.e., C07, C08, C09, C10, C13, and C16, decrease by 20.73, 31.39, and 20.73 ns on average, respectively. The OSBs of C2I, C6I, and C7I for the MEO and GEO satellites increase by 3.31, 5.01, and 3.30 ns on average, respectively. The fluctuations in the OSBs of the MEO and GEO satellites are consistent and obviously different from the fluctuations of the IGSO satellites. The variation in the B3I signal C/N0 value corresponding to the satellites of the IGSO of BDS-2 affects the B3I pseudo-range observation, which in turn affects the OSB estimation of C6I, leading to anomalous fluctuations in the OSB of the IGSO satellites. Figure 4 shows the temporal variations of the eight OSBs of BDS-3. The OSB variations in C1X, C2I, C5X, C6I, C7Z, and C8X tend to be consistent for all satellites. The OSB files provided by the CAS do not contain the OSBs of C1P and C5P for satellites C38 to C46 until 2021. As the observables of the new satellites are involved in the estimation of the OSBs, the OSBs of C1P and C5P have a significant jump in the beginning of 2021. All OSBs jump about 5 ns in February 2021, and the subsequent fluctuations are consistent with the changes in DCB. The fluctuation in the OSB of each satellite in this time period is mainly caused by the change in the B3I (C6I) pseudo-range signal. In the processing strategy of the OSB of the CAS, to ensure the derived OSBs are compatible with the IGS clock product, the constraint between the two signals, which is conventionally employed in the generation of satellite clock offsets, is  
Figure 1: Time series of CAS BDS-2 DCBs from July in 2020 to June in 2021. Figure 2: Time series of CAS BDS-3 DCBs from July in 2020 to June in 2021.  
applied [15]. When the C6I (B3I) OSB changes abnormally, it will also cause a change in the OSB of other signals. Thus, in February 2021, the OSB of all signals fluctuated. Figure 5 shows the mean values and STDs of DCBs and the combined OSB values of the BDS.