# An Analysis of Satellite Multichannel Differential Code Bias for BeiDou SPP and PPP
## 2 BDS Satellite DCB and OSB Parameters  
Occurring at both satellite and receiver ends, DCB is caused by differences in the time delays of signals between different channels, and it can be subdivided into inter-frequency bias and intra-frequency bias [16]. The clock of each satellite or receiver is relative to a certain frequency or combination of frequencies. When calculating the satellite and receiver clock of different frequencies, DCB or TGD correction is required due to the differences in hardware delays. The clock reference for BDS broadcast ephemeris is based on the B3I signal [17]. Different analysis centers may choose different combinations of observation values when generating the precise clock and orbit products of different systems, which need to be distinguished in practical applications. At present, the precise satellite orbit and clock products of GFZ and Wuhan University (WHU) are based on the ionosphere-free combination of B1I and B3I for both the BDS-2 and BDS-3 satellites [18]. The BDS-2 satellites transmit public signals at frequencies of B1I, B2I, and B3I, while the B1I, B1c, B2a, B2b, and B3I signals are transmitted on BDS-3 satellites [19]. The new signal structure for the BDS makes it possible to generate code and phase observations based on one channel or a combination of several channels. In order to simply reflect the detailed characterization of the actual signal generation, three characters are employed to indicate observation code types (C/L/D/S), frequency bands or frequencies (1, 2,..., 8),and trace modes or channels (I, Q, etc. ); relevant information concerning all BDS signals is listed in Table 1.  
DCB parameters can usually be estimated together with ionospheric model coefficients from international GNSS service stations. The DCB products of the BDS are mainly currently provided by two analysis centers, i.e., the Wuhan Institute of Geodesy and Geophysics (IGG) of the CAS and the German Aerospace Agency (DLR). The CAS has been providing multi-system DCB products in the format of Bias SINEX (BSX) to the IGS based on the extended IGGDCB method since 2015 [20]. The DCB products provided by the CAS are updated daily with a latency of 2-3 days and contain 11 main DCB parameters that are listed in Table 2. The DCB products provided by the DLR are updated every three months, including GPS, GLONASS, Galileo, and BDS data. The types of DCB observed by the DLR include C1X-C2I, C2I-C5X, C2I-C8X, C2I-C6I, C2I-C7I, and C2I-C7Z DCB [21]. Although MGEX DCBs from the CAS and DLR exhibit consistency at the level of 0.3 ns and 0.4 ns for C2I-C7I and C2I-C6I, respectively [22], there is no specific analysis of the short- and long-term time-varying characteristics of the DCB and OSB parameters of BDS-3. Actually, the defined DCBs are not completely independent from each other. Taking the OSBs of BDS-3 as an example, the B3I signal is selected as the reference signal and data component of each signal to act as the reference observation type to characterize the intra- and inter-frequency code bias parameters in the form of absolute parameters [23]. In this way, the code bias parameters are independent and uncorrelated with each other. The code \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{c} BPSK \\ BPSK \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Information for BDS-2 and BDS-3.