# An In-Depth Assessment of the New BDS-3 B1C and B2a Signals
## 5 Quality of Code Observation
### Code-minus-Carrier (CC)  
The typical theoretical accuracy of code observation is 1/100 of the chip width, and the range is generally from tens of centimeters to several meters. However, due to factors related to navigation satellites, propagation paths and receivers, the reduced accuracy causes degradation of user positioning. The combined observations of the two carriers and code pseudo-ranges are often used to evaluate the accuracy of code observations. The first linear combination observation is code-minus-carrier (CC). The basic observations in a GNSS receiver are the code and carrier phase. This difference of the range by code and range calculated by carrier phase (phase multiplied by wavelength) can be expressed as CC. The CC at a certain epoch \(t\) is expressed as follows:  
\[CC_{t}=\rho_{t}-\phi_{t}=2I_{t}-N_{t}+M_{pt}-M_{pt} \tag{2}\]  
where \(\rho\) and \(\phi\) is the code and carrier phase observation, respectively. \(I\) is the ionospheric delay. \(M_{pt}\) and \(M_{pt}\) include multipath and modeling errors in the code and carrier phase observations, respectively. This method makes a difference between the pseudo-range and carrier phase observations at a certain frequency and then eliminates the effects of the ionosphere by quadratic polynomial fitting to obtain CC observations [20; 21]. The statistical results of all GEO and IGSO satellites are shown in Tables 7 and 8, respectively. The CC of GEO satellites on the B3I frequency point is less than 0.2 m, while it is less than 0.1 m for IGSO satellites. The CC of both B1I and B3I of the BDS-3 GEO satellites was significantly improved compared to the BDS-2 satellites. On the other hand, the CC of the new signals B1C and B2a from the IGSO satellites are between the B1I and B3I, while the CC of B2a is significantly lower than B1C. There is a comparison of CC values between C14 and the C24 satellites in Figure 8. It can be found that the CC value on B3I is significantly lower than that of B1I and B2I, while the CC values on the B1I and B1C are significantly higher than the other two frequencies. The statistical results of all IGSO satellites are shown in Table 9. The CC value of B2a is the best with 0.13 m, and the CC value of B3I is sub-excellent with 0.14 m. The CC values of both B1I and B1C are higher than 0.2 m. By contrast, the CC values of the B1I/B3I of the \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & & **B1I** & **B1C** & **B2a** & **B2b** & **B2I** & **B3I** \\ \hline \multirow{3}{*}{GEO} & BDS-2 & 37.59 & & & & 40.34 & 42.62 \\  & BDS-3 & 42.82 & & & & & 44.57 \\  & BDS-2 & 43.35 & & & & 43.76 & 46.08 \\  & BDS-3 & 45.36 & 45.76 & 49.31 & & & 46.18 \\  & BDS-2 & 40.68 & & & & 43.02 & 44.93 \\  & BDS-3 & 44.42 & 43.87 & 47.23 & & & 44.67 \\ \hline \end{tabular}
\end{table} 
Table 6: \(\rm{C}/\rm{N}_{0}\) statistical results of all types of satellites. Table 10 shows the average values of CC for different types of satellites on different frequencies (B2b vacancies), including all BDS-2 and BDS-3 satellites. It can be found that the CC value of BDS-3 satellites on B1I is significantly higher than that of the BDS-2 satellites, while on B3I, there is a smaller degree of improvement. The CC value of the new B2a signal broadcast on IGSO and MEO of BDS-3 satellites is significantly lower than that of B1C (Figure 9). The B1C /B2a frequency of the BDS-3 MEO satellites has good interoperability with other GNSS. Therefore, the CC values on the same frequency are compared. In this contribution, the CC values of C25 (BDS-3), G04 (GPS), E13 (Galileo) and J01 (QZSS) on the same are compared. Figure 10 shows that the CC values of B1C, L1, E1 and L1 (QZSS) are relatively close, while the CC values of B2a, L5, E5a and L5 (QZSS) are also relatively close. At the same time, the CC value at the frequency point B2a (L5/E5a) is significantly lower than that of B1C (L1/E1) more than 0.3 m. Figure 11 shows the elevation changes of these satellites in one day.