# An In-Depth Assessment of the New BDS-3 B1C and B2a Signals
## 6 Quality of Carrier Phase Observations Based on Zero Baseline
### DD Phase Measurements  
In order to evaluate the carrier accuracy of BDS-2 and BDS-3 on different frequency points, seven sets of double difference combinations were selected for evaluation. From Figure 14 and Table 16. It can be found that for BDS-2 satellites, the carrier accuracy of the B1I, B1I and B3I is about 2-2.5 mm, while MEO satellites have the highest accuracy among different types of satellites, followed by IGSO, GEO has the worst accuracy. For BDS-3 satellites, the carrier accuracy of the two frequency points of B1I and B2I was close, and the accuracy of MEO satellites was also the highest, followed by IGSO and GEO had the worst accuracy. As for the carrier accuracy of the same type of satellites at the same frequency, BDS-3 satellites are 0.3 m higher than BDS-2 satellites.  
Figure 13: Comparison of MP value between BDS-3 and other GNSS signals at the same frequency.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**GNSS** & **Type** & **Sat** &
\begin{tabular}{c} **Elevation** \\ **(Average)** \\ \end{tabular} & **MP12** & **MP21** \\ \hline
**BDS-3** & MEO & C24 & 39.30 & 0.18 & 0.16 \\
**GPS** & MEO & G04 & 33.86 & 0.27 & 0.12 \\
**Galileo** & MEO & E13 & 37.78 & 0.16 & 0.22 \\
**QZSS** & QZS & J01 & 47.53 & 0.23 & 0.11 \\ \hline \hline \end{tabular}
\end{table}
Table 15: CC statistics of different GNSS satellites.  
In this contribution, the double-difference values of C24-32 (BDS-3), C25-32 (BDS-3), G04-G16 (GPS), E13-E08 (Galileo) and J01-J03 (QZSS) on the same frequency (BIC/L1/E1/L1 (QZSS) and B2a/L5/E5a/L5) were compared. Figure 15 shows the double-difference comparison of BIC/L1/E1/L1 (QZSS) and B2a/L5/E5a/L5, from which we can see that the accuracy of the dual-frequency zero-baseline observation value of the navigation system at the same frequency point was very close to each other. In Table 16, the observation accuracy of BIC/L1/E1/L1 (QZSS) was in the range of 1.2-1.4 mm, while the accuracy of B2a/L5/E5a/L5 was slightly worse, which was in the range of 1.3-1.6 mm. The statistical characteristics of the accuracy of the single-frequency DD are very prominently shown in Table 16. The carrier phase accuracy of the B1/B2/B3 frequency was higher than 2 mm, while the accuracy of the new signals B1C and B2a was better than 1.5 mm, which was very close to the L1/E1 and L5/E5 signals of GPS, Galileo and QZSS. Therefore, when using the carrier phase for high-precision positioning, B1C and B2a are both better choices.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**GNSS** & **Sat** & **Type** & **B1I** & **B2I** & **B3I** & **B1C** & **B2a** \\ \hline \multirow{3}{*}{**BDS-2**} & C02 & GEO & 2.33 & 2.52 & 2.52 & & & \\  & C08 & K50 & 2.22 & 2.41 & 2.38 & & \\  & C14 & MEO & 2.07 & 2.27 & 2.22 & & & \\  & C24 & MEO & 1.97 & & 2.20 & 1.12 & 1.31 \\  & C25 & MEO & 1.84 & & 2.03 & 1.14 & 1.47 \\  & C38 & IGSO & 2.13 & & 2.29 & & \\  & C59 & GEO & 2.08 & & 2.25 & & \\ \multirow{3}{*}{**GPS**} & G04 & MEO & & & 1.26 & 1.41 \\  & E13 & MEO & & & 1.34 & 1.54 \\ \multirow{3}{*}{**QZSS**} & J01 & MEO & & & 1.20 & 1.35 \\ \hline \hline \end{tabular}
\end{table}
Table 16: DD observation statistics of different GNSS satellites.  
Figure 14: Comparison of DD values between BDS-2 and BDS-3 at different frequency points.  
Figure 16: BDS-3 zero-baseline calculation results at different frequency points.  
Figure 17: GPS zero-baseline calculation results at different frequency points.