# The Effect of BDS-3 Time Group Delay and Differential Code Bias Corrections on Positioning
## 4 Result and Discussion
### Performance of SPP
#### 4.1.2 Dual-Frequency  
Figures 15-18 separately show the positioning errors scatters for B1I/B3I and BIC/B2a SPP on 1 April 2020. It is obvious that the performance of SPP for dual-frequency BDS-3 also can achieve the positioning accuracy of the metre-level. The positioning errors of the TGD and DCB correction schemes are less than 5 m; however, the positioning errors of \"non-corr\" scheme are within 10 m. Interestingly, comparing positioning errors of dual-frequency SPP with that of single-frequency in Figures 5-11, we can draw that the positioning accuracy of SPP based on dual-frequency is slightly worse than that of single-frequency. In addition, in terms of noise impact, the noise of dual-frequency positioning error is significantly larger than single-frequency. Actually, this phenomenon is reasonable, when we realize that the observation noise of dual-frequency LC will increase significantly due to various interesting factors, and the noise amplification factor of B1I/B3I dual-frequency SPP scheme is 3.5 [20]. Moreover, the ionospheric delay correction model of BDS-3 adopts the BDGIM, and the BDS-3 satellites broadcast a set of ionospheric parameters per hour, which makes the accuracy of single-frequency ionospheric correction error better. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Scheme**} & \multicolumn{3}{c}{**B1I**} & \multicolumn{3}{c}{**B3I**} & \multicolumn{3}{c}{**B1C**} & \multicolumn{3}{c}{**B2a**} \\ \cline{2-13}  & **E** & **N** & **U** & **E** & **N** & **U** & **E** & **N** & **U** & **E** & **N** & **U** \\ \hline ddb-corr & 0.85 & 1.42 & 2.26 & - & - & - & 0.98 & 1.97 & 3.53 & 1.08 & 2.93 & 3.14 \\ tgd-corr & 0.79 & 1.39 & 2.25 & - & - & - & 1.26 & 2.23 & 3.70 & 1.05 & 2.88 & 3.11 \\ non-corr & 1.52 & 2.45 & 5.92 & 0.94 & 1.74 & 2.86 & 1.66 & 2.54 & 6.08 & 1.42 & 3.85 & 5.48 \\ Improvement (\%) & \multirow{2}{*}{44.1} & \multirow{2}{*}{42.0} & \multirow{2}{*}{61.8} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{40.9} & \multirow{2}{*}{22.4} & \multirow{2}{*}{41.9} & \multirow{2}{*}{23.9} & \multirow{2}{*}{24.0} & \multirow{2}{*}{42.7} \\ (dcb-corr/ & & & & & & & & & & & & \\ non-corr) & \multirow{2}{*}{48.2} & \multirow{2}{*}{43.3} & \multirow{2}{*}{62.0} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{24.1} & \multirow{2}{*}{12.2} & \multirow{2}{*}{39.1} & \multirow{2}{*}{26.1} & \multirow{2}{*}{25.2} & \multirow{2}{*}{43.2} \\ Improvement (\%) & \multirow{2}{*}{48.2} & \multirow{2}{*}{43.3} & \multirow{2}{*}{62.0} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{24.1} & \multirow{2}{*}{12.2} & \multirow{2}{*}{39.1} & \multirow{2}{*}{26.1} & \multirow{2}{*}{25.2} & \multirow{2}{*}{43.2} \\ (tgd-corr/ & & & & & & & & & & & & \\ non-corr) & \multicolumn{3}{c}{} & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: RMS in BDS-3 single-frequency SPP performance with different schemes (m). In further verification and analysis, the boxplot of the positioning errors distribution are with \"tqd-corr\", \"dcb-corr\" and \"non-corr\" schemes in April 2020, as given in Figure 19. As can be seen intuitively from the figure, there are very few outliers in different frequency combinations, which further prove the reliability of BDS-3 positioning. Furthermore, the positioning errors of dual-frequency (B1I/B3I, B1C/B2a) are basically consistent for the six selected stations. In addition, the medians and 75 th percentiles for the TGD and DCB correction schemes of the selected stations are within 10 m. In contrast, the medians and 75 th percentiles of the TGD and DCB unmodified scheme are much larger than the other two schemes, which further demonstrate the need to be corrected TGD and DCB parameters when positioning with BDS-3.