# The Effect of BDS-3 Time Group Delay and Differential Code Bias Corrections on Positioning
## 4 Result and Discussion
### Performance of PPP  
As we mentioned earlier, the BDS-3 precise satellite clock and orbit products of WUM and BDS-3 DCB products of CAS are taken the B1I/B3I ionospheric-free LC as a reference at present. Thence, the \"non-corr\" scheme was only implemented for B1I/B3I PPP, and Figure 21 illustrates the PPP solutions B1I/B3I PPP performance for the selected stations. Additionally, Figure 22 shows the positioning errors of B1C/B2a PPP with and without DCB correction with each color representing the E, N and U components, respectively. Moreover, the positioning accuracy of PPP was calculated and summarized in Table 6. Combining Figure 21, Figure 22 and Table 6, we can obtain that the B1I/B3I PPP exhibits positioning performance similar to B1C/B2a. For dual-frequency PPP, the positioning accuracy of the horizontal component is up to 2-3 cm, and that of the vertical component is up to 3-4 cm. Due to the increasing contribution of the high-precision carrier phase in the  
Figure 19: Boxplot of 3D positioning errors for dual-frequency SPP distributions in the non-corr (green), tgd-corr (blue) and dcb-corr (red) schemes for the period of April 2020.  
Figure 20: Pseudorange residual scatters of B1I/B3I and B1C/B2a SPP with different schemes at BRCH station.  
subsequent epochs, the positioning accuracy in the horizontal and vertical components is within 0.1 m in the last. However, we can clearly see that the DCB parameter has a greater effect on the convergence time on the three components. Figure 23 gives the average convergence time and the improvement of BIC/B2a PPP with different schemes. The convergence time for B1C/B2a based on the \"dcb-corr\" scheme is significantly shorter than the \"non-corr\" scheme. Furthermore, the convergence time can be max increased by up to 50% when the DCB parameters are corrected for the six stations.  
Although the effects of differential code biases can be mitigated as time goes by, and similar positioning accuracy can be obtained after convergence, the bias will be absorbed and affected by other parameters. Take BRCH station as an example; Figure 24 illustrates the pseudorange residuals of BIC/B2a PPP obtained with or without DCB correction. The residuals of the \"non-corr\" scheme in Figure 24 shows significant systematic bias and a satellite-related offset range of several metres. By contrast, the residuals of the other schemes look more reasonable and behave like zero-mean random noise. For further research of the effect of code biases to the additional parameters, Figure 25 shows the time series of B1C/B2a PPP receiver clock obtained with or without DCB correction for BRCH and HMNS stations. Most of the unmodified code biases were absorbed into the receiver clock and other parameters. The differences of receiver clock of the \"non-corr\" scheme range up to 10-20 ns, compared with the \"dcb-corr\" scheme. Thence, this should be considered with great caution for time and frequency transfer applications based on BDS-3 high performance.  
Figure 21: Positioning errors of B1I/B3I PPP without DCB correction at six stations.  
Figure 22: Positioning error of B1C/B2a PPP with and without DCB correction at six stations (unit: m).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Scheme**} & \multicolumn{4}{c}{**B11/B31**} & \multicolumn{4}{c}{**B1C/B2a**} \\ \cline{2-7}  & **E** & **N** & **U** & **E** & **N** & **U** \\ \hline dcb-corr & - & - & - & 0.021 & 0.031 & 0.031 \\ non-corr & 0.023 & 0.028 & 0.039 & 0.032 & 0.029 & 0.038 \\ \hline \hline \end{tabular}
\end{table}
Table 6: RMS of BDS-3 PPP with and without DCB correction (m).  
Figure 23: Average convergence time and the improvement of B1C/B2a PPP with different schemes for six stations.  
Figure 24: Pseudorange residual scatters of PPP with different schemes at BACH station.