# BDS-3 Time Group Delay and Its Effect on Standard Point Positioning
## 5 Conclusions  
China's BDS-3 system is rapidly developing; from November 2017 to May 2019, 18 MEO, 1 GEO and 1 IGSO BDS-3 satellites (excluding experimental satellites) have been successfully launched. As a global system, BDS-3 has already provided basic services to the countries along the Belt and Road and to the neighbouring regions since the end of 2018. This work aims to investigate the impact of TGD on BDS-2, BDS-3 and BDS-2 + BDS-3 positioning. In this contribution, the single- and dual-frequency positioning accuracy of the BDS-3 signals were analysed. Six iGMAS stations, which can receive all the BDS-2 and BDS-3 signals at the current stage, with 41-day observation data, were selected to support our findings.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Scheme** & **B1I** & **B3I** & **B1I/B3I** \\ \hline \multirow{2}{*}{**E**} & tgd-corr & 0.86 & - & 1.18 \\  & non-corr & - & 0.90 & - \\ \hline \multirow{2}{*}{N} & tgd-corr & 0.85 & - & 1.08 \\  & non-corr & - & 1.07 & - \\ \hline \multirow{2}{*}{U} & tgd-corr & 1.84 & - & 2.17 \\  & non-corr & - & 2.56 & - \\ \hline \hline \end{tabular}
\end{table}
Table 7: The mean RMS values of all the SPP solutions with TGD correction (m).  
Figure 34: Daily RMSs of BDS-2 + BDS-3 at the BACH station with TGD correction.  
Single- (B1I, B3I, B1C and B2a) and dual-frequency (B1I/B3I and B1C/B2a) BDS-2, BDS-3 and BDS-2 + BDS-3 combined system signals were assessed with and without TGD correction. The results showed that the RMS value of B1I SPP with the \"tgd-corr\" scheme was increased by approximately 50% compared with the \"non-corr\" scheme based on BDS-2 satellites. For the positioning accuracy of all BDS-3 signals, the single- and dual-frequency-based SPP solutions without TGD correction present systematic errors deviating from the true positions. The accuracy of B1I SPP is improved by approximately 68.0%, 69.2% and 49.2% with TGD correction in the N, E and U components respectively, compared to SPP without TGD correction. The positioning accuracy of B1C SPP with the \"tgd-corr\" scheme is reduced by 64.8% from 1.18 m to 3.35 m and by 64.4% from 1.48 m to 4.16 m and by 51.2% from 2.91 m to 5.96 m in the E, N and U components, respectively. The RMS of B2a SPP is significantly reduced by 60.2% from 2.84 m to 1.13 m, by 55.4% from 3.59 m to 1.60 m and by 19.8% from 5.09 m to 4.18 m in E, N and U components, respectively. Compared with the \"non-corr\" scheme, the RMS of B1I/B3I SPP is reduced by 75%, 71.0% and 45.4% in the horizontal and vertical components, and the positioning accuracy can reach 1 m-2 m after TGD correction. The positioning accuracy of B1C/B2a SPP with the \"tgd-corr\" scheme is improved by 66.1% and 62.1% in the horizontal components. The N and E components can reach 1.66 m and 1.85 m, respectively, and the vertical component is improved by 43.3%, from 6.31 m to 3.58 m. The regional limitations of BDS-2 were overcome when BDS-3 satellites were put into use; hence, the global positioning accuracy of the BDS-2 + BDS-3 combined system is better than that of the BDS-2 or BDS-3. We expect that a better performance of BDS-3 can be reached when the global system completed.  
[PERSON], [PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments, analyzed the data, and wrote the paper; [PERSON] and [PERSON] helped in the discussion and revision.  
This work was supported by the National Natural Science Foundation of China (No. 11703033), Natural Science Foundation of Shaanxi (No.2018)Q1020) and Youth promotion committee of Chinese Academy of Sciences. The authors acknowledge iGMAS for funding.  
The authors gratefully acknowledge iGMAS and IGS for providing broadcast ephemeris, data, and satellite information.  
The authors declare no conflict of interest.