## 5 Conclusions  
In this contribution, we implied the tightly coupled integration of BDS-3/GPS, low-cost IMU, and odometer based on the inter-satellite differenced PPP model and the orbit/clock corrections of PPP-B2b. A vehicle experiment in urban circumstances was implemented to validate the performance of positioning and attitude determination of the developed model. The following conclusions can be obtained. (1) With the addition of INS, the improvements of BSSD PPP position accuracy on average are more than 31.2%, 23.3%, and 27.3% in the north, east, and down directions. Further enhancements in position accuracy are achievable with the aid of an odometer, especially while suffering GNSS outages. (2) By using the odometer, the accuracies of pitch and heading angles are improved by about 2.04% and 25%. (3) In comparison with the PPP/INS/ODO TCI based on the undifferenced PPP model, the developed BSSD model can provide results with higher accuracy, especially in the re-convergence periods. For attitude determination, comparable results can be obtained by both the BSSD model and the undifferenced model.  
**Author Contributions:** Conceptualization, Y.M. and Z.G.; data curation, Y.M., Q.X. and R.L.; funding acquisition, C.Y. and Z.G.; investigation, Y.M. and Z.G.; software, Y.M. and Z.G.; visualization, Y.M. and J.L.; writing--original draft preparation, Y.M.; writing--review and editing, Y.M., Z.G. and C.Y. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was partly supported by the National Key Research and Development Program of China (Grant No. 2021 YFB3901301) and the National Natural Science Foundation of China (NSFC) (Grant No. 42274022).  
**Data Availability Statement:** The datasets adopted in this paper are managed by the School of Land Science and Technology, China University of Geosciences Beijing and can be available on request from the corresponding author.  
**Acknowledgments:** The authors would like to thank anonymous reviewers who gave valuable suggestions that helped to improve the quality of the manuscripts.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Mean RMS (cm)**} & \multicolumn{3}{c}{**Mean Maximum Drifts (cm)**} \\ \cline{2-7}
**Positioning** & **North** & **East** & **Down** & **North** & **East** & **Down** \\ \hline BSSD PPP & 64.33 & 53.47 & 154.11 & 107.96 & 59.90 & 78.22 \\ BSSD PPP/INS TCI & 44.54 & 41.29 & 121.66 & 88.52 & 54.54 & 62.00 \\ BSSD PPP/INS/ODO TCI & 44.01 & 40.79 & 118.83 & 66.29 & 45.84 & 49.17 \\ Undifferenced PPP/INS/ODO TCI & 47.69 & 42.09 & 119.15 & 82.38 & 50.66 & 52.30 \\ \hline  & \multicolumn{3}{c}{**Mean RMS (\({}^{\circ}\))**} & \multicolumn{3}{c}{**Mean Maximum Drifts (\({}^{\circ}\))**} \\ \cline{2-7}
**Attitude** & **Roll** & **Pitch** & **Heading** & **Roll** & **Pitch** & **Heading** \\ \hline BSSD PPP/INS TCI & 0.025 & 0.049 & 0.184 & 0.047 & 0.077 & 0.313 \\ BSSD PPP/INS/ODO TCI & 0.026 & 0.048 & 0.138 & 0.049 & 0.076 & 0.174 \\ Undifferenced PPP/INS/ODO TCI & 0.031 & 0.048 & 0.135 & 0.058 & 0.082 & 0.163 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Statistic of position and attitude errors.