## 3 Tests, Results, and Discussion
### BSSD PPP/INS/ODO TCI Attitude Determination  
The attitude errors of BSSD PPP/INS TCI with and without odometer aid are shown in Figure 18. The results of the roll, pitch, and heading angles in the first 500 s are significantly different from the results in other periods. That is because the vehicle kept static in the first 500 s, which provided no observabilities for gyroscopes. Then, the accuracy of attitude determination during these times mainly depended on the accuracy of initial attitudes. While the vehicle moved ahead, the motions increased the observabilities of gyroscopes and upgraded the accuracy of attitudes. According to the statistics in Table 6, the average RMSs of BSSD PPP/INS TCI are 0.025\({}^{\circ}\), 0.049\({}^{\circ}\), and 0.184\({}^{\circ}\) in roll, pitch, and heading angles, respectively. Visibly, the accuracies of roll and pitch angles are more precise than heading angles because of the poor observability of heading angles measured by the gyroscope in the vertical direction [43]. Furthermore, attitude RMSs calculated using different GNSS are close to each other. That is because the attitudes are majorly determined by the gyroscope and are slightly affected by GNSS positioning accuracy while GNSS observations are available [28]. Therefore, the attitude solutions obtained by using different GNSS systems are somewhat different in accuracy. According to the RMSs of BSSD PPP/INS/ODO TCI listed in Table 6, the average RMSs of roll, pitch, and heading angles are 0.026\({}^{\circ}\), 0.048\({}^{\circ}\), and 0.138\({}^{\circ}\). In contrast, the odometer provides about 25.00% in heading angle and invisible enhancements in roll and pitch angles. It is due to the observability improvement on the vertical gyroscope by adding an odometer. To assess the impact of the odometer on attitude determination at each epoch, we also provided the differences between the solutions with and without the odometer in Figure 19. From it, there are visible differences in the three directions at every epoch. It means that the odometer affects the estimation of roll, pitch, and heading angles. Such effects on pitch and roll would become invisible after using the statistic index (i.e., RMS). Anyway, to furtherly illustrate the influence of the odometer on attitude determination under GNSS outages. The attitude differences of BSSD PPP/INS and BSSD PPP/INS/ODO during the 1500 s and 2500 s are plotted in Figure 20. It can be seen that the humps of heading angles appearing at the profile of attitudes errors of PPP/INS TCI can be restrained effectively by the addition of an odometer. According to the average attitude RMSs in Figure 21, the reduction in attitude drifts can be significantly constrained by using an odometer. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{2}{c}{**BSSD PPP/INS TCI**} & \multicolumn{2}{c}{**BSSD PPP/INS/ODO TCI**} \\ \hline  & **Roll (\({}^{\circ}\))** & **Pitch (\({}^{\circ}\))** & **Heading (\({}^{\circ}\))** & **Roll (\({}^{\circ}\))** & **Pitch (\({}^{\circ}\))** & **Heading (\({}^{\circ}\))** \\ \hline GPS & 0.025 & 0.050 & 0.178 & 0.025 & 0.049 & 0.137 \\ BDS-3 & 0.027 & 0.049 & 0.194 & 0.028 & 0.048 & 0.140 \\ GPS/BDS-3 & 0.024 & 0.048 & 0.181 & 0.025 & 0.048 & 0.138 \\ \hline \end{tabular}
\end{table} 
Table 6: Attitude RMS of BSSD PPP/INS TCI with and without odometer aid. Figure 18: Attitude errors of BSSD PPP/INS TCI (**left**) and BSSD PPP/INS/ODO TCI (**right**) with different GNSS. Similarly, the differences between the attitudes calculated by the undifferenced PPP-based integration (Figure 22) and the BSSD PPP-based integration are plotted in Figure 23. The attitudes RMSs based on undifferenced PPP are 0.031\({}^{\circ}\), 0.048\({}^{\circ}\), and 0.135\({}^{\circ}\) in three components, which are close to the solutions based on BSSD PPP. As the result shows, the two solutions are close to each other actually in terms of statistics index (RMS) but different in time series, which is due to the accuracies of attitudes being mainly determined by IMU ([28,46]).