*4.2. Experiment B*

To fully demonstrate the performance of BDS-3 PPP-B2b/INS loosely coupled integration in an urban scenario, we conducted Experiment B in a complex urban environment. Experiment B was carried out from 09:00:00 to 09:49:59 on 5 December 2021 in GPS time. The trajectory of the land vehicle is shown in Figure 7.

**Figure 7.** The trajectory of the land vehicle in Experiment B.

Figure 8 presents the number of visible BDS-3/GPS satellites and the values of PDOP. The number of available BDS-3/GPS satellites dropped frequently, and at the same time the PDOP value significantly increased as several typical city features such as tall buildings, trees, and over-bridges appeared along the vehicle route.

During the first 35 min, there were enough satellites for the implementation of PPP-B2b positioning. The positioning errors of PPP-B2b only, PPP-B2b/MEMS-IMU loosely coupled integration and PPP-B2b/tactical-IMU loosely coupled integration are plotted in Figure 9. It can be found that all three schemes provided similar positioning accuracy in this 35-min period. The RMS values of positioning errors were 20.7 cm, 17.3 cm, and 47.6 cm in the east, north, and up directions for PPP-B2b only. For PPP-B2b/INS loosely coupled integration, the corresponding RMS values were 20.8 cm, 17.4 cm, and 47.5 cm for the PPP-B2b/MEMS-IMU scheme, and 20.3 cm, 17.4 cm, and 47.5 cm for the PPP-B2b/tactical-IMU scheme. It should be noted that a convergence period of 15 min was removed when we

computed the positioning accuracy. Clearly, the equipped IMU sensors could not improve the performance of the PPP-B2b service when there was no GNSS signal blockage.

**Figure 8.** Number of visible BDS-3/GPS satellites and PDOPs in Experiment B.

**Figure 9.** Positioning errors of PPP-B2b only (**top** panel), PPP-B2b/MEMS-IMU loosely coupled integration (**middle** panel), and PPP-B2b/tactical−IMU loosely coupled integration (**bottom** panel) during the period from 09:00:00 to 09:34:59.

From 09:35:00 to the end of Experiment B, the land vehicle crossed two over-bridges, as shown in Figure 10. Two GNSS signal outages appeared when the land vehicle crossed the bridges, lasting 15 s and 5 s, respectively. During these two GNSS signal outages, PPP-B2b positioning could be implemented, because the number of visible satellites was less than four. Therefore, the PPP-B2b positioning was re-converged after crossing the bridges. Figure 11 shows the positioning errors of PPP-B2b only, PPP-B2b/MEMS-IMU loosely coupled integration, and PPP-B2b/tactical-IMU loosely coupled integration. It can be seen that the positioning accuracy of PPP-B2b/INS loosely coupled integration exhibited better performance than PPP-B2b positioning during the re-convergence phase. The overall positioning accuracies for the three schemes were calculated from 09:35:00 to 09:49:59 and are presented in Table 5. The RMS values of positioning errors for the east, north, and up components were 114.2 cm, 200.0 cm, and 244.3 cm for PPP-B2b only. These figures improved to 41.0 cm, 44.5 cm, and 64.0 cm for PPP-B2b/MEMS-IMU loosely coupled integration, and to 41.2 cm, 41.0 cm, and 62.5 cm for PPP-B2b/tactical-IMU loosely coupled integration. Obviously, the performance of PPP-B2b/INS loosely coupled integration was significantly improved compared to PPP-B2b only in the last 15-min period of Experiment B. The positioning accuracy of PPP-B2b/tactical-IMU loosely coupled integration was slightly better than that of PPP-B2b/MEMS-IMU loosely coupled integration.

**Figure 10.** Two bridge scenarios during the period from 09:35:00 to 09:49:59.

**Table 5.** The positioning accuracy during the period from 09:35:00 to 09:49:59 (unit: cm).


**Figure 11.** Positioning errors of PPP-B2b only (**top** panel), PPP-B2b/MEMS-IMU loosely coupled integration (**middle** panel), and PPP-B2b/tactical−IMU loosely coupled integration (**bottom** panel) during the period from 09:35:00 to 09:49:59.

#### **5. Conclusions**

Since July 2020, PPP-B2b has served as a featured service of BDS-3 to support real-time PPP. Compared to the IGS RTS service, PPP-B2b is a satellite-based service, which is not limited by Internet communication. Its high-precision positioning performance shows great potential in real-time GNSS applications. However, the BDS-3 PPP-B2b service continues to encounter challenges in urban environments due to GNSS signal outages. In order to overcome the drawbacks of the BDS-3 PPP-B2b service in urban environments, we set up a BDS-3 PPP-B2b/INS loosely coupled integration system and evaluated its performance in different urban scenarios.

The experimental results indicate that PPP-B2b/INS loosely coupled integration cannot improve the positioning performance in open-sky environments where there are enough GPS/BDS-3 satellites for PPP-B2b positioning. However, PPP-B2b/INS loosely coupled integration can show its superiority when GNSS signal outage occurs. During GNSS signal outages, the INS mechanization can provide continuous positioning. Therefore, the performance of PPP-B2b positioning can be improved by adopting PPP-B2b/INS loosely coupled integration in GNSS blockage environments. In the first experiment, we simulated four 30-s periods of GNSS signal outages. At the last epoch of the simulated outages, the averaged

positioning errors in the east, north, and up directions were 300.0 cm, 498.0 cm, and 41.0 cm, respectively, for PPP-B2b/MEMS-IMU loosely coupled integration, and 18.6 cm, 21.8 cm, and 6.1 cm, respectively, for PPP-B2b/tactical-IMU loosely coupled integration. In addition, we also evaluated the performance of PPP-B2b/INS loosely coupled integration in a real complex urban environment. When the land vehicle crossed bridges, two GNSS signal interruptions appeared that lasted 15 s and 5 s, respectively. Compared to PPP-B2b only, the respective improvement of positioning accuracy in the east, north, and up components was 64.1%, 77.8%, and 73.8% for PPP-B2b/MEMS-IMU loosely coupled integration, and 63.9%, 79.5%, and 74.4% for PPP-B2b/Tactical-IMU loosely coupled integration.

It can be concluded that when there are sufficient valid satellites for PPP-B2b positioning, the positioning accuracy of PPP-B2b/INS loosely coupled integration depends mainly on the accuracy of PPP-B2b. In urban environments, positioning performance can be significantly improved compared to PPP–B2b alone, by adopting PPP-B2b/INS loosely coupled integration especially in GNSS signal blockage environments.

**Author Contributions:** X.X.; Conceptualization, methodology, software, visualization, and writing original draft preparation. Z.N.; Conceptualization, software, writing—review and editing, and funding acquisition. Z.W.; writing—review and editing, supervision, project administration, and funding acquisition. B.W.; validation and formal analysis. Q.D.; validation and formal analysis. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the National Natural Science Foundation of China (No. 42104011), the Shandong Provincial Natural Science Foundation (No. ZR2021QD069), the National Key Research and Development Program of China (No. 2019YFC1509205), and Sinopec Science and Technology Entry Program (JP210004).

**Data Availability Statement:** The datasets used in this study are managed by the College of Oceanography and Space Informatics, China University of Petroleum, Qingdao, China and can be available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
