# Assessment of Affordable Real-Time PPP Solutions for Transportation Applications
## 4 Discussion  
At present, multi-GNSS positioning techniques are extensively used in the position, navigation, and timing (PNT) domain, being crucial for intelligent transportation system applications. A number of GNSS techniques, including RTK, PPP, and PPP-RTK, are employed for this purpose; however, both RTK and PPP-RTK approaches have the drawback of being expensive due to their dependence on two receivers or one receiver with network-based corrections. Furthermore, the PPP technique is usually applied in the post-processed mode. Consequently, we propose using ultra-low-cost RT-PPP scenarios in transportation applications, utilizing multi-GNSS ZED-F9P observations and SSR corrections from BKG, CNE, and WHU streams. Our proposed RT-PPP solution position estimates were validated against traditional differential kinematic counterparts, allowing for an overall statistical analysis of the two-dimensional positioning performance, including the mean bias error (MBE), RMSE, and standard deviation (STD). These statistical parameters were computed for the horizontal positioning components; they are summarized in Figure 10 for the RT-PPP and RTK processing strategies. Figure 10 shows that adding another satellite system (i.e., GLONASS, Galileo, or both) to the GPS solution improves the positioning accuracy, except for the RT-CNE-GR solution. This is due to the accuracy of the GLONASS SSR products obtained through the CNE stream. Additionally, the RT-PPP positioning accuracy is within 1-3 dm with three products; nevertheless, it is within a few centimeters for all RTK solutions, and notable enhancements in the horizontal components are achieved when the GE solution is used in all RT-PPP processing strategies. This is due to the Galileo SSR product's accuracy, which comes from the three analysis centers. For the RT-BKG processing model, the two-dimensional positioning accuracy is enhanced from 0.262 m to 0.216 m, 0.145 m, and 0.172 m when the G, GR, GE, and GRE solutions are utilized, respectively; in addition, using the GE and GRE solutions improves the horizontal positioning accuracy for the RT-CNE-G processing model from 0.317 m to 0.137 m and 0.227 m, respectively. Furthermore, the RMSE values of the RT-WHU-G solution are reduced from 0.311 m to 0.220 m, 0.173 m, and 0.192 m by RT-WHU-GR, RT-WHU-GE, and RT-WHU-GRE, respectively. For the RTK scenarios, the RMSE values are within the one-centimeter level, superior to the Galileo-based solution. Meanwhile, for the precision investigation, the STD values for all RT-PPP solutions are less than 0.177 m, with outperformance in the GRE scenarios; moreover, the STD values for the RTK models are less than 0.007 m.  
Figure 10: _Cont._The following conclusions can be drawn from the findings for the RT-PPP and RTK solutions:  
* A one-to-three-decimeter positioning accuracy level can be obtained using the RT-PPP solution, utilizing both the cost-effective ZED-F9P module and satellite orbit and clock correction products from the BKG, CNE, or WHU streams. * Incorporating GLONASS, Galileo, or both into the GPS RT-PPP solution improves positioning accuracy. * The BKG and WHU SSR products increase the GPS/GLONASS RT-PPP two-dimensional positioning accuracy by about 18% and 29%, respectively, compared with their counterparts in the GPS RT-PPP solution. * The enhancement percentages of the GPS/Galileo RT-PPP's 2D accuracy using orbit and clock products from BKG, CNE, and WHU are around 18%, 57%, and 44%, respectively, compared with their counterparts in the GPS RT-PPP solutions. * Compared with their counterparts from the GPS RT-PPP solutions, satellite orbit and clock products from the BKG, CNE, and WHU centers enhance the GPS/GLONASS/Galileo RT-PPP positioning accuracy by about 34%, 28%, and 38%, respectively. * BKG orbit and clock products offer the best solutions for the GPS and GPS/GLONASS/Galileo RT-PPP solutions, while CNE SSR products are superior for the GPS/Galileo  
Figure 10: Statistical analysis parameters for the RT-PPP and RTK solutions.