# On Mitigating the Effects of Multipath on GNSS Using Environmental Context Detection
## 3 Performance Evaluation  
The performance evaluation results of a GNSS receiver in quad-constellation mode in terms of fundamental quality indicators, i.e., the satellite availability, PDOP and RMS error over the entire experimental route of Figure 1 with a complete observation period of 80 min are shown in Figure 3. Figure 3a shows the number of tracked satellites, Figure 3b represents the PDOP, whereas the RMS error is shown in Figure 3c. As already mentioned, the experimental route comprises a real-world driving profile consisting of a clear open-sky, semi-urban and dense urban areas with small lane widths, high-rise buildings, flyovers and bridges. The results in Figure 3 show that significant deviations may occur in the fundamental quality indicators (PDOP, RMS error, satellites in-view and average satellite availability) when navigating across different types of operating contexts. In observation window P1, an average of 34 satellites were locked by the receiver during the 8 min time-slot, and this was expected due to negligible obstruction at the route. The availability of a large number of satellites ensures more accurate and seamless navigation because redundant measurements can help in selecting satellites with strong signal strengths and thus constitute good satellite geometry. In fact, excellent PDOP values (average PDOP \(<\) 1) were observed during this observation window as shown in Figure 3b. The RMS error in P1 is depicted in Figure 3c. As expected, with adequate satellites in view and good geometry, the average RMS error was found to be 1.3 m with lower and upper bounds of 1 and 2.4 m, respectively. The results suggest that lane-level accuracy (\(\leq\)1.5 m) can be achieved at highways or suburban areas with standalone multi-constellation GNSS without any correction services. As the test vehicle moved closer to the city center and passed through Lloyd's Barrage (observation window P2), the quality parameters began to deteriorate rapidly due to the surroundings (i.e., the structure of the bridge) resulting in multipath/NLOS environments. The average number of tracked satellites reduced to 27 from 34 (Figure 3a), and the maximum PDOP value increased to 2.21 in Figure 3b. Although the satellite availability was adequate at this point (i.e., a minimum of 17 satellites were acquired), the positioning performance was degraded. Figure 2: Instrumented test vehicle containing a roof-mounted antenna along with a PolaR5s multi-frequency and multi-constellation GNSS receiver as used for field experimentation and data acquisition. The average and maximum RMS errors in Figure 3c for the P2 observation window were found to be 4.14 and 7.83 m, respectively; however, this level of accuracy is not sufficient in many cases. The degraded positioning performance in P2 was due to the multipath and NLOS signal reception because there was no significant drop in the PDOP value, which was less than 2 most of the time as observed in Figure 3b. The PDOP values indicate the impact of the spatial distribution of the satellites on the final accuracy of the positioning solution, and a scale of 0 to 10 is typically used to compare the severity of this effect. A PDOP value of less than 2 indicates excellent satellite geometry, a value between 2 and 4 is acceptable, and PDOP values of greater than 4 refer to a poor geometrical distribution of satellites. Based on the PDOP values, it can be concluded that multi-constellation GNSS enhances the probability of better satellite geometry and lower PDOP values; however, the effects of multipath and NLOS signal reception lead to increased RMS errors in the positioning solution as can be seen in Figure 3c in the P2 observation window. This shows that the PDOP alone is not a good indicator for evaluating the GNSS performance. In Figure 3, the third observation window, i.e, P3, highlights the GNSS performance in the dense urban areas with congested roads, high-rise buildings, towers and a cantilever bridge.