# On Mitigating the Effects of Multipath on GNSS Using Environmental Context Detection
## 4 Context-Aware Navigation (CAN)
### Mitigating Multipath/NLOS Effects in Different Environmental Contexts
It is the most difficult to apply mitigation models to this type of route as the environmental context changes in a matter of seconds, e.g., the receiver may be working in one environment to apply the mitigation strategy when the context changes to another. In order to effectively counter this problem, the CAN algorithm uses 1 min intervals for estimating the parameter values (i.e., \(nS_{arg}\) and CF) for environmental characterization and then initiates the mitigation strategy accordingly. In the P3 observation window, the maximum RMS error was found to be 7.31 m as shown in Figure 8c, which was 14.3 m without using CAN in the same observation window as can be seen in Figure 3c. Similarly, the average RMS error using CAN dropped to around 2.93 m compared to 5.3 m without using CAN. This is a large improvement in the positioning accuracy. The PDOP values are almost same in the observation window P3 with and without using CAN. our comparison of the parameters used for the performance evaluation of CAN in different environments is summarized in Table 6 below. The table compares the parameters, including the satellites in-view, PDOP values and RMS errors. The degraded and highly degraded environments are the ones most affected by the multipath/NLOS signal reception resulting in reduced positioning accuracy. In the case of a highly degraded environment, the range errors were either large or there were not sufficient number of satellites available to estimate the position resulting in no position at all. Since the P3 observation window contains both the degraded and highly degraded environmental contexts, the P3 observation window was taken here as a case study to compare the overall performance of a dynamic mode GNSS receiver in achieving lane-level accuracy using a quad-constellation configuration with and without using CAN by considering the total number of positioning points during the P3 observation window. As mentioned earlier, the P3 observation window is a densely populated area with high-rise buildings, congested roads, flyovers and two cantilever bridges (Lansdowne Bridge). The window spans over 25 min (37-62 min) in Figure 3 without using CAN and is 22 min (36 to 58 min) when using CAN as can be seen in Figure 8. There are approximately 1500 measurement points in the P3 observation window if we take 25 min as the complete observation time in P3 with and without using CAN. The number of occurrences of RMS errors in the P3 observation window on a scale of 1 to 10 m is shown in Figure 9. In the case of CAN, more than half of the RMS errors fall within 2 m--consisting of almost 800 points out of 1500 measurement points. On the other hand, without initiating CAN, only 489 points out of 1500 measurement points suffered \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Site** & **Satellites in View** & **PDOP** & **RMS Error** \\  & **max/avg/min** & **max/avg/min** & **max/avg/min** \\ \hline P1 & 36/33/26 & 1.14/0.93/0.77 & 1.7/1.4/1.3 \\ P2 & 31/22/13 & 3.11/2.17/1.74 & 4.44/3.16/3.34 \\ P3 & 32/23/07 & 4.40/1.41/0.82 & 7.31/2.93/1.24 \\ P4 & 38/34/19 & 7.31/2.93/1.24 & 2.3/1.3/1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of the quad-constellation GNSS receiver performance with CAN enabled under different types of environments denoted by P1, P2, P3 and P4 observation windows. Figure 8: The performance evaluation results of a CAN-enabled GNSS receiver in quad-constellation mode in terms of fundamental quality indicators, i.e., (**a**) satellite availability, (**b**) PDOP and (**c**) RMS error over the entire experimental route containing different operating environments. errors less than 2 m while more than 1000 measurements suffered errors greater than 3 m, which is approximately 16 min out of the total route length of 25 min.