# On Mitigating the Effects of Multipath on GNSS Using Environmental Context Detection
## 4 Context-Aware Navigation (CAN)
### Environment Detection and Characterization Using CAN  
This section explains the strategy used by CAN to distinguish between different types of environments. In order to validate the performance of CAN in real-world scenarios, a field experiment was performed by enabling CAN in a GNSS receiver in quad-constellation mode. The experiment was performed on the same route and at the same time as used for the performance evaluation study in Section 2.1 above. Although it is difficult to maintain the exact same trajectory, we attempted to make it as close as possible by driving along the specific lane of the road over the same observation time. It should be noted that the total field experiment time was kept the same (i.e., 80 min) with and without using CAN; however, the times of each observation window (i.e., P1, P2, P3 and P4) and vehicle lane slightly differ from those used previously due to the overall route length, traffic density and vehicle speed. The context detection along with the categorization results are shown in Figure 7. Figure 7a shows the instantaneous and average number of satellites, Figure 7b represents the change factor (CF) in each observation window, and Figure 7c shows the context classification/categorization. The environment is categorized as standard, degraded and highly degraded based on the parameter values given in Equation (1). If a mitigation strategy is activated using CAN, it will only be initiated for the degraded and highly degraded cases because the standard environment has good accuracy and is taken as a reference to differentiate between the degraded and highly degraded environments. In Figure 7c, the thick green horizontal line is the region of the standard environment, and the green markers indicate the points when the vehicle environment was characterized as standard by CAN. Similarly, the thick purple horizontal line shows the degraded environment region, and the blue dotted markers indicate the time of the degraded environment, whereas the thick pinkish horizontal line is the highly degraded region, and the red markers indicate the points when CAN declared it as a highly degraded environment. In Figure 7c, from 0 to 16 min, 27 to 34 min and 61 to 77 min, the environment is mostly characterized as standard because the average number of satellites, \(nSV_{avg}\), is greater than 30 (Figure 7a), and the CF is less than 0.2 (Figure 7b) during these time slots. This means that the moving vehicle may have direct LOS contact with a majority of satellites locked by the receiver, which can lead to good positioning accuracy. On the other hand, between 17 and 26 min, 40 and 45 min and 47 and 60 min and at the 35 th, 37 th, 78 th and 79 th min, the environment is characterized as degraded by CAN because, as shown in Figure 7a,b, the average number of satellites, i.e., \(nSV_{avg}\), falls between 15 and 30 leading to a CF value of greater than 0.2 and less than 0.6, respectively, for most of the time. The degraded environment is usually a semi-urban or urban area where the blockage has a limited effect when using all four constellations. This is because there always exists a sufficient number of satellites that a GNSS receiver can hold lock on. However, multipath/NLOS still remains a major hurdle in degraded environments for accurate localization and navigation. The CAN algorithm leverages the redundancy of range measurements when using multi-constellation GNSS because it provides the user with a much greater choice of signals and, thus the accuracy can be improved by selecting only those signals that are least contaminated by multipath environments and NLOS to improve the positioning accuracy. The last case is the environment that is characterized as highly degraded by CAN at some points in P3. This type of environment may be a dense and congested urban area with blockages or high multipath/NLOS signal reception.