## 3 Results and Discussion
### Radar Sensor Operation in Different Driving Situations
#### 3.2.1 Visual Verification of Maximum Range  
Figure 8 shows three excerpts from _Driving_. While no tunnels or roundabouts are shown, statistics from this category in Table 4 also include those durations, thus showcasing diverse, complex driving segments. While serving as followers for 7 h, trucks 2 and 3 drove with their respective preceding trucks within radar range for approximately 6 h. All excerpts in Figure 8 had the preceding truck within range, so trends for measured distances appear mostly continuous. The top excerpt stems from an old, narrow road section without centerlines. Trucks often adjusted their speeds, including when encountering opposing trucks, as revealed by reduced inter-vehicle distances. The middle excerpt stems from the traversal of a flat and wide high-quality road with a 90 km/h speed limit and gentle horizontal curves. Here, the trucks maintained constant distances over long time periods, and opposing trucks did not influence the platoon. The individual data points from truck 2 which are located below the general trend were chosen by the algorithm since the sensor did not detect other data points during that logging instance. In general, truck 2 had more such erroneous measurements than truck 3 did, but it is unclear why this is the case. The lower excerpt illustrates the descent of a challenging mountain pass (negative 6% gradient), where trucks 2 and 3 reduced their speed repeatedly, to avoid speeding and becoming located too close to their respective preceding truck. These excerpts suggest that truck platooning is more suitable on wide, modern roads than on old roads with adverse horizontal and vertical alignment. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Condition**} & \multicolumn{2}{c}{**Trucks**} \\ \cline{2-3}  & **2** & **3** \\ \hline _Driving_* & 85\% & 83\% \\ _Tunnels_ & 96\% & 95\% \\ _Roundabouts_ & 88\% & 89\% \\ \hline \hline \end{tabular}  
* _Driving includes Tunnels and Roundabouts._  
\end{table} 
Table 4: Proportions of curated radar data retained vs. _LWRR_-filtered aggregated event durations. Figure 9 shows three excerpts during which, for both trucks 2 and 3, the inter-vehicle distance appears to have occasionally exceeded maximum radar ranges. Blue (truck 2) and green (truck 3) horizontal lines illustrate the farthest distances detected, which differ somewhat between excerpts. Nonetheless, for radar sensors in trucks 2 and 3, respectively, the maximum distances are approximately 75 and 70 m, which support the aforementioned statistics-based radar range estimates. The three excerpts stem from two different mountain passes. The two upper excerpts correspond to traversal of Mountain pass A, with very difficult combinations of sharp horizontal curves and vertical gradients. Videos were also useful in inspecting the radar data after curation. The combination of long inter-vehicle distances and horizontal curves occasionally caused the preceding truck to be obscured by rock walls at the inner part of right-turn curves (pale shading). Mountain pass B, shown in the lowermost excerpt, was more forgiving in terms of road alignment. In the upper excerpt, the data from truck 3 (green) at timestamp 16:56 potentially reveals the presence of a phenomenon which, together with the high average SNR values measured at large distances, suggests that sensors are capable of measuring the preceding truck far away. It appears as if measurements which naturally belong to the top of the green curve are folded down, instead of occurring at 90-100 m, where extrapolation would place them. Thus, it looks like the radar in truck 3 does measure the preceding truck, despite it being located beyond the maximum range imposed by the chosen parameters. Stated otherwise, the radar appears to measure points beyond its unambiguous range, referring to the maximum distance a target can have while it can be guaranteed that the reflected pulse from that target corresponds to the most recent transmitted pulse [14,39].