## 3 Testing and Results
### Test-1: Hovering by a Tunnel Entrance
Note the partial obstruction given by the building facade, which makes the operational conditions sub-optimal along the path back to the starting position, which simulates a real scenario. Figure 4: The chart reports the horizontal coordinates measured along the same path as in the test-1 runs. The black, blue, and red dots are used to represent fixed, floating, and single solutions, respectively. The (**top-left**) chart shows the runs from the starting position to the tunnel, while the trajectories of the way back to the initial position are reported in the (**top-right**) plot. Triangles show the positions of the last fixed solution approaching the tunnel or the first fixed solution on the way back to the initial position. The (**bottom**) chart focuses on the runs 5 and 21, which were affected by the false fixing of the phase ambiguities. The bottom chart in Figure 4 focuses on a couple of runs, namely the 5 th and the 21 st, where cases of the false fixing of the initial phase ambiguities occurred. These can be also identified in Figure 3 (orange zoom-in in the right sub-figure), where the elevation error reaches about two meters. False fixes can be spotted since the distance between these positions to the other points marked as \"fix\" are far above the decimeter level, which is incompatible with the known repeatability of fixed ambiguity solutions (see Figure 15 and Figure 16 in [28; 29; 30]). Both cases occurred while the cart was moving away from the obstacle, and they lasted until it started the following run. The coordinates are also strongly biased in the horizontal components, especially for run 21. Focusing on the time the system takes to lose and re-gain the fixing conditions, the histogram in Figure 5, on the left, reports the time (seconds) that passed between the start of the cart from its initial position and the instant the solutions switched from fixed to floating. On average, this time is 10 s (\(\sigma\) = 1.2). In Figure 4, top left, the positions where the solutions switch are marked using red triangles. This happens at variable distances from the entrance of the tunnel, mainly ranging from 0 to 5 m. The loss of fixing conditions before reaching the complete sky occlusion under the tunnel is due to the height of the building facade above the tunnel itself. Figure 5 on the right reports the histogram of the times that the receiver needed to fix again the solution while going back to the initial position. This delay ranges between 2 and 204 s, and on eight occasions, it is longer than 25 s. No correlation between longer fixing delays and a poor number of satellites on sight was found. Nevertheless, cutting off these cases, the average time to re-fix is about 9 s, with a 5 s variability in terms of the RMS. In Figure 4, the positions where the fixing conditions were re-obtained by the system are depicted as green triangles. From this image, the fixing happened only when the cart was already back to the starting position in many cases. In these cases, the system needed several seconds to re-fix again, probably because of both the presence of the main building facade, which partially obstructs the sky and the multipath caused by the metal shelter around the point.