## 4 Discussion  
We assume that the goal of the navigation system is to provide real-time positions that are accurate within the 10-centimeter level. The performed tests gave relevant information on the behavior of a state-of-the-art GNSS installed on a lightweight UAV. As is already  
Figure 10: Statistics related to test-3 about the time required to re-gain the re-fixed solution after having left the tunnel. Figure 9: Horizontal coordinates recorded during test-3 in fixed (black dots), floating (blue dots), and single-point positioning (red dots) conditions. The red and green triangles highlight the last and first fixed solution, respectively, for each run. known, only RTK GNSS modules allow such a positioning accuracy, while stand-alone (i.e., single-point) positioning solutions can only generate metric accuracies, if not worse. RTK will enable the real-time differencing of the observable phase so that kinematic baselines between the master and the rover receiver can be calculated. In that regard, it is fundamental for the pilot to have the real-time information concerning the status of the ambiguity fixing, since only with the set of phase ambiguities fixed as natural numbers can the system provide the required accuracies, while in the floating conditions, the errors can reach the meter level or more. In almost all the performed tests, a solution marked as an \"ambiguity fix\" provides reliable coordinates at a precision level of a few centimeters. In a few cases, tests evidenced the so called \"false fix\", meaning the cases when the statistical algorithm used to choose the set of natural numbers for the initial phase ambiguities failed. In these cases, despite the \"fix\" mark on the solutions, the given coordinates may have had errors at the meter level, making this condition one of the most dangerous for a drone flight close to obstacles. False fixes occurred during test-1 and test-2, which simulate the hovering condition of a drone, while no evident failure on the ambiguity fixing was observed in test-3. This is because a set of wrong phase ambiguities became statistically incoherent when the receiver changed its position: the moving condition prevented the system from keeping a false fix. In contrast, hovering drones in the same position led the GNSS system to ignore these inconsistencies: in this case, only the change in the satellite positions can reveal a wrongly defined set of initial phase ambiguities, but this takes several minutes. In principle, to shorten this time, the drone should move sufficiently to cause variations of a whole wavelength cycle along the distance between the receiver and all satellites: in this case, a wrongly estimated set of ambiguities could reveal its incoherency. In practice, this could be reasonably obtained with a displacement in the vertical direction larger than 0.5 m or a 1-meter shift in planimetry. According to the performed tests, there are no easy and reliable ways to recognize a false fix in real time. In most cases, like the ones highlighted in Figure 7, related to test-2, the real-time estimation of the positioning accuracy (which the software provides in terms of HRMS/VRMS) is a good proxy to determine false fixes. These parameters are correlated with the satellite geometry (DOP, number of satellites), in addition to multipath and other parameters, and they constitute an attempt to estimate the positioning quality. Thus, they can be exploited by setting a threshold to define the unsuitability of the RTK solution: from the performed tests, a 4 cm (see the green line in Figure 7 central plot) value could be a balanced choice to reject false positives without being too small to exclude reliable solutions, too. Reasonable values could range between 3 and 5 cm. A higher positioning quality should be guaranteed by a lower value, with the drawback of considering unreliable correct solutions with higher probability. In contrast, setting the threshold to five should move the balance to a less conservative choice, also accepting solutions more prone to significant errors.