## 2 Materials and Methods  
The aim of the paper is to test the GNSS navigation performance in extreme conditions, as already mentioned. It was therefore decided to test the navigation system in a \"controlled\" setting, avoiding the flight of the drone. This setting had two advantages: (i) to make the tests more easily replicable and (ii) to prevent possible crashes of the drone platform. The GNSS was disassembled, taking apart the drone's PX4 board, the RTK module, and the power supply (Figure 1), and then installed on a cart to better control the location and displacements. The cart was following predefined and reproducible paths, moving the receiver close to tall walls and under building tunnels (see the next section). This allowed us to repeat the same trajectories with an accuracy of a few (2 to 5) centimeters in the plane directions and less than 1 cm in the vertical one. This setting was considered sufficient to detect significant deviations (more than 10 cm) of the receiver localization during the tests. As mentioned, the GNSS board used in the test is the Ublox-F9P, built up and configured by the Holybro company (Hong Kong, China) in its \"H-RTK F9P\" kit ([[https://holybro.com/products/h-rtk-f9p-gsns-series](https://holybro.com/products/h-rtk-f9p-gsns-series)]([https://holybro.com/products/h-rtk-f9p-gsns-series](https://holybro.com/products/h-rtk-f9p-gsns-series)) accessed on 11 September 2024). In particular, the Helical antenna was chosen to be coupled to the rover receiver. In each test, the antenna position relative to the cart remained constant during data acquisitions. Possible rotations occurring on a real drone when accelerating and maneuvering were considered negligible since the Helical antenna used has no shield isolating signals coming from elevations lower than the cutoff angle. Moreover, for applications close to obstacles, a drone is likely flying at a slow speed with limited attitude changes. Figure 1: The Holybro Helical antenna mounted on the rover receiver based on the Ublox-F9P board, assembled with the power supply and radio transmission system all linked to the PX4 board inside the box. In the performed tests, we decided not to generate signal interruptions by acting in post-processing on the raw data [23; 24], but instead to simulate realistic working conditions to account for possible effects linked to multipath and changing sky visibility conditions. The data collection was carried out using the QGround Control software, version 4.2.0 [25], one of the well-known and most used packages for drone piloting, by recording the log files containing navigation parameters as they came from the RTK module. The master receiver was placed close to the test area (below 100 m distance) to avoid possible communication problems due to the radio connection. Moreover, the use of a master station as close as possible to the rover should ensure the most accurate estimation of the space-correlated GNSS biases. This makes the RTK operating conditions the best possible and leaves all possible sources of error to the inspected obstacles. Nevertheless, it is possible to navigate drones using Network RTK, which should provide corrections with similar quality and whose impact should be negligible compared to obstructions to the sky visibility and the multipath. Master's coordinates were set up using the QGround Control standard procedure to initialize the navigation system, namely the \"Survey-in\" mode, with 3 min of observation time. This procedure leads the rover's coordinates to be aligned to the WGS4 at the meter level of accuracy. Note that biases in the alignment to the reference frame due to the initialization process would equally affect all the coordinates of the same survey. Therefore, the intrinsic precision of the RTK positioning is preserved, whereas the surveys performed in different sessions cannot be compared at the centimeter level due to the re-initialization of the system.