Reliability of Real-Time Kinematic (RTK) Positioning for Low-Cost Drones' Navigation across Global Navigation Satellite System (GNSS) Critical Environments  
[PERSON]  
1 Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), Alma Mater Studiorum, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy; [EMAIL_ADDRESS]  
[PERSON]  
2 Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, 7500 AE Enschede, The Netherlands; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), Alma Mater Studiorum, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy; [EMAIL_ADDRESS]  
Footnote 1: email: {trancesco.s.s.g. over at least two different frequencies. Besides the higher number of observables that can be exploited, the use of different signal frequencies enables more efficient strategies to solve cycle slips, fix phase ambiguities, and deal with the ionospheric delay using proper linear combinations of observables [7; 8; 9]. Therefore, compared to single-frequency receivers, the double-frequency ones have a major advantage in those applications where the rover operates in critical conditions or several kilometers from the master station. The availability of a second signal and the related observables is also an advantage in terms of the redundancy of the system to be solved to retrieve the rover's position. The use of drones has surged in the last decades in several applications. Besides the classical surveying applications in open environments, drones have been increasingly used for the monitoring of infrastructure such as buildings or bridges. Despite the use of high-resolution cameras, these operations are normally executed very close to the construction for inspection, thus facing problems of multi-path and a reduced number of visible satellites. These monitoring activities require, therefore, both outdoor and indoor localization set-ups. The transition between these two environments is the most critical step, where we move from mainly satellite positioning to the integration of other sensors (e.g., inertial, visual, and range measurements) to safely navigate the drone. In addition, these applications require precise real-time tracking of the drone, meaning a few centimeters of accuracy, where the full functionality of RTK positioning is mandatory. The failures of the positioning system can cause large errors, often not perceived by the pilot from the feedback received on the controller, which is a danger for the drone integrity and for the people in the surroundings. Besides the need to avoid collisions, precise localization is fundamental in several applications such as the close-range inspection of infrastructure, where cracks need to be precisely located. Since April 2018, the Ublox company (Thalwil, Switzerland) has produced and sold F9P-class receivers. These are double-frequency and multi-constellation GNSS receivers that enhance the RTK performances allowed by the previous generation of low-cost devices [10; 11; 12; 13; 14]. In that regard, F9P instruments represent the state-of-the-art device to track a UAV in critical situations, and they are also fully portable on small drones and relatively cheap and affordable in most deployable solutions. This paper aims to provide useful insights on the performance of these instruments in critical operational conditions, simulating situations where the number of satellites is strongly limited by the occlusions (or in some cases totally absent).