## 2 Navigation System  
### Gnss  
UASs are systems capable of precise navigation due to the presence of GNSS receivers on board. To date, only the following GNSS systems provide global coverage: Global Positioning System (US system), GLONASS (Russian system), Galileo (European system), and BeiDou (Chinese system). There are also regional systems, called SBASs (satellite-based augmentation services) that provide augmentation services to position accuracies through a network of fixed stations that constantly monitor GNSS signals and allow errors to be controlled within predefined thresholds for navigation applications even in critical situations. In particular, GPS signals, and in some cases GLONASS, are \"augmented' at the regional level by services such as SBAS WAAS (US regional service), SDCM (Russian regional service), EGNOS (European regional service), MSAS (Japanese regional service), GAGAN (Indian regional service), and Beidou (Chinese regional service). GNSS positioning can be achieved in different ways. In the field of aerial photometry, two techniques are most widely used, namely, so-called \"Point Positioning\" and \"Differential Positioning\". The point-positioning technique (absolute positioning of a single point in the assigned reference system, also known as \"Single Point Positioning-SPP\") is a method in which observations are made by a single receiver acquiring data from different satellite constellations; the position of the receiver is determined through the use of coded pseudorange measurements [35]. In the last decade, a technique called \"Precise Point Positioning\" (PPP) has emerged that provides position accuracies at the decimetre level. In PPP systems, the user's receiver does not use the clock and empirical data transmitted by GPS satellites but precise clock and satellite position estimates calculated and provided by an external network, such as that organised by the International GPS Service [36]. The differential positioning technique requires observations to be carried out simultaneously by at least two receivers. In the latter method, the so-called \"baseline\", i.e., the vector connecting the antenna centres of two receivers, is calculated. In the acquisition process, the receiver is referred to as the base receiver and the rover receiver. In the case the GNSS system is mounted on the UAV, it represents the \"rover\", while the \"base\" can be represented by a single GNSS or a multi-GNSS. In recent years, GPS has often been replaced by continuously operating reference station services (CORS) [37; 38]. With differential kinematic GNSS solutions, rover positioning can reach an accuracy of a few centimetres [39]. To achieve high-precision real-time positioning using GNSS, several signal enhancementtechniques have been developed, such as real-time kinematic, RTK, and post-processed kinematic, PPK. The latter two approaches, using different frequencies (L1-L2-L5 for GPS, L1-L2 for GLONASS, E1-E5a-E5b for Galileo, B1-B2l for BeiDou), allow phase ambiguities, atmospheric propagation delay, and receiver clock errors to be resolved. RTK positioning also requires a radio or Internet link between the base and the UAV platform, which is not always available due to link interruptions. On the other hand, in PPK positioning, data processing takes place at the end of the aerophotogrammetric flight mission and allows the accuracy to be improved through post-processing of the GNSS data collected directly on site [40]. Recently, hybrid variants have been developed (e.g., PPK-RTK) with the advantage that when the drone is no longer receiving real-time corrections [41], it continues the pre-set flight mission by taking all the planned photos; once the drone mission is completed, the photos are stored on a digital device, together with a *.RINEX (Receiver Independent Exchange Format) file, i.e., a file containing GNSS observations and navigation of the entire route taken by the drone. The data will then be post-processed to acquire the photo coordinates with high accuracy.