### Structure and Configuration of UAS  
Recent technological development has enabled the construction and implementation of various UAV platforms. The numerous classifications of UAS are, in general, defined according to the different nomenclature adopted (civil and scientific use) for their capacity, size, and flight endurance. In 2010, the American Department of Defense (DoD) also classified UASs into five categories, as shown in Table 1.  
[PERSON] [28], according to several parameters such as endurance, range, altitude, and weight, classified UAVs into high altitude long endurance (HALE) UAVs, medium altitude long endurance (MALE) UAVs, tactical UAVs, small UAVs, and mini and micro UAVs. In addition, [PERSON] and [PERSON] and [PERSON] [29] proposed a further classification considering UAV platforms as unpowered and powered [30,31], as well as lighter or heavier than air (Table 2). Therefore, as shown in Table 1, several aerial platforms can be used in the field of photogrammetry; the most widely used platforms for acquiring photogrammetric datasets \begin{table}
\begin{tabular}{c c c c} \hline \hline
**UAV Category** & **Max Takeoff Weight (Gross)** & **Normal Operation Altitude (ft)** & **Airspeed** \\ \hline Group 1 & \textless{}20 pounds (9.07 kg) & \textless{}1200 AGL (365.76 m) & \textless{}100 knots (\textless{}185.20 km/h) \\ \hline Group 2 & 21–55 pounds (9.53–24.95 kg) & \textless{}3500 AGL (1066.8 m) & \textless{}250 knots (\textless{}463.00 km/h) \\ \hline Group 3 & \textless{}1320 pounds (\textless{}598.74 kg) & \textless{}18,000 MSL (5486.4 m) & \\ \hline Group 4 & \textgreater{}1320 pounds & \textless{}18,000 MSL (5486.4 m) & Any airspeed \\ \hline Group 5 & & \textgreater{}18,000 MSL & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: UAV categorization used by the American Department of Defense [27]. Figure 1: Historical evolution of drones. are single-rotor helicopters, multicopters, and fixed-wing aircraft. A multicopter is also called multirotor and can be considered a type of helicopter which has three or more propellers [32]. This basic design can give rise to various configurations. The various possible configurations must be realised in such a way as to balance the reaction torque by suitably alternating clockwise and anticlockwise rotating motors; furthermore, the configuration must not create problems with the centre of gravity and mass arrangements. The most popular multicopter is the quadcopter; it has four control inputs, which are the four propeller angular speeds. In general, the task of being able to take off and move through the air is entrusted to the propellers, usually two-bladed (sometimes three- and four-bladed). The propellers are then operated by means of a brushless-type DC motor that is controlled by a microprocessor and regulates the rotation speed; in the case of quadcopters, two motors move clockwise, and the other two anticlockwise. Other types of multicopter are hexacopters and octocopters: in the first case, the mechanism works with six motors where three work clockwise and three others move anticlockwise, while in the second case, power is provided by eight motors that transmit power to eight functional propellers. These latter configurations provide greater lifting power and stability than quadcopters, with the advantage of being able to land extremely safely [33]. A completely different category is fixed-wing UAVs, which are very similar in shape to an aeroplane and characterized by flight mechanics based on the lift of their wings. One of the advantages of this type of UAV is their longer flight times compared to multicopters, as well as their stability even in unfavourable wind conditions; in fact, they are mainly used in the field of land surveying and mapping, where flights can last up to an hour. Compared to multicopters, some fixed-wing models have totally different take-off and landing systems.