## 2 Materials and Methods
### Performance Evaluation by Simulation of the UAV Network  
#### 2.3.1 UAV Characteristics and Experimental Flight Tests  
A rotary-wing micro-UAV of a quadcopter [45] type was considered in this study. Thus, a DJI Mavic 2 Pro (DJI, Shenzhen, China) UAV was used. The main UAV's characteristics are presented in Table 3[46]. A flight altitude of 30 m was chosen based on the literature survey of the micro-UAV missions [47]. The UAVs can be operated remotely or in a pre-programmed way. The proposed UAV network is operated in a pre-programmed way and communicates with data exchange points (with each other) via wireless links. The DJI Mavic 2 Pro quadcopter uses eight high-resolution and two infrared sensors that allow omnidirectional obstacle sensing that determines the relative speed and distance between the UAV and the object and assures good stability in forward and hovering flight. Omnidirectional obstacle sensing includes left, right, up, down, forward, and backward obstacle sensing. The average charging time of a UAV battery is around 90 min. During this time, the UAV is locked in a UAV charging station. There are some techniques used to allow the battery-based UAVs to increase endurance [48], such as swapping the battery, laser-beam in-flight recharging, solar cells, wireless recharging, and tethered UAVs. An automated way of wire charging of the battery can be performed using a charging platform installed on the ground and one UAV retrofit-kit mounted on the UAV [49; 50]. The electricity needed at each station can be provided by a solar panel that charges a battery located at the station. The landing gear of the UAV can be electrically connected by touch with the charging platform after the UAV landing, and charging starts automatically. The main disadvantage of this system consists of locking the UAV on the ground during the battery charging. A battery swapping and recharging system in an automatic way was proposed to be used for the UAVs, similar to the one from [51]. The battery is automatically recharged after it is swapped at the station. Swapping time depends on the efficiency of the swapping mechanism having values of 15 s [51] to 60 s [52]. A maximum swapping time before the UAV is ready to take off of 60 s is considered. Usually, in practice, a minimum of 3-5 tests in the design of experiments are considered statistically relevant. Five preliminary flight tests were performed for the triangular-shaped flight mission in the Brasov area in Romania. The results of the flight tests were statistically analyzed, and the average values were used in the simulation of the UAV networks. Experimental flight times for each segment of the UAV flight mission were measured and used in the simulation of the UAV network. A Samsung S8 smartphone device with DJI Go \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Dimensions & \(214\times 91\times 84\) mm (length \(\times\) width \(\times\) height) \\ Max. Ascent/Descent Speed & 4 m/s; 3 m/s \\ Max. flight time (no wind) & 31 min (at a consistent 25 km/h) \\ Max. flight distance (no wind) & 18 km (at a consistent 50 km/h) \\ UAV battery & 3850 mAh, 1800 mA, 3.83 V \\ Weight with battery & 905 g \\ Approx. price & 1600 USD \\ Operating Temperature Range & 0–40 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 3: UAV parameters [46]. 4 app software (DJI, Shenzhen, China) installed on it, connected to a DJI remote controller (Figure 9), was used to remotely control the UAV. The preliminary tests were performed at 2 \({}^{\circ}\)C temperature, 69% humidity, and 3.5 km/h wind speed. The programming of the flight mission segments for preliminary flight tests was performed within the DJI Go 4 app software. The transfer between UAVs and stations is assured by a WiFi Arduino development board NodeMCU Lua WiFi, V3, ESP-12E, CP2102 (Espressif Systems, Shanghai, China) [53], Figure 10. ESP-12E offers a complete and self-contained Wi-Fi networking solution. The data (files) are stored on a micro SD card.