## 2 Materials and Methods
### Hardware, Software, and Methodology under Use Case  
Many test flights have been performed at Barcelona Drone Center (BDC) [25] testing site facilities. BDC, located in Moia (Catalonia, Spain), manages a Temporary Segregated Area (TSA) of 2500 ha specifically conceived for BVLOS flights. Their facilities are located in a rural area, without the ubiquitous 4G antennas and signal overlapping of urban areas, making it an excellent site to test coverage and performance indicators. Also, other flights were carried out in different locations of Catalonia to apply the developments with different traffic and infrastructure scenarios, such as roundabouts and roads. The implemented hardware and system architecture (namely EXO Data) have been integrated and developed by Exodronics, in collaboration with other institutions under the StratoTrans project. EACOM [26] has collaborated with telecommunications issues; UAVMatrix [27] provided a reference commercial product to develop 4G drone-to-ground services; the server to store the mission's telemetry data and the video hosting, has been developed in collaboration with NEXIONA [28]; the CV algorithms specifically designed for tracking elements from the video imagery where developed by Eurecat Multimedia department [29]. The aircraft used to carry out the telecommunication experiments is an EXO C2-L+ fixed wing developed by Exodronics [30]. This lightweight platform (1.3 kg Maximum Take-off Weight (MTOW)) provides around 75 min flight autonomy, which is appropriate to test under BVLOS conditions. It is, then, a specific case example of where it is appropriate to use such cellular network, capable to transmit FPV video, EO imagery and telemetry data from the aircraft via the 4G network. However, for hovering traffic monitoring in roundabouts, we used multicopter platforms (Figure 3). The specific hardware housed within the drone is a Pixhawk 2.1 Cube Black Flight Controller [31; 32], and a telemetry module with an antenna to locally radio-link (Spread Spectrum at 868 Mhz) the FC with the GCS. In the EXO Data implementation, an extra microcomputer (Raspberry Pi Zero W [33]), a 4G/LTE USB-stick a modem with a data rate of 150 Mbps [34], and a multioperator SIM card [35; 36] are used to connect the drone to Internet, with an aggregated extra weight of 40 g. The onboard power system is based on a Li-ion 4S 3.4 Ah battery and a power module that feeds independently the FC and the EXO Data hardware. The EXO Data system video cameras, compatible with the microcomputer ports, are a ZeroCam [37] for FPV video, and for downwards Earth Observation video cameras we use a PiCam V2 [38] or a remote sensing camera with video functionalities with a HDMI--CSI converter connected to the Raspberry video input port. The video encoding and packaging can be carried out with H264 compression (1:125 ratio), accounting for a video quality of 1280 \(\times\) 720 pixels, 720 p and 30 fps, which can be decoded and reconstructed in widely used video software (e.g., VLC, Gstreamer). The complete hardware implementation in the EXO C2-L+ is shown as follows (Figure 4). The implementation uses a secured VPN [39] to switch the 3 endpoints: The drone, the GCS and the EXO Data server. The telemetry link is based on a MAVLink decoders, with an open-source software in the GCS (Mission Planner [40]) that can be linked to the onboard FC thorough local radio-link or thorough TCP/IP protocol. Moreover, a specifically developed software [28] in the EXO Data server is used to store the telemetry  
Figure 4: **(A)** On board hardware devices. (**B**) Complete hardware implementation in the EXO C2-L+ fixed wing drone. (**C**) The implementation is adaptable to multicopters and fixed-wing drones, useful for different purposes. Figure 3: Complete scheme of the 4G cell network functions and connections. and imagery data in separated dockers. The complete implementation is detailed as follows (Figure 5).