## 3 Prototype Description and Implementation
### Prototyping of Hardware Components
The base station will also enable the operator to program a series of alerts whenever telemetry values are abnormal (i.e., programming the return of the UAV to the takeoff point when the battery level is low). In addition to this, the display and analog video receiver provide the UAV operator with visual imaging of the terrain the aircraft is flying over, which are pivotal to carry out monitoring and grazing of livestock. This image is transmitted through a video system integrated in the UAV that will be described in the next section. Aside from the base station, which is mandatory to send and receive data, the actual UAV components used to build it up must be considered as well, so that a UAV that is compliant with the requirements previously formulated can be used. Such components are as follows:  
1. Frame F450 [28]: this frame is one of the most popular in UAV prototyping. Its main advantages are its light weight, its resistance to impacts and its ease of repair, which are of great usefulness for the application domain formulated in this paper. Furthermore, its size makes it a suitable option for assembling modular UAVs since it has more than enough room for the integration of many components. The material from which it is made of is reinforced plastic and its low cost are added values that make it the best option. 2. Navio2 controller [29]: the flight controller is the component in charge of ensuring that all the other ones that make up the UAV will work properly. Together with the Raspberry Pi 3B [30] on which it is integrated, the controller governs the modular systems integrated in the aircraft (that is to say, autonomous stabilization, GNSS navigation, flight by waypoints or displacement points, failsafe systems or rear door for greater security, camera control, UAV manual control functions). The Raspberry Pi 3B is also an extremely useful tool for hardware integration, as it offers several interfaces (four Universal Serial Bus -USB- ports, one High-Definition Multimedia Interface -HDMI- interface and, more importantly, a 40-pin extended General Purpose Input/Output) that can be used to connect any hardware component required. Overall, the flight controller used offers a series of features that come in handy for the purpose of this demonstrator. The Global Navigation Satellite System (GNSS) receiver provided by Navio2 is compatible with various positioning systems, such as the American GPS [31], Russian GLONASS [32] and European Galileo [33]. One of the reasons why this controller was selected is that its GPS chip is compatible with the GNSS European civil positioning system. As far as the purpose of this system proposal is concerned, Galileo offers several advantages: it is a free and open service, it offers greater positioning accuracy (explicitly described as Galileo High Accuracy Service or HAS [34]) and, more importantly for the purpose of this manuscript, it offers a higher level of security for data encryption that are not present in the other GPS solutions. Galileo also offers a high precision, fully encrypted service freely available for government-authorized users [35], which is equivalent to the one that GPS offers as military P(Y) or M code. 3. Video transmission subsystem: the designed aircraft has an integrated analog video system that offers the operator a first-person view of the drone's flight (which is referred to as First Person View or FPV). This feature will provide the pilot with a precise view of the UAV state at every moment of the mission that is taking place. For the implementation of this system, a series of hardware components are required. As mentioned, an analog video camera is needed, which is integrated on board the UAV and provides images with enough quality for the operator so that they can orient themselves on the terrain over which the UAV is flying. Additionally, an analog video transmitter is required, which receives the signal captured by the camera and transmits it to the ground base station that has a receiver synchronized with the transmitter and connected to a screen.