## 4 System Description
### Algorithm  
In this subsection, the functioning algorithm for both the drone and the field nodes is explained. Furthermore, the message exchange between the elements of the architecture is presented as well. The PA scenario is comprised of multiple sensor nodes deployed in several agricultural fields. No networking structure is established among the nodes. Thus, the nodes do not communicate in any way among themselves. A sensor network where the nodes deployed on the field forward the data through relay nodes located in between the soil monitoring nodes would address the problem of sending the information to the base station, but it would significantly elevate the cost of the deployment--more so for node densities where large distances separate the nodes. The nodes incorporate various soil monitoring sensors that are able to obtain data on the physical parameters necessary to manage and monitor the crops. Depending on the node model, similar to the ones previously presented in Table 2, the node may use an embedded antenna to communicate, or it may need a separate communication module from the ones available on the market. The data are stored in the SD (Secure Digital) cards connected to each one of the nodes until the remote sensing drone retrieves the information. Figure 5 presents an example of a soil monitoring drone with a temperature and a humidity sensor. The drone is located on a base station, where the route begins and ends. The drone takes photographs of the fields and the terrain during its flight. When the drone reaches the base station, all the retrieved information is forwarded to the data center where the information is stored on a database for further analysis. A connection between the sensor node and the drone, with a good enough quality to ensure that the data is transmitted successfully, is one of the requirements of a PA system that employs a drone to gather data from the sensor nodes deployed on the field. Other requirements include low energy consumption for the drone to complete its route, the availability of the operating frequency band, and good weather conditions to avoid signal attenuation and damages to the drone and sensor nodes. Regarding the data rate, it is crucial to consider that the parameters measured in PA systems vary at a slow pace. Therefore, continuous monitoring is not necessary, and the data can be captured each minute or at larger time intervals such as 5, 10, or 30-min intervals. Therefore, the information stored in the SD card is not a large amount, and the necessary data rate would hardly surpass  
Figure 5: Design example of the soil monitoring node. 1 Mbps. Furthermore, works such as [50] show real data rates obtained at different flying heights and distances that surpassed 1 Mbps for heights of 20 m and distances of 45 m, with the drone acting as an intermediary node and therefore having more chances of experiencing interference and signal attenuation. A fixed-winged drone is utilized in [39] to capture images from the fields and forward them using WiFi, where a UDP (User Datagram Protocol) throughput up to 176 Mbps was obtained from real experiments. Therefore, although parameters such as flying height and speed are undoubtedly determinant in the success of the data transmission, once the connection between the sensor node and the drone is established, the necessary data rate is not high. Low energy consumption should be sought on both the drone and the sensor nodes. The sensor nodes can go into sleep mode during idle time to reduce energy consumption. Furthermore, if the settings of the drone indicate a fixed time for the drone to gather the data from the sensor nodes, the nodes can activate at that time to avoid being in sleep mode when the drone flies over them. Lastly, the lifetime of sensing nodes can surpass a year if the energy consumption and battery choice are optimized. Using solar panels to power the sensor nodes would even nearly cease the problem of battery-less sensor nodes.