## 3 Results and Discussion
### Telecommunications Analysis and Applied Results
At distances in the range of hundreds of meters in the horizontal plane, the gain of the antenna is expected to be more than 20 dB below its maximum gain. As the distance increases, the vertical angle decreases and the gain of the antenna gets closer to its maximum gain, compensating the loss due to free space propagation. Figure 7 illustrates the above described:  
Taking a scenario considering the most restrictive frequency (2.6 GHz), a carrier power of 20 Watt, the gain of a typical base station antenna in the direction of maximum radiation (18.5), the free space loss the level of the main lobe to the secondary, and a distance of 1376 m, the resulting expected signal level at the drone antenna is \(-59.79\) dBm. A good signal level in 4G is considered above \(-75\) dBm so there are more than 15 dB of margin (75 dBm-59.79 dBm), that would increase the distance beyond the 2 km. The frequency of 2.6 Ghz is likely to be used in an urban area or an environment with small cells, but for rural environments it is much more likely to use lower frequencies (in the band of 800 Mhz) that would increase the coverage range even further. Therefore, the drone can receive a good quality 4G signal at distances of around 2 km from the network antenna node, being the lack of coverage more probable in the GCS that in the drone, as explained below. In practice, we found that at flying heights under 120 m the coverage was better than at ground level. The explanation for such behavior is that the topography and the surface elements (vegetation, buildings) are not shadowing the signal when the drone is in the air, while it was compromised in ground before taking off. Moreover, if the 4G network antenna covering the flight area is located on a hill exceeding the height of the drone top altitude (as usual, for instance, in rural areas), the main antenna lobule is covering the drone 4G receiver and the signal is excellent. Nevertheless, if the 4G network antenna is located within a basin position (not common), or the drone if flying at higher altitudes than 120 m above ground level (commonly not allowed), the coverage quality drops. However, the movement of the drone will cause at some point to enter a zero radiation from the network antenna node. This will force the start of a handover protocol to transfer communication to another node on the network, which will interrupt for some seconds the communication. The antenna handovers and  
Figure 7: 4G antenna beam lobule distribution pattern: Theorical connectivity range from 4G antennas. (Source: Modified from [PERSON] [43]). the performance indicators depending on the coverage have been analyzed in [7; 14], with similar results, but they are dealing with copters and in urban areas with more 4G signal density. In other circumstances, such as in [13], the coverage is worse than in our study, but they are supported with mobile ground station to aid the drone. There are some telecommunication companies that have agreements with the main telecommunication operators and provide multioperator SIM cards (e.g., Wirelesslogic [36]). Having a multioperator SIM card in the drone on-board modem enables us to connect the operator network with the best receiving signal at the drone antenna. In most developed countries, there are three or more physical network managers (e.g., Vodafone, Orange, Telefonica) and they share the infrastructure. Nevertheless, there will always be a signal better than the others, for instance due to network management parameters, so the multioperator feature adds a significant value. The multioperator SIM analyses the coverage when it becomes active and selects the operator with the best signal to perform the specific flight.