## 5 Discussion of the Results
### Communications Conditions  
Since the moment that the balloons are released, they start to move away from the ground station, reaching a maximum value of 95 km. The FSSExp payload is designed to establish a communication link between two modules separated up to 400 km. Therefore, the FSSExp payload can communicate with the ground station during the entire flight. However, other aspects may also impact this effective communication range. This communication is feasible only if the Signal-to-Noise Ratio (SNR) of the received signal is greater than the minimum required. In the laboratory, the noise floor measured with the Received Signal Strength Indicator (RSSI) from the FSSExp payload, and connecting a matched load as an antenna was \(-113.0\) dBm (average). Considering the rural area of the launchpad, the expected noise floor should be \(-104.7\) dBm according to the International Telecommunication Union Radiocommunication Sector (ITU-R) recommendations [22]. However, the noise floor in the launchpad was \(-85.0\) dBm (average), \(19.7\) dB greater than the expected one. Figure 8 presents the difference between these three noise floors. The blue line corresponds to the measured noise floor, the red line corresponds to the measured noise floor in the laboratory, and the green line corresponds to the estimated one in the launchpad. This increase generates an antenna temperature of \(2.9\)\(\times\)\(10^{5}\) K, two orders of magnitude larger than the expected one (i.e., \(2900\) K). The existence of an interference signal is the cause of this increase. The link budget is thus impacted by reducing the maximum communications range to 55 km, which entails the loss of communication between 13:00 and 13:15. Although the increase of this noise floor, the communication with the balloon and the ground station is feasible during almost all the ascending phase. To take some measurements of the received signals, an additional Software Defined Radio (SDR) module was installed near the ground station. Figure 9 presents a short fragment of the spectrum history retrieved with this device while the balloons were flying. At the top of the figure, the spectrum of the current received signal is plotted. The presented curve shows the shape of a Gaussian Minimum-Shift Keying (GMSK) modulated signal centered at 437.35 MHz. Additionally, at the bottom of the figure, a history of the spectrum is presented over time. A color scale is used in this history to highlight the power of the signal, being orange the great values, and blue the lowest ones. Figure 8: RSSI measurement of the noise floor on the launchpad (blue line), the expected one (green line), and in the laboratory (red line). The record of all the sniffed packets in the ground station enables the evaluation of the impact of this separation in the performance of the communications. Figure 10 shows the evolution of the SNR of Balloon A packets, which decrease over time. Before the release of the balloon, the SNR remains around 40 dB. After it takes off, the SNR rapidly goes down until 16 dB (around 12:50). Then, the communications between the balloons and the ground station is lost. Please note that this situation is coherent with the degradation of the link budget due to an interference signal, which determines that the communications should be lost after 13:00. Although the communication with the ground station is lost, the balloons keep interacting between them as Figure 11 highlights. This figure presents the SNR of the publish (blue dots), and the accept (orange dots) packets received in the balloons. Three different time slots are identified depending on their location: The former slot (purple box) is delimited between 11:53 and 12:12 when the balloons were placed on the ground. Because of the flying dynamics are not present, this scenario becomes the performance reference. The following slot (red box) is between 12:12 and 12:30, when the balloons are released sequentially.