## 2 Air-to-Space Channel Models
### Experimental Campaigns  
The A2S link was investigated in [13], in terms of airframe shadowing events. More specifically, experiments took place with a manned aircraft receiving the pilot-tone signal from a satellite (Italsat beacon) at a frequency of 18.685 GHz. The results revealed that in the LOS condition, no shadowing was induced during the flight, and the received signal was modeled by the Rician probability density function (pdf) with the K-factor reaching almost 34 dB. By contrast, in non-LOS (NLOS) cases, the received signal exhibited both multipath fading and shadowing effects, in which the signal level depended directly on the flight maneuvers. For example, when the aircraft formed a 25-degree roll angle when performing a typical U-turn, the signal level decreased, since the wing compromised the received signal. Furthermore, in extreme scenarios, in which 45-degree rolls were tested, signal-shadowing events up to 12.5 dB occurred when the wing crossed the LOS level. Another observation was the difference of 0.78 dB in the signal reception when the aircraft flew above and below the clouds. Finally, when the aircraft flew in a wavy line, in line with the propagation path from the satellite, there was a periodical block of the propagation path from the satellite from the aircraft's tail. This accounted for the diffraction and shadowing incidences with fade depths of up to 2-3 dB.  
Throughput- and latency-measurement tests between a satellite and a UAV were carried out in [14], in order to assess the reliability and robustness of the A2S link and investigate the potential of a satellite network to remotely control a BLOS drone. For this purpose, the Inmarsat I4 satellite network was exploited, along with an Aviator 200 UAV  
Figure 2: Article layout and organization.  
moving at a speed of 140 km/h. To test the throughput, the download time of a 1.2-MB file was examined using a continuously transmitting channel with a data rate of 200 kbps. The measurements took place in the greater area of Clinceni in Romania, and data were collected at 24 preselected waypoints. The test outcome revealed that the file downloading was completed with an average throughput of 140.4 kbps and an average latency time of 254.2 ms.  
Measurements of the A2S channel at the L-band (1575.42 MHz) using four different aircraft types were conducted in [15]. A dual polarized antenna array (i.e., an array with right- and left-hand circular polarized hemispherical patch antennas) was mounted on top of the aircraft to receive signals from the satellite, and a 2 \(\times\) 2 antenna array was applied on the bottom of the aircraft in order to capture the multipath signals reflected from the ground. The addressed channel model was based on a physical-statistical approximation, which took into account, in addition to the LOS component between the satellite and the aircrafts, the ground-reflected component, estimating for this purpose the ground-reflection coefficient. The specific parameter was determined as a function of the elevation angle and its statistical parameters (mean and standard deviation) for both polarizations, which are available in [15]. The corresponding layout of the final channel model is provided in Figure 3.