## 8 System Feasibility Discussion
### Feasibility of the Aircraft-SB System Compared with Aircraft-Satellite Systems  
Conventional mobile satellite systems are the main source of internet services for aviation and maritime. The low-orbit (LEO) satellites are considered feasible systems for mobile-satellite communications compared with geostationary (GEO) satellites. However, the low orbital altitudes of the LEO satellites provide low orbital periodic time and hence short visible time duration for airborne or maritime stations and requires frequent handover. For example, the most recently high-speed Internet Starlink satellites [8] orbit at altitude of 550 km in inclined orbits where each has about 66 satellites. Each satellite in this system can be visible for a few minutes and the ground receiving stations must perform handover to another nearby visible satellite. On the other hand, moving transceivers result in Doppler frequency shift which is directly proportional to the relative speed between transceivers. LEO satellites have very high orbital speed which if its direction is opposite to the aircraft will result in higher Doppler shifts. The communication performance is expected to be improved in \"GeoSurf\" constellation proposed in [11] where zenith paths are considered for the communication link, so the atmospheric losses and FSL are reduced compared to other LEO constellations. However, an aircraft-SB system provides many advantages and become more feasible compared to most mobile-satellite systems. For example, SB is kept fixed in position and there is no self-initiated handover or Doppler frequency shift. A single SB at 20 km high can serve an aircraft for about a distance of 1200 km or more during flight. If the average aircraft speed is 800 km/h, this means that the aircraft can be served by a single SB for approximately 90 min without any required handover during cruise. The Doppler frequency shift in an aircraft-SB network is only initiated by the aircraft movement and, therefore, may be predicted and manipulated. Also, the SB can be easily managed, controlled, and maintained compared with LEO satellites as SBs can be positioned and adapted to the coverage regions where aircraft always pass and can be landed for maintenance and upgrading. On the other hand, as the SB is located at lower altitudes than any broadband mobile satellite, therefore, the propagation delay is expected to be lower in SB networks than satellites. The signal propagation delay, \(\tau_{p}\), is given by:  
\[\tau_{p}=\frac{1}{c}\Bigg{(}\sqrt{\left(R_{e}+h_{s}\right)^{2}-\left(\left(R_ {e}+h_{p}\right)\cos(\alpha_{n})\right)^{2}}-\left(R_{e}+h_{p}\right)\sin( \alpha_{n})\Bigg{)} \tag{36}\]  
where \(c\) is the speed of light. Figure 19 displays the variation of the maximum propagation delay with the basestation altitude which can be considered as SB or mobile satellite. The small squares on the curves corresponds to  
the delay at the altitudes of current Starlink satellites while the small circles correspond to the SB altitude which is 20 km. The data in this figure show that the SB systems can provide less propagation delay compared to LEO satellites especially for satellite altitudes greater than 550 km. If the minimum elevation angle is 10\({}^{\circ}\), then the SB network will always provide less of a propagation delay compared to Starlink satellites at any altitude. In addition to the improved propagation delay in aircraft-SB networks, the FSL is also improved compared to LEO mobile satellites. Table 2 summarizes the feasibility comparison between an aircraft-SB system and an aircraft-mobile-satellite system.