## 5 Cellular SB-Aircraft Network  
As shown in the previous section, to provide all-time flight connection, there should be a network of SBs. The size of each SB coverage area depends on the maximum communication range shown in Equation (7). For an aircraft, and before crossing this distance, there should be a standby link with another nearby SB that is ready for the handover process. One of the most important features of civil air traffic is the scheduled flights and regular aircraft paths which helps in achieving a scheduled resource management. This property gives good expectations about when and where the handover should occur between aircraft and SBs. The information about changes in scheduled flights should be provided to the system to reschedule the handover process. As shown in Figure 3, a cellular aircraft-SB network is designed over the main traffic paths. The distance between neighbored SBs is determined according to the expected aircrafts altitudes and the maximum communication range without any loss of the line-of-sight between SB and aircraft. If two neighbouring SBs are at altitudes \(h_{s1}\) and \(h_{s2}\) with maximum geometrical communication ranges of \(d_{cmax1}\) and \(d_{cmax2}\) respectively, then, the inter-SB central distance can be given by:  
\[r_{BB\ 1,2}=\frac{\sqrt{3}}{2}(d_{cmax1}+d_{cmax2}) \tag{13}\]  
Figure 4 displays the variation of the inter-SB distance with the aircraft height at different values of ground elevation angles. The value of \(\alpha_{g}\) can be chosen as zero for regions where the aircraft is unlikely to land or takeoff such as over regions where oceans, wide deserts or forests exist, and in this case, the inter-SB distance can be extended to 1500 km for aircraft flying at 11 km cruise altitude. However, for aircraft security, control, and continued connection, it is recommended that the minimum elevation angle is greater than zero even over rural areas to achieve an acceptable quality of communication between aircraft and SB.  
Figure 3: Cellular SB network for aviation.  
On the other hand, the practical value of the inter-SB distance is smaller than that given in Equation (13) to reduce the atmospheric losses resulting from the Earth's atmospheric gases over the path between SB and aircraft. This problem arises near the end-of-coverage regions where the signal propagates through longer distances in the atmosphere. Also, the impact of weather conditions is severe during heavy rain or in the presence of taller clouds and high-humidity atmosphere which provide higher attenuation than other clear-sky regions. A detailed discussion of atmospheric losses will be presented and modelled in the next section.  
For a cellular hexagonal SB-aeronautical system, a single SB has an area of coverage that can be determined by a hexagon of side length that is equal to \(d_{cmax}\) and the area of the aerial cell is, therefore, given by:  
\[A_{SB}=\frac{3\sqrt{3}}{2}\;d_{cmax}^{2} \tag{14}\]  
Figure 5 displays the variation of the SB cell coverage area in square kilometers with the aircraft height for SB located 20 km high. The area of a single SB may cover an area up to 1.5 million square kilometers for an aeronautical communication network which may extend over many countries.  
Figure 4: Variation of inter-SB distance with the aircraft height at different minimum elevation angles.  
Figure 5: Variation of SB coverage area with the aircraft height at different minimum elevation angles.