## 6 Atmospheric Gaseous Losses Profile for Aircraft-Stratospheric Basestation (SB) Link
### Long Flight Handover Scenarios  
As shown in the previous section, the impact of atmospheric losses is very high near ground especially at higher frequencies, while it becomes insignificant at the cruise altitudes. For long flight distances, handover should be performed between aircraft and neighbouring SBs. The inter-SB distance  
Figure 12: Flight signal attenuation performance for 5G mm-wave frequencies from 26 up to 50 GHz in dB: (**a**) atmospheric gaseous attenuation, (**b**) free-space loss (FSL), (**c**) combined atmospheric gaseous attenuation and FSL performance. Figure 13: Flight signal attenuation performance at 66 GHz 5G mm-wave frequency. is determined according to the level of received signals at the aircraft. Long flights stay at the cruise level most of the time, however the spherical surface of the Earth means that, over the horizon, the line-of-sight between an aircraft and SB will be inserted into the troposphere again and is subjected to higher atmospheric attenuation. The situation becomes more complicated if the troposphere contains clouds/fog along this path which means a huge signal attenuation may occur especially at high mm-wave frequencies. This scenario is shown in Figure 14, where the line-of-sight between an aircraft and SB at which a handover may be performed lies between two limits \(d_{minAtt}\) and \(d_{maxAtt}\). A compromise between the inter-SB distance and the acceptable minimum signal level at the aircraft should be performed to reduce the number of SBs in the network while providing acceptable communication performance. The best communication performance can be achieved when the handover is performed at a cruise distance where the aircraft is at the tangential point to the circular path of altitude \(R_{e}+h_{cr}\), a point under which the minimum elevation angle exits on the ground. This scenario leads to denser SB network where the number of required SBs is the largest. On the other hand, the worst communication performance is obtained at a handover distance \(d_{maxAtt}\) at which the line-of-sight is almost tangential to the Earth's surface which corresponds to paths at minimum elevation angles as shown in Figure 14. If the line-of-sight becomes closer to the Earth surface than \(d_{maxAtt}\), severe blocking will occur, and the link will be eventually lost. The atmospheric attenuation at distances close to \(d_{maxAtt}\) can be calculated by splitting the path between an aircraft and SB into two main segments; one from ground point \"g\" to the aircraft and the other from this point to the SB or:  
\[d_{CmaxAtt}\approx d_{Pmax}+d_{SBmax} \tag{30}\]  
The overall path atmospheric attenuation will be the sum of the two atmospheric attenuations exhibited in the two segments or:  
\[L_{o,aird_{maxAtt}}\approx\sum_{n=0}^{N_{p}}(l_{o}+l_{w})p_{n}+\sum_{n=0}^{N_{s }}(l_{o}+l_{w})\rho_{n} \tag{31}\]  
This equation is expected to give a little bit overestimation for the atmospheric attenuation due to the approximation of \(d_{CmaxAtt}\) as the sum of the two segments between ground and both aircraft and SB. However, it gives a good estimation at an elevation angle that is equal to zero where \(d_{CmaxAtt}\) is exactly equal to the sum of \(d_{Pmax}\) and \(d_{SBmax}\). If the flight is spanning very long distance over areas with no airports such as wide desert or oceans, the communication slant range may be extended to its maximum allowable value in order to reduce the required number of SBs in the network provided that it does not affect seriously the communication performance especially the atmospheric losses. As shown in Figure 15, the aircraft at  
Figure 14: Geometry for handover scenarios in SB network for aviation. the maximum line-of-sight distance, the signal will be exposed to denser atmosphere and hence more atmospheric attenuation. The atmospheric attenuation at \(d_{max,Att}\) is approximately double the value at \(d_{CminAtt}\) due to the re-insertion of the line-of-sight into the troposphere.