Why is the aeronautical radio path between the ground and an elevated platform different to say a path between two stations on the surface? Surely the same principles are at play? Well yes they are, but it is more a question that the terrestrial path is often simplified when modelling stations near the surface of the Earth compared to the aeronautical case.
A simplification often applied, is to model of the atmosphere as uniform rather than inhomogeneous. Of course the density of air is greatest at the surface and ‘thins out’ with altitude. This may be fine for short range applications, or for those close to the surface of the Earth, but for a path that is slanted between a terrestrial station on the surface and an aircraft at altitude, the radio wave will encounter different densities of atmosphere, and so it will refract causing the ray to bend with a magnitude that varies with altitude.
To model the ray bending, ray tracing is often used with an exponential model of the atmosphere. An example of this is contained within the ITS IF77 model published in 1983 by Johnson and Gierhart. IF77 is the basis for the ITU recommendation P.528-3 on propagation for aeronautical mobile and radionavigation services using the VHF, UHF and SHF bands. The diagram below depicts an exaggerated example of the ray tracing used to generate key metrics for estimate of propagation losses for IF77.
Straight line geometry over estimates the ray bending leading to horizons being predicted as further away, so a longer line-of-sight region and thus smaller propagation losses. In IF77, the ray is traced using iterative methods to find the radio horizons, and then to correct the end station heights to lower effective heights. This is one aspect of the model that is by no means unique to aeronautical, but more significant than for say land mobile over short paths. Other effects that are significant in an aeronautical context include the role of antenna systems in attenuating the Earth reflected path, some of the principles of which were discussed in the post Plane old Earth on the sea!
Despite the age of the Johnson Gierhart work, it is a current topic of discussion in ITU SG3, because it was a well researched piece, and provides a solid basis from which to model the propagation path, which is required to help satisfy the need for improved spectrum planning, mostly for sharing and coexistence studies. This need arises from the relentless pressure for more spectrum to serve both land and aeronautical mobile systems.
