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Tropospheric Propagation Modelling with the Parabolic Equation

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Under the influence of synoptic processes such as advection, subsidence, coastal fronts, or nocturnal cooling, stratification of the troposphere in the form of refractivity layering can occur. If the resulting refraction of propagating electromagnetic waves is sufficiently great to cause a ray curvature that exceeds the curvature of the earths surface, then the propagating wave can be channelled in a duct. At VHF and higher frequencies, surface and elevated tropospheric ducts can cause extended propagation well beyond the radio horizon with little attenuation relative to free space. The received radio signals can then be markedly stronger or weaker than would be expected for a standard atmosphere. The anomalous propagation of energy via a duct can thereby lead to the volatile problem of inter-system interference, whilst radar detection ranges can be enhanced for targets located within the duct. Propagation prediction models tend to fall into two categories those methods based on optical ray tracing techniques and those relying on mode theory. Although qualitative predictions can be made using ray theory, problems associated with the focusing of rays and identification of ray families render these methods unsuitable for modelling diffraction effects in non-standard atmospheres. Mode theory, which involves a full-wave solution to Maxwells equations, can be applied to ducting problems but becomes intractable when complex refractive structures are involved. Australia

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  • Radiofrequency Wave Propagation

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