## 6 The ITU-R P.452-17 Wave Propagation Model
The correction formulas for LoS multipath propagation and atmospheric focusing are as follows:  
\[E_{sp}=2.6\left[1-e^{-0.1(d_{\rm fs}+d_{\rm fs})}\right]\log_{10}\left(\frac{ p}{50}\right), \tag{40}\]  
\[E_{s\beta_{0}}=2.6\left[1-e^{-0.1(d_{\rm fs}+d_{\rm fs})}\right]\log_{10} \left(\frac{\beta_{0}}{50}\right), \tag{41}\] \begin{table}
\begin{tabular}{l l} \hline \hline \multicolumn{1}{c}{**(a) Long-Term Propagation Mechanisms**} \\ \hline Line-of-Sight (LOS, basic) & Direct propagation under normal atmospheric conditions, typically stable over time. Applies mainly to short paths (\(<\)5 km). \\ Diffraction & Dominant beyond LOS under standard conditions, especially over terrain obstacles. Influences median and background levels. \\ Tropospheric scatter & Scattered signals from the troposphere dominate long paths (\(>\)100-150 km) when stronger mechanisms are absent. \\ \hline \multicolumn{1}{c}{**(b) Short-Term (Anomalous) Propagation Mechanisms**} \\ \hline LOS multipath and focusing & Rapid temporal enhancements due to atmospheric stratification \\  & causing multipath and focusing, especially for paths\(>\)5 km. \\ Surface ducting & Anomalous propagation along ducts near the surface, especially over sea/coasal paths; may exceed free-space levels (\(>\)500 km). \\ Elevated-layer reflection/refraction & Refraction or reflection from elevated layers (up to several hundred meters); can bridge diffraction gaps. \\ Hydrometeor scatter & Rain or hydrometeor-induced scattering; weak but potentially omnidirectional, may affect off-axis systems. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Classification of interference propagation mechanisms according to ITU-R P452-17. where \(d_{lt}\) and \(d_{lr}\) denote the distances, in kilometers, from the transmitting and receiving antennas, respectively, to their corresponding radio horizons. Based on the components above, the basic transmission loss not exceeded for a given time percentage \(p\) [%] due to LoS propagation is given by the sum of the gaseous-augmented free-space loss and the correction for multipath and focusing:  
\[L_{b0p}=L_{\text{bfgg}}+E_{sp}, \tag{42}\]  
where \(L_{b0p}\) is expressed in decibels. This equation provides the basis for estimating the signal level at the receiver under line-of-sight conditions, taking into account both the average atmospheric effects and short-term enhancements due to favorable refractive conditions. Similarly, the basic transmission loss not exceeded for the time percentage \(\beta_{0}\) [%], which corresponds to the typical fraction of time during which anomalous refractive conditions (e.g., ducting or elevated-layer reflection) may occur, is given by:  
\[L_{b0\beta}=L_{\text{bfgg}}+E_{s\beta_{0}}. \tag{43}\]  
This formulation is applied regardless of whether the path is actually line of sight or transhorizon. It enables the model to estimate the minimum  
Based on the plotted sets of curves, it can be concluded that the frequency has the most significant impact, followed by altitude and atmospheric pressure. The effect of water vapor content is the least pronounced, though it is by no means negligible. The curves still diverge visibly in this case as well, particularly when propagation distances of several hundred kilometers are assumed. As previously mentioned, beyond direct line-of-sight propagation, the model also accounts for the diffraction of radio waves around terrain obstacles. To this end, it employs an extended version of the Bullington method [59] in combination with the spherical-Earth model. This is the so-called the Delta-Bullington method [60]. The extended approach first uses the actual terrain profile between the transmitting and receiving points, and then a so-called zero-height or \"smoothed\" profile, in which the terrain elevations are set to zero and the antenna heights are adjusted to represent the effect of Earth curvature. This dual approach enables a more effective treatment of the transition between theoretical and practical diffraction scenarios.