## 6 The ITU-R P.452-17 Wave Propagation Model
The model identifies the most significant obstacle along the path--that which causes the highest diffraction loss--and estimates the loss using a single knife-edge approximation. Tropospheric scatter is modeled as a separate component of ITU-R P.452-17, estimating the associated loss based on frequency, antenna heights, terrain profile, and tropospheric refractivity. This model is empirically derived and represents a form of background interference level, which although weak, remains relatively constant and may still be relevant for highly sensitive receivers, such as radar systems. Short-term propagation mechanisms included in the model are associated with anomalous atmospheric conditions: surface ducting or elevated refractive layers. Although these conditions occur less frequently, they can result in unexpectedly low path losses with significant levels of interference. These are empirical components derived from statistical observations of anomalous atmospheric conditions such as surface ducting, elevated-layer reflection, and atmospheric focusing. Rather than relying on deterministic physical modeling, this part of the model is based on long-term measurement data and expresses the likelihood and impact of extreme propagation enhancements that can occur under favorable refractive conditions. The components and several characteristics of the ITU-R P.452-17 propagation model are summarized concisely in Table 7.  
This overview also illustrates why the model is referred to as a hybrid propagation model: its components are partly deterministic, based on closed-form equations derived \begin{table}
\begin{tabular}{l l l} \hline \hline
**Propagation Mechanism** & **Type** & **Description** \\ \hline Free-space path loss & Deterministic & Based on frequency, distance and antenna height; calculated using the classical formula \(L=92.45+20\log f+20\log f\). \\ Gaseous attenuation & Deterministic & Computed from water vapor density, pressure, temperature and path length using ITU-R P.676 absorption models. \\ Multipath and focusing & Empirical & Corrected based on distance to radio horizon and time percentage; \(p\) accounts for LOS enhancements from atmospheric layering. \\ Diffraction & Semi-deterministic & Combines terrain profile, antenna heights and effective Earth radius via the Delta–Bullington method (deterministic core + empirical smoothing). \\ Tropospheric scatter & Empirical & Derived from measurements; depends on frequency, antenna heights, distance and surface refractivity \(N_{0}\). \\ Ducting/elevated-layer reflection & Empirical & Based on refractivity gradient \(\Delta N\), median refractivity \(\beta_{\text{U}}\) and path angle; models anomalous propagation due to ducts or layer reflection. \\ Hydrometeor scatter & Empirical & Based on statistical scattering distributions related to rain rate, reflectivity, and antenna radiation patterns. \\ Clutter (local obstructions) & Semi-empirical & Terminal environment correction (urban, suburban, forest) using antenna height and clutter type; can introduce up to \\  & +20 dB attenuation. \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of empirical and deterministic components in the ITU-R P.452-17 model. from physical laws, and partly empirical, incorporating corrections developed from measurements and observations.