## 2 The ITU-R P.1546-6 Wave Propagation Model
To give a simple example: if the value determined by the model is 60 dB\(\mu\)V/m at 10% time, then for 36.5 days in 1 year, i.e., 876 h, the actual measured field strength level may exceed the calculated value, while for the remaining time, i.e., 7884 h, the maximum value is 60 dB\(\mu\)V/m. The model performs interpolation and extrapolation between the propagation curves, except for the extreme values, according to the variables frequency (\(f\)), distance (\(d\)), and as \(h_{1}\). This procedure is illustrated in Figure 3.  
This method [30] is based on the principle of linear interpolation, which assumes a linear relationship between the independent variable (\(x\)) and the dependent variable (\(y\)). Let the inferior and the superior values of \(x\) be \(x_{inf}\) and \(x_{sup}\), and similarly, for the dependent variable \(y_{inf}\) and \(y_{sup}\). For a given \(x_{inf}\leq x_{0}\leq x_{sup}\), the task is to find  
Figure 2: Propagation curves for 100 MHz, 600 MHz, and 2000 MHz, land, 50% time. \(y_{inf}\leq y_{0}\leq y_{sup}\), such that we know that the \(x\)-derivative of \(y\) is constant due to the linear relationship. From this,  
\[\frac{\Delta y}{\Delta x}=\frac{y_{0}-y_{inf}}{x_{0}-x_{inf}}=\frac{y_{sup}-y_{ inf}}{x_{sup}-x_{inf}}\Leftrightarrow y_{0}=y_{inf}+\frac{\Big{(}y_{sup}-y_{inf} \Big{)}\Big{(}x_{0}-x_{inf}\Big{)}}{x_{sup}-x_{inf}}. \tag{2}\]  
Considering the wide range of interpretation of the variables, it is appropriate to proceed to logarithmic scaling, which leads to the following relationship:  
\[y_{0}=y_{inf}+\frac{\Big{(}y_{sup}-y_{inf}\Big{)}\log_{10}\Big{(}\frac{x_{0}}{ x_{inf}}\Big{)}}{\log_{10}\Big{(}\frac{x_{sup}}{x_{inf}}\Big{)}}. \tag{3}\]  
The base field strength level value read from the propagation curves by interpolation or extrapolation must then be corrected by various constants to take into account the characteristics of the propagation path [6], in order to ensure that the determined field strength level value is as close as possible to the real conditions of the terrain profile [31]. Of these, Terrain Clearance Angle (TCA) correction plays a very important role. TCA is defined as the elevation angle of the line from the receiving antenna that just clears all terrain obstructions in the direction of the transmitter antenna over a distance up to 16 km but not exceeding the transmitter antenna. The range is limited between \(+0.55^{\circ}\) and \(+40.0^{\circ}\), and the calculation does not take Earth curvature [32] into account. The calculation method is illustrated in Figure 4, assuming that the transmitting antenna is located further than 16 km from the receiving antenna. The difficulty in the implementation of determining \(\theta\) is not in determining the specific value of the angle itself, but in finding the line \(u\) connecting the antenna to the appropriate point on the terrain from which it can be calculated. In Figure 4, \(h_{terr}\) denotes the altitude above sea level of the antenna's geographical location, \(h_{ant}\) represents the height of the antenna above the ground, and \(h_{p}\) is the altitude above sea level of the geographical point  
Figure 3: Method of interpolation between propagation curves. that lies directly on the \(u\) line. The task is to find the line \(u\) connecting the elevation point of the antenna \((P_{a}(d_{a},h_{a}))\) with the point \(P_{b}(d_{b},h_{b})\) of the terrain for which it is true that  
\[h_{i}\leq h_{j}\quad\forall h_{j}\in u,\;\forall h_{i}\in[h(d_{a}),h(d_{max}=16 \text{ km})], \tag{4}\]  
where \(h_{j}\) is the altitude above sea level of the line \(u\) at the \(j\)-th evaluation point, and \(h_{i}\) refers to the altitude above sea level of the terrain at the \(i\)-th point between the transmitter and the point located at a maximum distance of 16 km from it. There are many algorithms that can be used to find \(u\), some useful methods are described in [33].