The ITU terrain loss model is a radio propagation model that provides a method to predict the median path loss for a telecommunication link.
Developed on the basis of diffraction theory, this model predicts the path loss as a function of the height of path blockage and the First Fresnel zone for the transmission link.
This model accounts for obstructions in the middle of the telecommunication link, and therefore, is suitable to be used inside cities as well as in open fields.
These losses are summed and then added to the predicted line of sight path loss which is calculated using Friis transmission equation or a similar theoretical or empirical model.
In the event that the loss is less than 6 dB or is negative (i.e., gain), this A-value should be discarded.