The Longley-Rice model, also known as ITS irregular terrain model, is a general purpose model intended to be of use in a very broad range of problems. It is flexible in application and can actually be operated as either an area prediction model or as a point-to-point model.
The main input parameters are the following ones:
•Frequency: 20 MHz to 20 GHz.
•Distance: 1 km to 2000 km
•Antenna heights: 0.5 m to 3000 m
•Polarization: Vertical or horizonal.
•Terrain irregularity parameter ∆h, which is the interdecile range of terrain elevations - that is, the total range of elevations after the highest 10% and lowest 10% have been removed.
•Dielectric constant and conductivity of the ground.
•Surface refractivity: 250 to 400 N-units.
The model computes in the first place a reference attenuation, which is a certain median attenuation relative to free space. As treated by the model, this reference attenuation is most naturally thought of as a continuous function of distance. It is defined piecewise in three regions, called the line-of-sight, diffraction, and forward scatter regions. The "line-of-sight" region is somewhat misnamed; it is defined to be the region where the general bulge of the earth does not interrupt the direct radio waves, but it still may be that hills and other obstructions do so. In other words, this region extends to the "smooth-earth" horizon distance, which is probably farther than the actual horizon distance. In this region the reference attenuation is computed as a combined logarithmic and linear function of distance; then in the diffraction region there is a rather rapid linear increase; and this is followed in the scatter region by a much slower linear increase. Parameters other than distance enter into the calculations by determining where the three regions fall and what values the several coefficients have.
Then from this median attenuation further allowances are subtracted to account for time, location, and situation variability. Time variability accounts for the expected variations of the signal level in time, due to different propagation conditions mainly associated to changes in the atmosphere. Location variability represents the changes expected when the radio terminal is moved between different locations. Finally, the situation variability models the differences that appear between links deployed in similar areas.
 NTIA Report 82-100 “A Guide to the Use of the ITS Irregular Terrain Model in the Area Prediction Mode “, by G.A. Hufford, A.G. Longley and W.A. Kissick, 1982.