A17.9.1 Description of the model

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Last updated: Jan 21, 2020 by Zeljko Tabakovic
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ITM propagation model also known as Longley-Rice was developed to estimate radio transmission loss over irregular terrain for VHM, UHF, and SHF frequency bands. Computing of signal loss is based on electromagnetic theory and on statistical analysis. It has been complemented by empirical dependences, obtained during tests and measurements.

ITM model has two modes of working. Point-to-Point mode needs details of terrain profile. Area prediction mode  - which is described below - uses estimation of parameters based on empirical medians. 


The basic input parameters such as: Antennas height, frequency, terrain irregularity determines computing method. Algorithm can be divided into three areas (determined by a function of the distance): A) Line-of-sight range (understood as uninterrupted visibility of the bulge of the Earth Plane, but which may be obscured by obstacles on the route), B) Diffraction range, C) Scatter range. In the first range to loss computing used Geometric-optics rays theory. In the second range used Fresnel-Kirchoff knife-edge diffraction theory. In the third range used Forward scatter theory.

The Longley-Rice (Area prediction mode) propagation model was originally implemented for SEAMCAT in the  National Telecommunications Institute (Poland) in co-operation with the  Wrocław University of Technology. The task was performed under Research Project of the Polish Ministry of Science and Higher Education entitled: Next generation teleinformatics services - technology, application and market aspects (no. PBZ-MNiSW-02/II/2007).

The SEAMCAT implementation of the Longley-Rice (Irregular Terrain Model) assumes a propagation model in area prediction mode (i.e. no need for terrain profile details to make a path loss calculation).The ITM model returns predicted median attenuation of a radio signal, regarding not only statistical terrain, system, climate parameters but also variability of the signal in time and in space, assuming a given level of confidence. Longley-Rice computes path loss into three regions:

  • Line-of-sight range;
  • diffraction range; and
  • scatter range (for distances between 1 – 2000 km). 

The ITM model is based on electromagnetic theory and on statistical analysis, the original model was developed in 1960’s in the USA.

At the beginning necessary geometry parameters are calculated, based on input parameters. Then, depending on the range in which the transceiver and receiver are, ITM model computes a reference attenuation Aref – the median attenuation relative to a free space. The reference attenuation is a function of distance d, what is illustrated below. The cumulative attenuation comprises reference attenuation and attenuation of quantiles A(­q­T, qL, qs) – which are calculated based on normal distribution, as variability of time, localization and reliability.

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Figure 475: Reference attenuation as function of distance