ITM propagation model also know as Longley-Rice was developed to estimate radio transmission loss over irregular terrain for VHM, UHF, SHF frequency band. Computing of signal loss based on electromagnetic theory and on statistical analyses. It has been complemented by empirical dependences, 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 describe below - use estimation of parameters based on empirical medians. Area prediction mode is interesting for using, due to it can be implements to SEAMCAT system.
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 level of confidence. Longley-Rice computes path loss into three regions:
- Line-of-sight range;
- diffraction range; and
- scatter range (In distance between 1 – 2000 km).
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 compute reference attenuation Aref – the median attenuation relative to a free space. Reference attenuation is a function of distance d, what is illustrated below. On cumulative attenuation comprises reference attenuation and attenuation of quantailes A(qT, qL, qs) – which are calculated based on normal distribution, as variability of time, localization, reliability.
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Figure 475: Reference attenuation as function of distance