Diffraction Module

Diffraction Algorithms
A diffraction loss calculation first characterizes the terrain using the following categories:

single knife edge

near single knife edge or isolated obstacle

multiple knife edge (using Epstein-Peterson or Deygout methods)

foreground loss between an antenna and its horizon (height - gain)

default irregular terrain (Longley-Rice) or rough earth diffraction

Three automatic diffraction algorithms are provided:

TIREM - Terrain Integrated Rough Earth Model
NSMA - National Spectrum Managers Association
Pathloss - a user configurable algorithm

Each of these algorithms follow a set of rules to characterize the terrain and are the basis of variable parameter calculations, area coverage and interference analysis.

On line of sight paths with less than 60% first Fresnel zone , the loss is determined using the following methods:

a series of isolated obstacles defined by the 60% Fresnel zone intersections with the terrain .
Longley-Rice
Longley-Reasoner

Interactive Diffraction Loss Calculation

In addition to the three automatic algorithms, the user can analyze any portion of the path using any combination of the following basic algorithms:

knife edge	isolated obstacle (knife edge with a radius)
average diffraction	2 ray optics
height gain	Longley and Rice

This feature permits such computations as:

replacing otherwise irregular terrain with an effective obstacle
determining the loss from a reflection point to either site.

Tropospheric Scatter Loss - Combined Loss

On obstructed paths, tropospheric scatter loss is automatically calculated and combined with the diffraction loss.

Time Variability

On non line of sight paths, the time variability of the transmission loss is analyzed using the statistical curves contained in Technical note 101. The complete set of these curves have been digitized into the program.

Variable Parameters

Diffraction loss can be calculated as a function of any of the following parameters:

site 1 or site 2 antenna height	earth radius factor (K)
frequency	distance along the path

Last Update: 08/06/99