What is the Longley-Rice Propagation Model?

The Longley-Rice Propagation model is a method for forecasting attenuation (signal loss) for a telecommunications link over a distance. New interest in this method is arising because the FCC has agreed to make the "white spaces" from unallocated TV channels available for ultra-WiFi and perhaps other services. This article discusses the essential concepts and assumptions required by the Longley-Rice "irregular terrain model" or ITM.