LongleyRice

Longley-Rice, or the Irregular Terrain Model (ITM), is a radio propagation model used to estimate signal strength for terrestrial links over irregular terrain. It was developed in 1968 by Longley and Rice. The model is designed to predict median field strength and time-varying fades for VHF/UHF and microwave paths, incorporating effects of terrain, atmospheric refractivity, diffraction, and tropospheric scattering.

Inputs to the model include the frequency, transmitter and receiver heights, transmitted power (optional), the path distance, and a terrain profile along the route (digital elevation data) plus a refractivity or climate profile (M-profile). The model computes path loss or received field strength at the receiver, providing estimates such as the median value and time-exceedance values (e.g., 50% and 90% probabilities). It covers a broad frequency range from tens of megahertz up to several gigahertz, depending on terrain data quality.

The computation combines line-of-sight concepts with diffraction over terrain and long-range atmospheric effects; it uses methods such as knife-edge diffraction approximations and a site-specific refractivity profile to produce predictions along the path. The output is typically presented as loss in decibels or field strength in decibels microvolts per meter.

Longley-Rice is widely used in broadcast, military, and telecommunications planning and is incorporated into ITU-R P.452 as the Irregular Terrain Model. Numerous software tools implement ITM, but results depend on input resolution and assumptions about clutter and atmosphere, so they should be interpreted with appropriate caution, especially in complex urban environments.