Longley/Rice Propagation Model

The Longley/Rice propagation model is also know as the Irregular Terrain Model (ITM). T3d uses version 1.2.2 of the Irregular Terrain Model. The Longley/Rice model is intended for use with frequencies from 20 to 20,000 MHz, and distances less than 2,000 km.

Model Inputs

Conductivity

This is the conductivity of the ground over which the signal propagates (measured in Siemens per Meter). [See Appendix B for conductivity values for the continental United States.]

Dielectric Constant

The dielectric constant is the relative ground Permitivity. [See Appendix C for suggested values for different types of ground.]

Refractivity

This is the atmospheric refractivity, measured in N-Units (parts per million). See Appendix D for suggested refractivity values for differently climate zones in the continental United States.

Climate Zone

The climate zone can be one of the following:

None

Equatorial

Continental Subtropical

Maritime Subtropical

Desert

Continental Temperate ( this is the most commonly used )

Maritime Temperate Overland

Maritime Temperate Over sea

Antenna Polarization

This is the polarization of the transmitting and receiving antennas. Longley/Rice supports horizontal and vertical polarization.

Receiver Height

This is the receiver height above ground (in meters). Default value is 9.1M.

Situation Variability

This is the situation variability expressed as a percentage (between 0 and 100). Generally a 50% situation variability is considered for coverage calculations.

Time Variability

This is the time variability expressed as a percentage (between 0 and 100).

ITM Mode

The ITM mode refers to the internal mode in which the model is run. Generally Broadcast mode is used, however the Satellite Home Viewers Act requires the use of the ILLR (Individual Location Longley-Rice) which runs the Longley/Rice model in the “individual” mode.

Urban Clutter Factor

The urban clutter factor is an additional loss factor determined by A. Longley to make the Longley/rice model results resemble the results for the Okumura propagation model (for use in urban areas).