Q: How can I use the Longley-Rice propagation model for specialized calculations, such as the "Longley-Rice Methodology for Evaluating TV Coverage and Interference" in OET Bulletin 69, or the "Individual Location Longley-Rice (ILLR) calculation in FCC 00-185?
A: The Longley-Rice model is a well-accepted propagation model in the industry and is specified as the standard for several FCC calculations. One of the benefits of Longley-Rice is the ability to set various parameters that affect the way the calculations are performed, so the model can be customized to a wide variety of circumstances and applications. Like any tool, the power and flexibility of the model are best used when you have a good understanding of how it works and the significance of the various parameters. Several articles are available on our web site that describe the general principles of Longley-Rice. Copies of the original documents can be ordered from the National Technical Information Service.
In the SoftWright implementation of the Longley-Rice model in the Terrain Analysis Package (TAP) software, parameter settings are defined in "templates". A propagation model template in TAP for Longley-Rice (as well as other propagation models) is a group of settings, containing a value for each of the model’s parameters. Multiple templates can be defined for any model, containing different parameter values for different applications or circumstances. The propagation model templates provide a convenient mechanism for standardizing settings, for example, when several engineers are using the software. The templates also can be used to set the specified, pre-defined settings required by certain FCC calculations.
NOTE THAT ALL VALUES USED IN THIS ARTICLE ARE FOR ILLUSTRATION ONLY TO EXPLAIN HOW THE SETTINGS ARE TO BE MADE. YOU SHOULD VERIFY THE SETTINGS THAT ARE APPROPRIATE FOR YOUR APPLICATION AND USE THOSE VALUES IN YOUR PROPAGATION STUDIES.
The Longley-Rice template displays the parameters for the model:
The values in the "LR Defaults" template supplied with the demo version of TAP are the values most closely fitting the typical settings in the earlier DOS version of TAP.
You can create new templates with the New button at the lower left corner of the form. For example, suppose you have the following parameters (for illustration only, based on FCC First Report and Order FCC00-185, released May 26,2000; you should verify the correct values for your application):
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| EPS | 15.0 | Relative permittivity of ground |
| SGM | .005 | Ground conductivity, Siemens per meter |
| ZSYS | 0.0 | Coordinated with settings of EN0 |
| EN0 | 301.0 | Surface refractivity in N-units (parts per million) |
| IPOL | 0 | Horizontal polarization |
| MDVAR | 1 | Individual mode of variability calculations |
| KLIM | 5 | Continental Temperate |
| XI | 0.1km | Distance between successive points along the radial from transmitter to individual reception point. [This is set elsewhere in TAP when entering the general parameters for the study.] |
| HG(1) | Height of radiation center above ground. [This is set elsewhere in TAP when entering the general parameters for the study.] | |
| HG(2) | 6m or 9m | Height of receiving antenna above ground. Use 6m for one-story building; otherwise 9m. [This is set elsewhere in TAP when entering the general parameters for the study.] |
| Time Variability | 50% | |
| Confidence Variability | 50% | Indicating median situations. |
These values could be used to create a Longley-Rice template as shown below:
Note that some applications may require other information, such as the use of USGS Land Use data. The settings for applying specified loss factors for different Land Use categories can be defined in the TAP Land Use module.
As another example, suppose you have the following parameters (for illustration only, based on FCC OET Bulletin No. 69, dated July 2, 1997; you should verify the correct values for your application):
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| EPS | 15.0 | Relative permittivity of ground |
| SGM | .005 | Ground conductivity, Siemens per meter |
| ZSYS | 0.0 | Coordinated with settings of EN0 |
| EN0 | 301.0 | Surface refractivity in N-units (parts per million) |
| IPOL | 0 | Horizontal polarization |
| MDVAR | 3 | Broadcast mode of variability calculations |
| KLIM | 5 | Continental Temperate |
| XI | 0.1km | Distance between successive points along the radial from transmitter to individual reception point. [This is set elsewhere in TAP when entering the general parameters for the study.] |
| HG(1) | Height of radiation center above ground. [This is set elsewhere in TAP when entering the general parameters for the study.] | |
| HG(2) | 10m | Height of receiving antenna above ground. [This is set elsewhere in TAP when entering the general parameters for the study.] |
| Time Variability | 10% | |
| Location Variability | 50% | |
| Confidence Variability | 50% | (also called "Situation Variability") |
These values could be used to create a Longley-Rice template as shown below:
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