Longley-Rice Variability Values

Longley-Rice Variability Values

Q: How do the variability settings affect the Longley-Rice field strength calculations?

A: The Longley-Rice propagation model includes inputs for time, location, and situation variability.

The variability factors introduce the ability to compensate statistically for signal variations that cannot be accounted for by analytical means.

For example, in real world circumstances, even when all factors such as transmitter power, antenna pattern, terrain, etc., are constant, there will be small (usually) variations in signal strength. The "variability" factors in the Longley-Rice model enable you to include these factors in your calculations. The "Longley-Rice Variability Effects" article discusses several sample paths and how different variability settings affect the predicted field strength on those paths.

The types of variability factors included in the Longley-Rice model are discussed in more detail in the Longley-Rice documentation and in other SoftWright FAQs. A summary is presented below.

Time Variability

Time variability (sometimes called "reliability") may be the easiest to understand. If you pick a location and place a recording field strength meter at that spot and leave if for several days, the results would show a variation in signal strength.

Consider the hypothetical illustration:

As the signal level varies, the 39dBu level represents the field strength that is available 90% of the time.

However, for the same signal, if the requirement is to know what signal level is available 95% of the time, the value to use would be lower:

Changing the Longley-Rice value for time variability ("reliability") will result in lower field strength values, but with the higher likelihood that value will be available at a given time.

Location Variability

Likewise, location variability represents the change in signal level when a field strength meter is moved around the target location. For example, if the hypothetical field strength meter described above is moved in a 10-meter radius around the location, there will be variations in the measured signal. For most paths, that slight variation in the target location will make no discernible difference in the path topography or intervening obstructions. But the location variability factor accounts for those variations in signal.

Note that the default mode for Longley-Rice calculations in TAP does not include location variability. The original development of the Longley-Rice model was before the availability of detailed topographic databases, and the location variability factor was intended to compensate for the uncertainty of topography in those circumstances.

Situation Variability

Situation variability (sometimes called "confidence") represents the statistical variation in field strength values for other reasons. Continuing our hypothetical example of a target point where field strength is being measured, suppose ten different engineers went to the exact location, at the same time (impossible, but this is just hypothetical) to measure the signal. There would be variations in the reported values for several very practical reasons: different meter calibration, different skill levels of the operators, different levels of care take by the different individuals.

The situation variability enables these factors to be included in the Longley-Rice field strength calculations. For example, if you specify a situation variability ("Confidence") of 90%, and the Longley-Rice prediction is 39dBu, you would expect that 90% of the engineers who measure the signal strength at that location for a performance test would report a value of 39dBu or higher.