Q: When should I use the Carey propagation model? When should I use Bullington? What about Broadcast and Longley-Rice?
A: Each of the propagation models available in the Terrain Analysis Package (TAP) software is described and compared in the FAQ "Propagation Comparison."
The Carey, Broadcast, and SMR models could be classified as a "statistical" approach. They use generalized information, such as Height Above Average Terrain (HAAT) and field strength curves based on statistical averages of predicted field values for the calculation.
Models such as Bullington, Longley-Rice, Okumura, Hata-Davidson, and Rounded Obstacle could be classified as using an "analytical" approach. These models typically use much more detailed and specific information, such as the individual elevation values along a path, for the field strength calculation.
As a result, the more analytical models usually give more realistic results when compared to specific measured values taken at particular locations. The statistical models give an overall presentation of the coverage.
An important factor in deciding the model to use is the applicable regulatory requirements. If the FCC or other jurisdiction requires the use of a particular model for filing an application for license, naturally that model must be used. When results from a statistical model are required, it may be helpful to do the same study with one or more other analytical models to get a better idea of what to expect for the "real-world" coverage.
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