In addition to a method of computing signal attenuation as a function of the rate of rainfall, Crane describes estimated rates for various regions around the world. TAP uses these regions and user-supplied coordinates (or the midpoint coordinates of the path in a link budget) to determine the climate category (Temperate-Continental, Tropical-Wet, etc.). The worldwide regions are shown in Figure 3 from Crane:

Ó 1980 IEEE; Used with permission.

For locations in the continental U.S., Figure 4 provides more detail:

Ó 1980 IEEE; Used with permission.

The TAP program will determine the region based on the coordinates, but you can select a different region for the calculation if you believe it better represents your particular application.

The other factor is the statistical measure of the frequency of the rainfall. For example, in considering a worst-case situation, a very high rainfall rate would be needed, but that rate might be relatively rare, only experienced for a small portion of any given year. For the same area, a more typical rate, experienced more commonly during the year, would be a lower value.

The Crane method in TAP allows you to define the percent of the year for estimating the rainfall rate (in millimeters per hour). The range in Crane is from 0.001% to 2.0%. For the relatively rare rainstorm (.001% of the year, or about 315 seconds) very heavy rainfall rates would be predicted, with the value depending on the climate region determined by the coordinates. On the other hand, a much more common storm that might be experienced for a total of 2% of the year (about a week – 7.3 days) would be a much lower rainfall rate.

When the estimated rainfall rate has been determined, other parts of TAP, such as the Microwave Link Budget program, can use the rate with any of the rain attenuation models to estimate the signal attenuation for the path.