Page 126 - Satellite Communications, Fourth Edition

106 Chapter Four

Figure 4.2 Total zenith attenuation
at ground level: pressure 1 atm,
temperature 20°C, and water
3
vapor 7.5 g/m . (Adapted from
CCIR Report 719-2, with permission
from International Telecommunication
Union.)

4.4 Rain Attenuation
Rain attenuation is a function of rain rate. By rain rate is meant the rate
at which rainwater would accumulate in a rain gauge situated at the
ground in the region of interest (e.g., at an earth station). In calculations
relating to radio wave attenuation, the rain rate is measured in mil-
limeters per hour. Of interest is the percentage of time that specified
values are exceeded. The time percentage is usually that of a year; for
example, a rain rate of 0.001 percent means that the rain rate would be
exceeded for 0.001 percent of a year, or about 5.3 min during any one
year. In this case the rain rate would be denoted by R 0.001 . In general,