Page 396 - Satellite Communications, Fourth Edition

376 Chapter Twelve

TABLE 12.2 Rain Attenuation for Cities and Communities
in the Province of Ontario
Rain attenuation, dB
Location 1% 0.5% 0.1%

Cat Lake 0.2 0.4 1.4
Fort Severn 0.0 0.1 0.4
Geraldton 0.1 0.2 0.9
Kingston 0.4 0.7 1.9
London 0.3 0.5 1.9
North Bay 0.3 0.4 1.9
Ogoki 0.1 0.2 0.9
Ottawa 0.3 0.5 1.9
Sault Ste. Marie 0.3 0.5 1.8
Sioux Lookout 0.2 0.4 1.3
Sudbury 0.3 0.6 2.0
Thunder Bay 0.2 0.3 1.3
Timmins 0.2 0.3 1.4
Toronto 0.2 0.6 1.8
Windsor 0.3 0.6 2.1

SOURCE: Telesat Canada Design Workbook.

the dB values given in each column. For example, at Thunder Bay, the
rain attenuation exceeds, on average throughout the year, 0.2 dB for 1
percent of the time, 0.3 dB for 0.5 percent of the time, and 1.3 dB for 0.1
percent of the time. Alternatively, one could say that for 99 percent of
the time, the attenuation will be equal to or less than 0.2 dB; for 99.5
percent of the time, it will be equal to or less than 0.3 dB; and for 99.9
percent of the time, it will be equal to or less than 1.3 dB.
Rain attenuation is accompanied by noise generation, and both the
attenuation and the noise adversely affect satellite circuit performance,
as described in Secs. 12.9.1 and 12.9.2.
As a result of falling through the atmosphere, raindrops are somewhat
flattened in shape, becoming elliptical rather than spherical. When a radio
wave with some arbitrary polarization passes through raindrops, the com-
ponent of electric field in the direction of the major axes of the raindrops
will be affected differently from the component along the minor axes. This
produces a depolarization of the wave; in effect, the wave becomes ellipti-
cally polarized (see Sec. 5.6). This is true for both linear and circular polar-
izations, and the effect seems to be much worse for circular polarization
(Freeman, 1981). Where only a single polarization is involved, the effect
is not serious, but where frequency reuse is achieved through the use of
orthogonal polarization (as described in Chap. 5), depolarizing devices,
which compensate for the rain depolarization, may have to be installed.
Where the earth-station antenna is operated under cover of a radome,
the effect of the rain on the radome must be taken into account. Rain