Page 397 - Satellite Communications, Fourth Edition

The Space Link 377

falling on a hemispherical radome forms a water layer of constant thick-
ness. Such a layer introduces losses, both by absorption and by reflec-
tion. Results presented by Hogg and Chu (1975) show an attenuation
of about 14 dB for a 1-mm-thick water layer. It is desirable, therefore,
that earth station antennas be operated without radomes where possi-
ble. Without a radome, water will gather on the antenna reflector, but
the attenuation produced by this is much less serious than that produced
by the wet radome (Hogg and Chu, 1975).

12.9.1 Uplink rain-fade margin
Rainfall results in attenuation of the signal and an increase in noise tem-
perature, degrading the [C/N ] at the satellite in two ways. The increase
0
in noise, however, is not usually a major factor for the uplink. This is so
because the satellite antenna is pointed toward a “hot” earth, and this
added to the satellite receiver noise temperature tends to mask any
additional noise induced by rain attenuation. What is important is that
the uplink carrier power at the satellite must be held within close limits
for certain modes of operation, and some form of uplink power control
is necessary to compensate for rain fades. The power output from the
satellite may be monitored by a central control station or in some cases
by each earth station, and the power output from any given earth sta-
tion may be increased if required to compensate for fading. Thus the
earth-station HPA must have sufficient reserve power to meet the fade
margin requirement.
Some typical rain-fade margins are shown in Table 12.2. As an exam-
ple, for Ottawa, the rain attenuation exceeds 1.9 dB for 0.1 percent of
the time. This means that to meet the specified power requirements
at the input to the satellite for 99.9 percent of the time, the earth sta-
tion must be capable of providing a 1.9-dB margin over the clear-sky
conditions.

12.9.2 Downlink rain-fade margin
The results given by Eqs. (12.53) and (12.54) are for clear-sky condi-
tions. Rainfall introduces attenuation by absorption and scattering of
signal energy, and the absorptive attenuation introduces noise as dis-
cussed in Sec. 12.5.5. Let [A] dB represent the rain attenuation caused
by absorption. The corresponding power loss ratio is A 10 [A]/10 , and
substituting this for L in Eq. (12.29) gives the effective noise tempera-
ture of the rain as

1
T a1 b (12.58)
T rain a
A