Page 123 - Satellite Communications, Fourth Edition

Chapter
4

Radio Wave Propagation

4.1 Introduction
A signal traveling between an earth station and a satellite must pass
through the earth’s atmosphere, including the ionosphere, as shown in
Fig. 4.1, and this can introduce certain impairments, which are sum-
marized in Table 4.1. Some of the more important of these impairments
will be described in this chapter.

4.2 Atmospheric Losses
Losses occur in the earth’s atmosphere as a result of energy absorption
by the atmospheric gases. These losses are treated quite separately
from those which result from adverse weather conditions, which of
course are also atmospheric losses. To distinguish between these, the
weather-related losses are referred to as atmospheric attenuation and
the absorption losses simply as atmospheric absorption.
The atmospheric absorption loss varies with frequency, as shown in
Fig. 4.2. The figure is based on statistical data (CCIR Report 719-1, 1982).
Two absorption peaks will be observed, the first one at a frequency of 22.3
GHz, resulting from resonance absorption in water vapor (H O), and the
2
second one at 60 GHz, resulting from resonance absorption in oxygen
(O ). However, at frequencies well clear of these peaks, the absorption is
2
quite low. The graph in Fig. 4.2 is for vertical incidence, that is, for an
elevation angle of 90° at the earth-station antenna. Denoting this value
of absorption loss as [AA] 90 decibels, then for elevation angles down to
10°, an approximate formula for the absorption loss in decibels is (CCIR
Report 719-1, 1982)

[AA] [AA] cosecEl (4.1)
90
103