b. For line-of-sight propagation, the earth's environment acts to
limit the performance of a communications system.
At ground level,
this impediment manifests itself by absorption in the lower
atmosphere, by multipath interference fading, and to a lesser extent,
by refraction (ray bending).
Figure 15 shows a potential multipath
interference condition where waves from two apparent sources can
reinforce or reduce each other, depending upon geometry and
propagation conditions.
For space-based systems, the dispersive
sensitive)
and
refractive
effects
of
the
entire
atmosphere, including the ionosphere, must be considered.
In all
systems, atmospheric precipitation affects the use and reliability of
the line-of-sight mode.
Figure 15.
Potential multipath-interference condition.
c. Theoretically, there are no definite limits on frequencies
associated with line-of-sight communications.
However, most surface
relay systems, as well as space links, utilize frequencies that
minimize the effects of the atmosphere and, at the same time, permit
construction of antennas which are both high gain (a measure
of
efficiency)
and
moderate
in
size.
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