b. For line-of-sight propagation, the earth's environment acts to
At ground level,
this impediment manifests itself by absorption in the lower
atmosphere, by multipath interference fading, and to a lesser extent,
Figure 15 shows a potential multipath
interference condition where waves from two apparent sources can
reinforce or reduce each other, depending upon geometry and
For space-based systems, the dispersive
atmosphere, including the ionosphere, must be considered.
systems, atmospheric precipitation affects the use and reliability of
the line-of-sight mode.
Potential multipath-interference condition.
c. Theoretically, there are no definite limits on frequencies
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