Line of Sight
Almost everyone has been driving while listening to the radio,
and heard the station become weak or fade out entirely due to
hills or other substantial geographic features(including man made
ones like concrete buildings and tunnels) blocking the signal.
We also know that signals fade out over long distances. These two
problems effect Very High Frequency(VHF) and Ultra High Frequency(UHF)
radio waves, such as the ones used in radar. The same factors limit
our vision, so the distance we are able to see from a given location
and direction is roughly equivalent to the distance VHF and UHF radio
waves can travel from that point. Since the Nike missiles were guided
by radar beams, the Launcher area had to be "line of sight" from
the IFC area.
For more information about radio waves, see the
American Radio Relay League.
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