By the end
of the first decade of powered flight, people began to realize
that aircraft, like automobiles, would crash into each other unless
some means was developed to control and direct them.
At first, because aircraft could
not fly very high, such control was accomplished using simple
hand signals. Later, control was exercised through radio communication.
By the end of World War II,
the invention of radar permitted visually displayed tracking of
several aircraft at once. When coupled with radio communication,
radar made it possible for the pilot to be warned when he was
in danger of colliding with another aircraft or when he was off
course. Today, aircraft are continually tracked from take-off