relatively slow rate (1,100 feet per second), you notice the time interval. The farther you
are from the cliff, the longer this time interval will be. If you are 2,200 feet from the cliff
when you shout, about 4 seconds will pass before you hear the echo. It takes 2 seconds
for the sound waves to reach the cliff and 2 seconds for them to return.
Figure 5-15 -- Echo principle.
In RADAR, the shout in our sound analogy is a series of short signal pulses from an
extremely high frequency transmitter that sends out high-power electromagnetic radio
waves. Electromagnetic waves travel much faster than sound waves. The speed of
radio energy is the same as that of light. An object in the path of these waves reflects
some of the radio energy. The reflected energy, or echo signal, is picked up by the
same antenna used by the transmitter and is fed to a receiver. The receiver amplifies
the signal, changes it to a usable voltage, and feeds it to an indicator.
Distance and Direction
The signal pulse is repeated at definite time intervals. There is a very short period
during which the transmitter sends out energy. There is a much longer period during
which the receiver waits to pick up the reflected signals. The farther away the object, the
longer it takes for the energy to reach the target and return. When each echo is
received, the time between transmission and return of the echo is measured
electronically. Because we know the time it takes to receive a reflection from an object
and the speed at which pulses of radio energy travel, we can calculate the distance to
an object. Because the energy travels to and returns from an object in straight lines, we
also know the target s direction.
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