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1-1
1. PRINCIPLE OF OPERATION
1.1 What is Radar?
The term "RADAR" is an acronym meaning
RAdio Detection And Ranging. It is a
device which measures the time it takes for
a pulsed signal to be reflected back from
an object.
1.2 How Ships Determined
Position Before Radar
The use of echoes to determine position
did not begin with radar. Ships would sound
a short blast on their whistles, fire a shot, or
strike a bell as an aid to navigation when
running in fog near a rugged shoreline. The
time between the origination of the sound
and the returning of the echo indicated how
far the ship was from the cliffs or the shore.
The direction from which the echo was
heard indicated the relative bearing of the
shore.
1.3 How Radar Determines
Range
Radar determines the distance to the target
by calculating the time difference between
the transmission of a radar signal and the
reception of the reflected echo. It is a
known fact that radar waves travel at a
nearly constant speed of 162,000 nautical
miles per second. Therefore the time
required for a transmitted signal to travel to
the target and return as an echo to the
source is a measure of the distance to the
target. Note that the echo makes a
complete round trip, but only half the time
of travel is needed to determine the one-
way distance to the target. This radar
automatically takes this into account in
making the range calculation.
1.4 How Radar Determines
Bearing
The bearing to a target found by the radar
is determined by the direction in which the
radar antenna is pointing when it emits an
electronic pulse and then receives a
returning echo. Each time the antenna
rotates pulses are transmitted in the full
360 degree circle, each pulse at a slightly
different bearing from the previous one.
Therefore, if one knows the direction in
which the signal is sent out, one knows the
direction from which the echo must return.