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1. PRINCIPLE OF OPERATION
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.
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.
How Radar Determines Range
Radar determines the range 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 range 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 range to the target. This
radar automatically takes this into account in
making the range calculation.
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.
Radar Wave Speed and Antenna
Rotation Speed
The speed of the radar waves out to the target
and back again as echoes is extremely fast
compared to the speed of rotation of the
antenna. By the time radar echoes have
returned to the antenna, the amount of antenna
rotation after initial transmission of the radar
pulse is extremely small.
The Radar Display
Targets are displayed on what is called a Plan
Position Indicator (PPI). This display is
essentially a polar diagram, with the transmitting
ship’s position at the center. Images of target
echoes are received and displayed at their
relative bearings, and at their distance from the
PPI center.
With a continuous display of the images of
targets, the motion of targets is also displayed.
See the figure on the next page for a
comparison of actual situation and radar picture.