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1-1
1.1 What is Radar?
The term “RADAR” is an acronym meaning
RAdio Detection And Ranging. Although the
basic principles of radar were developed dur-
ing World War II, echoes as an aid to naviga-
tion is not a new development.
1.2 How Ships Determined
Position Before Radar
Before the invention of radar, when running
in fog near a rugged shoreline, ships would
sound a short blast on their whistles, fire a
shot, or strike a bell. 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 rela-
tive 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 recep-
tion 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 dis-
tance 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 automati-
cally takes this into account in making the
range calculation.
1. PRINCIPLE OF OPERATION
1.4 How Radar Determines
Bearing
The bearing to a target found by the radar is
determined by the direction in which the ra-
dar scanner antenna is pointing when it emits
an electronic pulse and then receives a re-
turning echo. Each time the scanner rotates
pulses are transmitted in the full 360 degree
circle, each pulse at a slightly different bear-
ing 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.
1.5 Radar Wave Speed and
Scanner Rotation Speed
Note that the speed of the radar waves out
to the target and back again as echoes is
extremely fast compared to the speed of ro-
tation of the scanner. By the time radar ech-
oes have returned to the scanner, the amount
of scanner rotation after initial transmission
of the radar pulse is extremely small.
1.6 The Radar Display
The range and bearing of a target is displayed
on what is called a Plan Position Indicator
(PPI). This display is essentially a polar dia-
gram, with the transmitting ship’s position at
the center. Images of target echoes are re-
ceived and displayed at their relative bear-
ings, and at their distance from the PPI center.
With a continuous display of the images of
targets, the motion of the transmitting ship is
also displayed.