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1. PRINCIPLE OFOPERATION

1.1 What is Radar?

The term "RADAR" is an acronym mean-
ing RAdio Detection And Ranging. Al-
though the basic principles of radar were
developed during World War II, primarily
by scientists in Great Britain and the United
States, the use of echoes as an aid to navi-
gation 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 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 con-
stant 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 deter-
mine 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 scanner antenna is pointing when it
emits an electronic pulse and then receives
a returning echo. Each time the scanner ro-
tates pulses are transmitted in the full 360
degree circle, each pulse at a slightly differ-
ent bearing from the previous one. There-
fore, 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

Antenna Rotation Speed

Note that the speed of the radar waves out
to the target and back again as echoes is ex-
tremely fast compared to the speed of rota-
tion of the antenna. By the time radar echoes
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
received and displayed at their relative bear-
ings, and at their distance from the PPI cen-
ter.
With a continuous display of the images of
targets, the motion of the transmitting ship
is also displayed.