Furuno FR-7112 - Manual (Page 13)

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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 indi-
cated 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 ra-
dar 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 tar-
get. 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. 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 radar
scanner antenna is pointing when it emits an
electronic pulse and then receives a returning
echo. Each time the scanner 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 di-
rection 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 extremely
fast compared to the speed of rotation 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 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 dis-
played at their relative bearings, and at their
distance from the PPI center.
With a continuous display of the images of tar-
gets, the motion of the transmitting ship is also
displayed.