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9
BASIC RADAR THEORY
■ Multiple echoes
Multiple echoes may appear when a short-range
and strong echo is received from a vessel, bridge, or
breakwater.
Your vessel
Another vessel
Multiple echoes will appear beyond the target’s true
echo point on the same bearing of a large target.
They can be reduced with proper adjustment of the
/ control.
See page 9 for the
/ control.
■ Minimum range
Detection at short range is very important. Minimum
range is determined primarily by transmitter pulse
length, vertical beam width and height of the scanner
unit. The shorter the transmission time, the quicker
the return echoes can be received and their distance
measured.
The ability to see targets very close to the vessel is
decreased if the scanner is mounted too high off the
water, because the bottom of the vertical beam of the
scanner overshoots nearby targets.
This target cannot be detected by radar.
The target in this area cannot be detected.
Pulse length
Pulse starting point
Vertical beam width
■ Blind and Shadow sectors
Blind or Shadow sectors may exist because of
obstructions such as masts, derricks or other metal
objects. An obstruction may throw either a complete
or partial shadow as shown in the diagram below. If
a target is in a shadow sector, target echoes may not
appear on the screen.
Mast or
similar
obstruction
Shadow sector
When tall and massive targets such as a large island
are located at close range are also shadowed without
producing any echoes. This phenomenon is called
blind sector. It is very important to know the bearings
and widths of all shadow sectors caused by your own
vessel’s obstructions.
Blind sector
Your vessel
Large
island
Small
island
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