America Inc. Fish Finder User Manual

8
BASIC RADAR THEORY
23
■ Blind and Shadow sectors
Blind or Shadow sectors may exist because of ob-
structions such as masts, derricks or stacks. An ob-
struction 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.
When tall and massive targets such as a large island
are located at close range also shadowed without pro-
ducing 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
ship’s obstructions.
Blind sector
Own ship
Large
island
Small
island
Mast or
similar
obstruction
Shadow sector
■ Target resolution
Target resolution is determined by the horizontal beam
width and transmit pulse width. Sometimes it is difficult
to detect two targets which are separated by short dis-
tances or which are in the same direction.
D Distance resolution
When two targets are separated by more than the
pulse width, they appear as two echoes.
When two targets are not separated by more than
the pulse width, they appear as 1 echo.
D Direction resolution
When two targets are separated by more than the
horizontal beam width, they appear as two echoes.
When two targets are not separated by more than
the horizontal beam width, they appear as one echo.
Scanner
Target 2
Target 1
Target 1, 2
Target 2
Target 1
Scanner
Target 2
Target 1
Transmit
pulse
Echo
Target 1,2
Target 1, 2
Transmit
pulse
Echoes
Target 1
Target 2
Target 2
Target 1
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