Icom MR-1220 Series Instruction Manual page 21

Basic radar theory
D Minimum range
The minimum range of the radar is primarily
determined by transmitter pulse length, vertical
beam width, and the scanner unit height. The quicker
transmission, the faster the echo returns so that the
distance can be measured faster.
If the scanner is placed too high from the surface of
the water, the scanner's vertical beam overshoots the
targets that are closer to your vessel.
Out of the beam cannot be detected.
Pulse length
Shorter target than the pulse length
cannot be detected.
D Blind and shadow sectors
A blind or shadow sector may appear because of
obstructions such as masts or other metal objects.
An obstruction may throw either a complete or partial
shadow as shown below. If a target is in a shadow
sector, target echoes may not appear on the screen.
Mast or similar
obstruction
When tall and massive targets such as a large island
are located at close range 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.
Large Small
Your vessel
island island
Vertical beam width
Shadow sector
Shadow sector
D Target resolution
Target resolution is determined, by the horizontal
beam width and transmit pulse width. It can be difficult
to detect two targets that are separated by a short
distance or in the same direction.
Distance resolution
When the distance between two targets is more than
the pulse width, they are displayed as two echoes.
Target 1 Target 2
Transmit
Echoes
pulse
However, if the distance between them is less than
the pulse width, they are displayed as one echo.
Target 1 Target 2
Transmit Echo
pulse
Overlapped
targets
1 and 2
Direction resolution
When two targets are separated by more than the
horizontal beam width, they are displayed as two
echoes.
Target 1
Target 2
Scanner
However, if they are closer by less than the horizontal
beam width, they are displayed as one echo.
Target 1
Target 2
Scanner
Overlapped
target 1 and 2
BASIC OPERATION
Target 1
Target 2
Target 1
Target 2
2
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