Furuno FAR-21 7-BB Series, Operator'S Manual

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3. ARP OPERATION
3-38
Blind and shadow sectors
Radar shadow or blind areas caused by obstructions aboard ship, for example,
funnels and masts, in the path of the radar beam can result in reduction of radar
beam intensity in that particular direction. This may eliminate the detection of
some targets. The ARP system will lose track of targets shortly after they are lost
on the radar picture and if they remain in a blind zone. These targets will
however be acquired and tracked when they pass out of the blind zone and
again present normal radar echo. The angular width and bearing of any shadow
sector should be determined for their influence on the radar. In certain cases
false echoes in the shadow sector cause the ARP system to acquire, track, and
vector them. Shadow sectors should be avoided.
Indirect echoes
A target at close range is usually picked up directly, but it can also be received
as reflection from a large, flat surface. This will result in the radar presenting two
or more echoes on the display, each at a different range. The ARP can acquire
and track the false echo if it is detected by five consecutive scans. Reduction in
radar gain can eliminate the multiple echoing but care should be taken as range
detection also will be reduced.
Radar interference
If interference is extreme due to another radar operating at close range, spiral
“dotting” and/or false targets may appear momentarily. The interference rejector
can clear the display.
To receive radar beacon or SART signals, turn on “SART” in the ECHO menu.