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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 ARPA 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 ARPA 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 ARPA 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 off the radar interference rejection and echo
average which operate on the correlation technique.