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pulse. This gives an incorrect range indi-
cation. Second and third trace echoes can
be tracked if they are consistent enough
to meet acquisition and tracking criteria
but target course and speed data will be
in error.
Blind and shadow sectors
Radar shadow or blind areas caused by
obstructions aboard ship, for example,
funnels and masts, in the path of the ra-
dar 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 ac-
quired and tracked when they pass out of
the blind zone and again present normal
radar echo. The angular width and bear-
ing of any shadow sector should be de-
termined 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 "dot-
ting" 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.