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5-27
Low clouds
Usually no effect. If necessary, adjust the [A/C RAIN] control.
Non-synchronous emissions
No effect.
Low gain
Insufficient or low radar receiver gain will result in some targets not being acquired at long
distance. The ARPA display will be missing on one or more targets that could only be
visible if the radar sensitivity control (GAIN control) were increased.
The setting of the correct radar receiver gain is not critical but the target should be on the
radar PPI and be clearly visible and well defined.
Manual acquisition is done if a target is positively displayed more than once. Automatic
acquisition is done when the target is detected 5-7 times continuously. Tracking is achieved
when the target is detected five times (not necessarily continuously) out of 10 scans. If not
detected six times out of 10 scans, the target will become a “lost target.” The ARPA will
acquire a radar echo that is present once in every six antenna scans and continue tracking
if one in 10.
Second trace echoes
When the radar beam is super refracted, strong echoes may be received at such long
ranges that they appear on a different timebase sweep than the transmitted pulse. This
gives an incorrect range indication. 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 radar beam can result in reduction of radar beam intensity in
that particular direction. This may eliminate the detection of some targets. The ARPA sys-
tem 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.