Furuno FR-2105-B - Manual (Page 107)

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2.19 Factors Affecting ARPA Functions
Sea returns
If the radar anti-clutter control is adjusted properly, there is no serious effect because distant
wave clutter, not eliminated by this control, is filtered out by more than one bang correlation
and scan-to-scan matching of data.
Rain and snow
Clutter can be acquired and tracked as targets. Adjust the A/C RAIN control. If it is heavy rain,
switch to S-band if provided, or switch on the interference rejector on the radar. If heavy
clutter still exists, switch to manual acquisition. Accuracy can be affected.
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. 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 5 times (not necessarily continuously) out of 10 scans. If not
detected 6 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 1 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.