Furuno FR-2805 Series - Manual (Page 80)

2-19
Automatic acquisition areas and
suppression lines
Performance of auto-acquisition is en-
hanced by controlling the limit lines (sup-
pression line) in the former series of
FURUNO ARPAs. In this new series of
ARPAs, the automatic acquisition rings
are used instead of the limit lines.
Auto acquisition rings work as suppres-
sion lines when viewed from the opposite
direction. They should be placed clear of
a landmass or shoreline. The acquisition
areas may be a full 360 degree circle or
sector of any angles are gyro stabilize.
Qualitative description of
tracking error
The FURUNO ARPA accuracies comply
with or exceed IMO standards.
Own ship maneuvers
For slow turns there is no effect. For very
high turning rates (greater than 150
°
/
minute, depending on gyro), there is some
influence on all tracked targets which last
for a minute or two and then all tracked
targets revert to full accuracy.
Other ship maneuvers
Target ship courses, lag 15 to 30 seconds
at high relative speed, or 3 to 6 seconds
at low (near 0) relative speed. It is less
accurate during a turn due to lag, but ac-
curacy recovers quickly.
2.18 Factors Affecting ARPA
Functions
Sea returns
If the radar anti-clutter control is adjusted
properly, there is no serious effect be-
cause 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 con-
trol (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. Au-
tomatic acquisition is done when the tar-
get is detected 5-7 times continuously.
Tracking is achieved when the target is
detected 5 times (not necessarily continu-
ously) out of 10 scans. If not detected 6
times out of 10 scans, the target will be-
come a "lost target." The ARPA will ac-
quire 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 differ-
ent timebase sweep than the transmitted