False Echoes - Furuno 851 MARK-2 Operator's Manual

Radar is able to display two
distinct target echoes.
Target
Horizontal
beamwidth
Direction of
scanner rotation
Target
Horizontal
beamwidth
Direction of
scanner rotation
Radar cannot display targets as
separate echoes because they
are within the beamwidth.
Figure 3-3 Bearing resolution

3.5 False Echoes

Occasionally false echoes appear on the
screen at positions where there is no target.
In some cases the effects can be reduced
or eliminated. The operator should
familiarize himself or herself with the
appearance and effects of these false
echoes, so as not to confuse them with
echoes from legitimate contacts.
Multiple echoes
Multiple echoes occur when a short range,
strong echo is received from a ship, bridge,
or breakwater. A second, a third or more
echoes may be observed on the display at
double, triple or other multiples of the
actual range of the target as shown in
Figure 3-4. Multiple reflection echoes can
be reduced and often removed by
decreasing the sensitivity or properly
adjusting the STC.
Target
Own ship
Figure 3-4 Multiple echoes
Side-lobe echoes
Every time the antenna rotates, some
radiation escapes on each side of the
beam-called "side-lobes." If a target exists
where it can be detected by the side-lobes
as well as the main-lobe, the side-lobe
echoes may be represented on both sides
of the true echo at the same range, as
shown in Figure 3-5. Side-lobes show
usually only at short ranges and from
strong targets. They can be reduced
through careful reduction of the sensitivity
or proper adjustment of the STC.
Main-lobe
Side-lobe
Spurious
Antenna
Figure 3-5 Side-lobe echoes
True
echo
Multiple
echo
True echo
target
3-3