False Echoes - Furuno MODEL 1622 Operator's Manual

Range measurement
Measurement of the range to a target is also a
very important function of the radar. Generally,
there are two means of measuring range: the
fixed range rings and the variable range marker
(VRM). The fixed range rings appear on the
screen with a predetermined interval and
provide a rough estimate of the range to a target.
The variable range marker's diameter is
increased or decreased so that the marker
touches the inner edge of the target, allowing
the operator to obtain more accurate range
measurements.

2.2 False Echoes

Occasionally echo signals appear on the screen
at positions where there is no target or
disappear even if there are targets. They are,
however, recognized if you understand the
reason why they are
displayed. Typical false echoes are shown
below.
Multiple echoes
Multiple echoes occur when a transmitted pulse
returns from a solid object like a large 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 below. Multiple
reflection echoes can be reduced and often
removed by decreasing the gain (sensitivity) or
properly adjusting the [STC] control.
Target
Own ship
Figure 2-2 Multiple echoes
16
True
echo
Multiple
echo
Sidelobe echoes
Every time the radar pulse is transmitted, some
radiation escapes on each side of the beam,
called sidelobes. If a target exists where it can
be detected by the side lobes as well as the
main lobe, the side echoes may be represented
on both sides of the true echo at the same range.
Sidelobes show usually only on short ranges
and from strong targets. They can be reduced
through careful reduction of the gain or proper
adjustment of the [STC] control.
Target A
Target B
(Spurious)
Figure 2-3 Sidelobe echoes
Virtual image
A relatively large target close to your ship may
be represented at two positions on the screen.
One of them is the true echo directly reflected by
the target and the other is a false echo which is
caused by the mirror effect of a large object on
or
close to your ship as shown in the figure below.
If your ship comes close to a large metal bridge,
for example, such a false echo may temporarily
be seen on the screen.
Target ship
Own
ship
Mirror image
of target ship
Figure 2-4 Virtual image
Target B
(True)
True
echo
False
echo