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3-2
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 mul-
tiples 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 prop-
erly adjusting the A/C SEA control.
Own ship
Target
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. Side
lobes show usually only on short ranges
and from strong targets. They can be re-
duced through careful reduction of the gain
or proper adjustment of the A/C SEA con-
trol.
Target B
(True)
Target B
(Spurious)
Target A
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
mined by pulselength only. Practically, a
0.08 microsecond pulse offers the discrimi-
nation better than 25 m as do so with all
FURUNO radars.
Test targets for determining the range and
bearing resolution are radar reflectors hav-
ing an echoing area of 10 m
2
.
Bearing accuracy
One of the most important features of the
radar is how accurately the bearing of a
target can be measured. The accuracy of
bearing measurement basically depends
on the narrowness of the radar beam.
However, the bearing is usually taken rela-
tive to the ship’s heading, and thus, proper
adjustment of the heading marker at in-
stallation is an important factor in ensur-
ing bearing accuracy. To minimize error
when measuring the bearing of a target,
put the target echo at the extreme posi-
tion on the screen by selecting a suitable
range.
Range measurement
Measurement of the range to a target is
also a very important function of the ra-
dar. 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, al-
lowing the operator to obtain more accu-
rate range measurements.
3.2 False Echoes
Occasionally echo signals appear on the
screen at positions where there is no tar-
get or disappear even if there are targets.
They are, however, recognized if you un-
derstand the reason why they are dis-
played. Typical false echoes are shown
below.