False Echoes; Multiple Echoes; Sidelobe Echoes - Furuno FR-7041R-MARK-2 Operator's Manual

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.
2.2.1

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 [A/C SEA]
control.
2.2.2

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 A/C SEA control.
Target
Own ship
Figure 2-2 Multiple echoes
Target A
Target B
(Spurious)
Figure 2-3 Sidelobe echoes
2. INTERPRETING THE DISPLAY
True
echo
Multiple
echo
Target B
(True)
2-3