Furuno GD-1700, Operator'S Manual

Page: 140 / 249

3. RADAR OPERATION
3-46
3.26.2 False echoes
Occasionally echo signals appear on the screen at positions where there is no
target or disappear even if there are targets. False target situations may be
recognized, however, if you understand 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 [A/C SEA]
control.
Own ship
Target
True
echo
Multiple
echo
Multiple echoes
Sidelobe echoes
Every time the radar pulse is transmitted, some radiation escapes on each side
of the beam. This stray RF is called a “sidelobe.” If a target exists where it can
be detected by the sidelobes 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 B
(True)
Target B
(Spurious)
Target A
Sidelobe echoes