Furuno 1730 Operator's Manual page 15

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 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 Fig.3~4. Multiple reflection
echoes can be reduced and often
removed by decreasing the gain or
proper ly adjusting the A/C SEA.
Fig.3"4 Multiple Echoes
Side-Lobe Echoes
Every time the scanner rotates, some
radiation escapes on each side of the
beam—callcd "sidc-lobes." If a target
exists where it can be detected by the
side-lobes as well as ihe main lobe, the
side ecliocs niay bc represented on both
sides of tlic true echo at the same range,
as show ti in Fig.3-5. Side-lobes show
usually only on short ranges and from
strong targe Is. Thcy can be reduced
through careful rcduction of the gain or
proper adjustmcnl of the \(C ska control.
Fig.3"5 Side-Lobe Echoes
Blind and Shadow Sectors
Funnels, stacks, masts, or derricks in the
path of antenna may rcducc the intensity
of the radar beam. If the angle subtended
at the scanner is more than a few degrees
a blind sector may bc produced. With in
the blind sector small targets at close
range may not bc detected while 1 arger
targets at mueh greater ranges may be
detected. See Fig,3-6.
3-5
Indirect Echoes
Indirect echoes may be returned from
either a passing ship or returned from a
reflecting surface on your own ship, for
example, a stack. In both cases, the echo
will return from a legitimate contact to
the antenna by the same indirect path.
The echo will appear on the same bearing
of the reflected surface, but at the same
range as the direct echo. Fig.3-7
illustrates the effeet of an indirect echo.
Indirect echoes. may be recognized as
follows. (1) they usually occur in a
shadow sector; (2) they appear on the
bearing of the obstruction but at the
range of the legitimate contact; (3) when
plotted, their movements are usually
abnormal, and (4) their shapes may
indicate that they are not direct echoes.
True
Echo
False
Echo
Fig.3-7 Indirect Echoes
RADAR PICTURE AND CORRESPONDING CHART
Under normal conditions, a picture which is very similar to a chart can be obtained on
the radar display. The radar picture and corresponding chart shown in Fig.3-8 are from
the Kada Inland Sea, south of Osaka Bay, in Southwestern Japan.
3-6