Kongsberg Simrad RA83 Instruction Manual page 77

RA83/84/85/93/94/95 Series
Operation Manual
(2) False echo caused by multiple echoes
When a boat passes near a large boat, radio waves
are repeatedly reflected between own boat and the
boat nearby, causing several echoes at regular
intervals to appear in the same bearing (Multiple
reflections). These false echoes produced by
multiple reflections are called multiple echoes. In this
case, the real target is the closest one. The multiple
echoes soon disappear when your boat moves from
the boat that caused the multiple reflections or, the
boat has changed her course. For this reason, the
true image can easily be identified.
(3) False echo caused by the side lobes
The radio beam emitted from a radar
antenna contains some residual smaller
beams on both sides of the main beam.
These components are called the side
lobes. A closely located target with high
reflectivity can be displayed as an arc
caused by the side lobes. To reduce these
false echoes, reduce the gain slightly or
apply A/C sea control.
(4) Shadow and dead angle
If the funnel, mast or other structures are located near a radar antenna, the radar beam will be blocked,
causing a shadow area where no object will be detected by the radar. The angle of the shadow is called the
dead angle. To avoid the shadow, the radar antenna position should be changed to a better place to
minimize this effect. If this is not possible, the operator should be aware this dead angle exists while
operating the radar.
Height of antenna affects the short
range target detection.
antenna is mounted too high, the
radar beam may skip the target,
causing no echo detection. The
vertical beam width of the radar
antenna also affects the close range
detection.
93142105-02
Antenna
Picture of a close
in target
Shadow caused by a mast
and other structures nearby
These echoes are not shown
caused by the dead angle
The dead angle changes according to the
dimensions and distance of the structures.
If the
Antenna
height
Target
Boat
Other targets
Main lobe
Width of vertical
beam
Unilluminated
area
Chapter 5
Basic Operation
True echo
Multiple echoes
Real echo of a
strong target
False echoes of the
strong target caused
by the side lobe
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