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Radar Installation Manual Page 33
create a shdow zone behind it and prevent you from seeing targets on the other side. More
importantly, if a mast or some part of the boat’s superstructure is in the path of the antenna’s
sweep, this will also create a shadow zone. No targets will be recognized behind it and it
could create a dangerous situation.
A.1.2 False echoes
Sometimes radar will display targets on screen that do not exist in the real world. You should
be aware of how and why this happens.
Ghost echoes
Sometimes one large object very near your boat will appear as two different targets on
screen. One is the actual radar echo. The other is a ghost echo generated by a re-reflection
of the original signal. It comes back to your own boat, bounces back to the target, and then
is picked up by the antenna on the second bounce. The actual echo appears at the correct
distance and bearing on the screen. The ghost echo appears somewhere behind your boat.
This type of false echo is also generated by re-reflection of waves from bridges, break walls
or building along shore.
Target
Direct
reflection path
Secondary
reflection path
Mast etc.
Real echo
Ghost echo
Direction of ghost echo
3
1
HU
Figure A.1.2 - Ghost Echoes
Multiple echoes
If there is a large vertical reflecting surface near your boat, as in the case when you pass
alongside a large ship, radar signals are repeatedly bounced back and forth between your
boat and the other object. Two to four images appear on the screen at equal intervals in the
same bearing. This is called a multiple echo. The image appearing closest to you is the real
echo. Multiple echoes will disappear as you move away from the reflecting object or its
bearing changes.