MDP-621/641/640
Chapter1
Operation manual
General Information
A. 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, quay walls or building along shore.
Target
Direct reflection
path
Secondary
reflection path
Mast etc.
Real echo
Ghost echo
Direction of ghost echo
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1
HU
Figure 1. 9 False echoes of radar (Ghost echoes)
B. Multiple echoes
If there is a large vertical reflecting surface near your boat, as in the case when you pass along side 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.
3
1
HU
Real echo
Multiple
Figure 1. 10 False echoes of radar (Multiple echoes)
C. False echoes caused by side lobe
An antenna's side lobe emissions are low power, and will not register distant targets. However, if
there is a strong reflecting target near your boat, it sometimes may appear as a circular-arc false echo
on the screen.
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