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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.
A. Ghost echoes
Sometimes one large object very near your boat will appear as two dif-
ferent targets onscreen. 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 radar waves from
bridges, quay walls or buildings along the shore.
B. 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 may 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.
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.
D. Distant false echoes caused by duct phenomenon
The duct phenomenon sometimes occurs when meteorological condi-
tions create a temperature inversion between layers of air. When this
happens, radar waves propagate erratically and can reach a location
considerably farther away from your boat than the radar's maximum
distance range.
What appears onscreen is a false echo that looks to be nearer than the
actual target. Since the true echo from the distant target is outside the
measurement capabilities of the radar, its apparent distance will
change when you change ranges, and you can conclude that it's a false
echo.