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
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
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
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.
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