JRC JMA-5212-4, User Manual

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6.4 FALSE ECHOES
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6.4
FALSE ECHOES
The radar observer may be embarrassed with some echoes that do not exist actually. These false echoes
appear by the following causes that are well known:
[I] Shadow
When the radar scanner is installed near a funnel or mast, the echo of a target that exists in the direction of
the funnel or mast cannot appear on the radar display because the radar beam is reflected on the funnel or
mast. Whether there are some false echoes due to shadows can be checked monitoring the sea clutter
returns, in which there may be a part of weak or no returns.
Such shadows appear always in the same directions, which the operator should have in mind in radar
operation.
[II] Side Lobe Effect
A broken-line circular arc may appear at the same range as the main lobe of the radar beam on the radar
display. This type of false echo can easily be discriminated when a target echo appears isolated. (See
Figure 6.5.)
HL
Figure 6.5
[III] False Echo by Secondary Reflection
When a target exists near own ship, two echoes from the single target may appear on the radar display.
One of those echoes is the direct echo return from the target and the other is the secondary reflection return
from a mast or funnel that stands in the same direction as shown in Figure 6.6.
Radar
scanner
Funnel
HL
Direct microwave
Secondary reflection
of microwave
Actual target
False echo from funnel
Figure 6.6
False echo from funnel