JMA-2343/2344 - ProNav

4.3.1 Sea returns
On a wavy sea surface, an echo appears on the PPI screen as a bright defused image at the
center of the screen. This echo is developed by returns from the sea surface. The features of
the echo depend on the size and range of the wave, and the wind direction.
4.3.2 False echoes
There are cases in which nonexistent targets appear as echoes or in which existing targets
do not appear as echoes on the PPI screen. These echoes are called false echoes.
False echoes are produced by the factors explained below.
• Please observe carefully in consideration of there being always false echoes.
(a) Ghost
Depending on the location where the scanner unit is installed, radio waves are emitted from nearby
chimney stacks or masts, developing ghosts. Consequently, targets that are located in those direction
may not appear as echoes on the PPI screen.
The presence of these ghosts can be identified by observing sea returns and checking the returns for
dim areas or voids.
If a ghost is detected, remember the direction in which it appears and observe the target carefully.
(b) Side echo
There are cases in which an arc-shaped broken line appears over
the same range as the echo from the target. This image is caused by
the side lobes of the beam emitted from the scanner unit. This type
of false echoes can easily be identified if the target stands alone.
(c) Indirect echo
The direction of the radio wave
from the radar may be changed by a
reflection (secondary reflection) at a
chimney stack or mast of a ship,
developing a false echo in a direction
in which there should be no target.
The indirect echo developed by the
secondary reflection appears in the
direction of the chimney stack or
mast that reflected the radio wave.
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