89
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.
To Next Page
Loading...
