3-4
If the echo disappears while the boat is
turning, the target has entered a shadow
sector and it will again become visible
when the target emerges from the
shadow. Very quiet considerations of wind
and sea are essential to ensure reliable
results when this operation is carried out
on a small craft since a rough sea can
cause a buoy to be lost in the clutter or
boat to swing through a shadow sector
before the scanner has completed one
revolution. In any case an average of sev-
eral observations of each shadow sector
should be taken. It is a waste of time to
attempt the operation in anything other
than very smooth water with little wind.
2. Another method is to observe the shadow
sector against a background of sea clut-
ter. Any shadows will show as dark sec-
tors in the clutter.
Note that a shadow cannot be fairly esti-
mated in heavy clutter, as echoes from
either side of the sector may be spread
into it and give an illusion that objects in
the sector are being observed. Nor can it
be satisfactorily determined in confined
waters, because of the probability of indi-
rect, false or multiple echoes being pro-
duced from nearby buildings or other
vessels.
SCANNER
SHADOW SECTOR
BLIND SECTOR
90°
180°
270°
0°
Figure 3-6 Shadows caused by objects
Figure 3-7 Shadow sector diagram
