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Identifying False Echo
Returns
Indirect Echoes
Thereareseveraltypesofindirectechoesorghostimages- some examplesare
shown in Figure 3-4. These sometimes have the appearance of true echoes,
but in general they are intermittent and poorly defined.
Figure 3-4: Indirect Echoes
Multiple Echoes
Multiple echoes appear if there is a large target with a wide vertical surface at
a comparativelyshort range. Thetransmitted signal willbe reflectedback and
forth between the target and your own ship. Multiple echoes therefore appear
beyond the true target’s echo on the same bearing, as shown in Figure 3-5.
This is not very common.
Figure 3-5: Multiple Echoes
Blind Sectors or Shadow Effect
Funnels and masts located near the antenna, may cause shadows. In shadow
areas beyond the obstruction there will be a reduction of the beam intensity,
although not necessarily a complete cut-off. However, there will be a blind
sector if the subtended angle is more than a few degrees. There may also be
multiple echoes which extend behind the obstruction. This should be avoided
by careful selection of the scanner site prior to installation (Chapter 8).
False echo
True echo
Passing
ship
True echo
False echo
D1641-2
Mast
or funnel
Indirect (false) echoes
D1642-1
True echo
Multiple echoes
MULTIPLE ECHOES
81186_3.book Page 6 Thursday, August 22, 2002 8:23 AM