Furuno 841 User Manual download pdf (Page 34)

23

Indirect echoes

Indirect echoes may be returned from either

a passing ship or returned from a reflecting

surface on your own ship, for example, a

stack. In both cases, the echo will return from

a legitimate contact to the antenna by the

same indirect path. The echo will appear on

the same bearing of the reflected surface,

but at the same range as the direct echo. Fig-

ure 3-6 illustrates the effect of an indirect

echo. Indirect echoes may be recognized as

follows:

• they usually occur in a shadow sector

• they appear on the bearing of the obstruc-

tion but at the range of the legitimate con-

tact

• when plotted, their movements are usu-

ally abnormal, and

• their shapes may indicate they are not di-

rect echoes.

Figure 3-6 Indirect echoes

oes may be observed on the display at

double, triple or other multiples of the ac-

tual range of the target as shown in Figure

3-4. Multiple reflection echoes can be re-

duced and often removed by decreasing the

sensitivity or properly adjusting the STC.

Figure 3-4 Multiple echoes

Side-lobe echoes

Every time the scanner rotates, some radia-

tion escapes on each side of the beam—

called “side-lobes.” If a target exists where

it can be detected by the side-lobes as well

as the main-lobe, the side-lobe echoes may

be represented on both sides of the true echo

at the same range, as shown in Figure 3-5.

Side-lobes show usually only at short ranges

and from strong targets. They can be reduced

through careful reduction of the sensitivity

or proper adjustment of the STC.

Figure 3-5 Side-lobe echoes

Own ship

Target

True

echo

Multiple

echo

Spurious

target

True echo

Main-lobe

Side-lobe

Scanner

Heading

mark

True

echo

Indirect

echo

Heading

mark

True

echo

Indirect

echo

Indirect

path

Direct

path

Scanner

Obstruction

(mast, funnel,

etc.)

Own

ship

Target

Bridge

Direct

path

Indirect

echo

Target

Indirect

path

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