Furuno 841 MARK-2 manual Side-lobe echoes, 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.

True echo

Target

Own ship

Multiple echo

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.

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.

	Heading
Direct	mark
	Target
path
	Indirect
	path
	Obstruction
Scanner	(mast, funnel,
Target	etc.)	True
		echo
	Indirect

Main-lobe

True echo

Indirect	Direct	echo
path
	path
	Own	Heading
		mark
	ship

Side-lobe

Spurious target

Scanner

Figure 3-5 Side-lobe echoes

True echo

Bridge

Indirect

Indirectecho echo

Figure 3-6 Indirect echoes

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