Furuno FR-7112, FR-7252, FR-7062 manual Indirect Echoes, Blind and Shadow Sectors

4.3 Indirect Echoes

Indirect echoes may be returned from either a passing ship or returned from a reflecting sur- face on your own ship, for example, a stack. In both cases, the echo will return from a legiti- mate contact to the antenna by the same indi- rect path. The echo will appear on the same bearing of the reflected surface, but at the same range as the direct echo. Figure 4-3 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 usually abnormal, and

•their shapes may indicate they are not direct echoes.

	Direct	Heading
	path	line
	Target
	Indirect
	path
	Obstruction
	Antenna (mast, funnel.
	etc.)		True
Indirect	Target
		Indirect	echo
path	Direct	echo
	path
	Own	Heading
		line
	ship

True echo

Bridge

Indirect

Indirectecho echo

Figure 4-3 Indirect echoes

4.4Blind and Shadow Sectors

Funnels, stacks, masts, or derricks in the path of antenna may reduce the intensity of the ra- dar beam. If the angle subtended at the antenna is more than a few degrees a blind sector may be produced. Within the blind sector small tar- gets at close range may not be detected while larger targets at much greater ranges may be detected. See Figure 4-4.

Vessel taller

than wharf		Mast, etc. in
		path of radar
	Wharf
		beam

	Wharf
Blind sector
(no echo)
		Size of blind	sector
		depends on target
		size and range.

Figure 4-4 Blind and shadow sectors

4-2