Furuno 1942 MARK-2 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 surface on your own ship, for example, a stack. In both cases, the echo will return from a legitimate contact to the scanner 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. Figure 4-3 illustrates the effect of an indirect echo. Indirect echoes may be recognized as fol- lows:

•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.

Direct	Heading
path	marker
	Target

Indirect path

Obstruction

Scanner (mast, funnel. etc.)

Indirect	Target		True
		Indirect	echo
path	Direct	echo
	path

Heading

Ownmarker ship

True echo

Bridge

Indirect

Indirectecho echo

Figure 4-3 Indirect echoes

4.4 Blind and Shadow Sectors

Funnels, stacks, masts, or derricks in the path of antenna may reduce the intensity of the radar 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 targets at close range may not be detected while larger targets at much greater ranges may be detected. See Figure 4-4.

Figure 4-4 Blind and shadow sectors

4-2