Furuno FR-8251, FR-8111 manual Bearing Resolution, False Echoes, Indirect echoes, Multiple echoes

2.3 Bearing Resolution

Bearing resolution is a measure of the capability of a radar to display as separate targets the echoes re- ceived from two targets which are at the same range and are close together.

The principal factor which affects bearing reso- lution is horizontal beamwidth. Two targets at the same range must be separated by more than one beamwidth to appear as separate pips.

	Heading
Direct	marker
	Target
path
	Indirect
	path
	Obstruction
Scanner	(mast, funnel,
	etc.)

	Target		True
			echo
		Indirect
Indirect
	Direct	echo
path
	path

Target

Horizontal beamwidth

Direction of scanner rotation

Target

Radar is able to display two distinct target echoes.

	Heading
Own
	marker
ship

True echo

Bridge

Indirect

Horizontal beamwidth

Direction of scanner rotation

Radar cannot display targets as separateechoes because they are within the beamwidth.

Figure 2-3 Bearing resolution

2.4 False Echoes

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 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 di- rect echo. Figure 2-4 illustrates the effect of an indirect echo. Indirect echoes may be recognized as follows: (1) they usually occur in a shadow sector; (2) they appear on the bearing of the ob- struction but at the range of the legitimate con- tact; (3) when plotted, their movements are usually abnormal, and (4) their shapes may indi- cate they are not direct echoes.

Indirectecho echo

Figure 2-4 Indirect echoes

Multiple echoes

Multiple echoes occur when a short range, strong echo is received from a ship, bridge, or breakwa- ter. A second, a third or more echoes may be ob- served on the display at double, triple or other multiples of the actual range of the target as shown in Figure 2-5. Multiple echoes can be reduced and often removed by decreasing the sensitivity or properly adjusting the A/C SEA.

True echo

Target

Own ship

Multiple echo

Figure 2-5 Multiple echoes

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