Furuno 1932, 1942, 1832 manual Multiple Echoes, Side-lobe Echoes, Indirect Echoes

4. FALSE ECHOES

Occasionally false echoes appear on the screen at positions where there is no target. In some cases the effects can be reduced or eliminated.

sented on both sides of the true echo at the same range, as shown in Figure 4-2. Side-lobes show usually only at short ranges and from strong tar- gets. They can be reduced through careful re- duction of the sensitivity or proper adjustment of the A/C SEA.

The operator should familiarize himself or her- self with the appearance and effects of these false echoes, so as not to confuse them with echoes from legitimate contacts.

Main-lobe

True echo

4.1 Multiple Echoes

Multiple echoes occur when a short range, strong echo is received from a ship, bridge, or breakwater. A second, a third or more echoes may be observed on the display at double, triple or other multiples of the actual range of the tar- get as shown in Figure 4-1. Multiple reflection echoes can be reduced and often removed by decreasing the sensitivity or properly adjusting the A/C SEA.

True echo

Own ship Multiple

echo

Figure 4-1 Multiple echoes

4.2 Side-lobe Echoes

Every time the antenna rotates, some radiation 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 repre-

Side-lobe

Sprious target

Antenna

Figure 4-2 Side-lobe echoes

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.

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