Furuno FR-7252, FR-7112, FR-7062 manual Multiple Echoes, Side-lobe 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. 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.

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-

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- 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.

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.

Main-lobe

True echo

True echo

Own ship

Multiple echo

Figure 4-1 Multiple echoes

Side-lobe

Spurious target

Antenna

Figure 4-2 Side-lobe echoes

4-1