Furuno 1932 MARK-2 manual False Echoes, 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 herself 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 ech- oes may be observed on the display at double, triple or other multiples of the actual range of the target as shown in Figure 4-1. Multiple reflection echoes can be reduced and often removed by decreasing the sensi- tivity 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 scanner rotates, some radia- tion 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 represented 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 targets.They can be reduced through careful reduction of the sensitivity or proper adjustment of the A/C SEA.

Main-lobe True echo

Side-lobe

Sprious

target

Antenna

Figure 4-2 Side-lobe echoes

4-1