Furuno 1824C manual Interpreting the Radar Display, False echoes, Multiple echoes, Sidelobe echoes

2. RADAR OPERATION

2.27 Interpreting the Radar Display

2.27.1 False echoes

Occasionally echo signals appear on the screen at positions where there is no target or disappear even if there are targets. False target situations may be recognized, however, if you understand why they are displayed. Typical false echoes are shown below.

Multiple echoes

Multiple echoes occur when a transmitted pulse returns from a solid object like a large 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 target as shown below. Multiple reflection echoes can be reduced and often removed by decreasing the gain (sensitivity) or properly adjusting the A/C SEA control.

Sidelobe echoes

Every time the radar pulse is transmitted, some radiation escapes on each side of the beam. This stray RF is called a “sidelobe.” If a target exists where it can be detected by the sidelobes as well as the main lobe, the side echoes may be represented on both sides of the true echo at the same range. Sidelobes show usually only on short ranges and from strong targets. They can be reduced through careful reduction of the gain or proper adjustment of the A/C SEA control.

Virtual image

A relatively large target close to your ship may show at two positions on the screen. One of them is the true echo directly reflected by the target and the other is a false echo which is caused by the mirror effect of a large object on or close to your ship as shown in the figure below. If your ship comes close to a large metal bridge, for example, such a false echo may temporarily be seen on the screen. Virtual image

True echo

Target

Own ship

Multiple echo

Multiple echoes

Target A

Target B

(Spurious) Target B

(True)

Sidelobe echoes

Target ship

Own ship

True echo

Mirror image of target ship

False echo

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Virtual image