Simrad CR34, User Manual

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CR34/44/54 Radar menu Chapter 3-7
3.1.11 False echoes
Occasionally, echoes may appear on the screen at positions where
there is no actual target. This type of target is called a False Echo.
Sometimes they are known as Ghost Images, Indirect Echoes or
Multiple Echoes depending on how they are generated.
Ghost images usually have the appearance of true echoes, but, in
general, they are intermittent and poorly defined. A true ghost image
retains a fixed relationship with respect to the true image and char-
acteristically produces a more arc-like appearance with a tendency
to smear on the screen. Ghost images are sometimes caused by large
targets which have a wide, smooth surface as they pass by near your
own ship.
Ghost images sometimes are referred to as indirect echoes. Indirect
echoes may appear when there is a large target, such as a passing
ship at a short range, or a reflecting surface, such as a funnel or
spotlight on your own ship in line with the antenna. The signal, on
first striking the smooth side of the large target, will be reflected,
and these subsequent echo returns to the antenna are shown on the
display. However, the same reflection may also hit other masts or
obstacles and then be picked up by the radar antenna with enough
strength to appear as a target on the radar screen at various locations.
Multiple echoes could appear if there is a large target having a wide
vertical surface to your own ship at a comparatively short range. The
transmitted signal will be reflected back and forth between the wide
vertical surface of the target and your own ship.
Thus, multiple echoes will appear beyond the true target’s echo on
the same bearing as shown below.
True echo
Multiple
echoes