User's Manual

RADARPILOT / CHARTRADAR
ED 3038 G 232 / 01 (2002-06)
Operating Instructions
15 Evaluation of the Radar Video
15.3 Undesirable Echo Displays and Effects
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In the case of strong, extended areas of rain, the losses become so great that the achievable range is
reduced very severely. Useful targets might be completely concealed.
S-Band Radar
The S-band radar „sees“ through the rain with practically no attenuation losses, and even detects targets
in and behind a rain area. The echo signals caused by the rain are only displayed weakly. Similar behav-
iour is exhibited in the case of hail and snow.
In this case too, unusually heavy rain can reduce the achievable ranges and thus cause sudden loss of
targets.
Reflections from the Sea Surface
Clutter echoes from the surface of the sea can be recognised from the fact that a mostly circular but
eccentric area displaced in the wind direction and having non-uniform brightness is formed around the
own ship symbol (sea clutter). These echoes make targets in this region more difficult to detect and track,
but they have no effect on the display of targets situated behind the reflection zone.
These disturbances are more severe in the case of a radar antenna mounted high up than for one
mounted lower down. The area covered by these disturbances increases with increasing sea state, and
is increased particularly by steeper waves.
With the aid of the sea clutter suppression function, the disturbance echoes described above can be
reduced. However, since this also weakens the display of targets and because the target might not be
displayed with every revolution of the antenna, maximum concentration by the observer is necessary. In
the case of automatic target tracking, the sea clutter suppression function can lead to the loss of targets
due to the attenuation of the signals.
Disturbances Caused by Other Radars
Transmissions from other radars working in the same electrical frequency band occur as disturbances in
the form of small points or dashes. These move in rapid succession along straight or curved tracks to the
centre of the screen or in the opposite direction.
These disturbances appear particularly clearly at the larger range settings, but they impede radar obser-
vation to an insignificant extent only. They can be almost completely eliminated by means of the Inter-
ference rejection function.
False or Indirect Echo Displays
False or indirect echo displays ("ghost echoes") occur as a result of reflection of the radar pulses by the
ship's own superstructures of all kinds and by nearby objects that reflect very well. Therefore, a typical
feature of these echoes is that they appear at the bearing of the reflecting surface, regardless of the real
position of the targets.
When two ships pass one another on parallel courses, multiple echoes can often be observed as a result
of pulses reflected to and from several times between the ships' sides. On the screen, a series of equis-
paced echo displays of decreasing intensity appears at the bearing of the object.
Sectors of Reduced Radar Visibility
Sectors of reduced radar visibility occur as a result of own ship's superstructures which impair and
prevent rectilinear propagation of the radar pulses. Theoretical determination of the geometry of these
sectors and of their influence on the display of targets is possible to an inadequate extent only. An effec-
tive practical method consists of determining the shadow sectors against the background of disturbance
signals from the sea surface (sea clutter).