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Chapter 9 TRUE OR FALSE EHOS ON DISPLAY
9.3 SEA CLUTTER AND RAIN AND SNOW CLUTTER
In addition to the echo required for observing ships and land radar video image also includes unnecessary
echo, such as reflection from waves on the sea surface and reflection from rain and snow. Reflection from
the sea surface is called "sea clutter," and reflection from rain and snow is called "rain and snow clutter," and
those spurious waves must be eliminated by the clutter rejection function.
9.3.1 SEA CLUTTER
Sea clutter appears as an image radiating outwardly from the center of the radar display and changing
depending on the size and the shape of waves. Generally, as waves become larger, image level of the sea
clutter is intensified and the clutter far away is also displayed. When waves are large and the sea clutter
level is high, it is difficult to distinguish sea clutter from a small boat whose reflection intensity is weak.
9.3.2 RAIN AND SNOW CLUTTER
Rain and snow clutter is a video image that appears in a location where rain or snow is falling. The image
changes according to the amount of rain (or the amount of snowfall). As precipitation increases, the image of
rain and snow clutter becomes intensified on the radar display, and in the case of localized heavy rain, an
image similar to the image indicating land is displayed in some cases. Furthermore, because radio waves
tend to attenuate due to rain and snow, the ability to detect a target in the rain and snow clutter or a target
beyond the rain and snow clutter may decrease.
9.3.3 COPING WITH SEA CLUTTER AND RAIN AND SNOW
CLUTTER
When the weather is bad and the ocean is rough, reducing the pulse width will reduce the influence by
spurious waves, and also the spurious wave rejection function effectively works; therefore, the use of short
pulse is effective when the weather is bad. By using image processing functions "3Scan CORREL" to
"5Scan CORREL", it is expected that spurious waves are further suppressed. Since optimal settings for
those items can be automatically made by using the function mode, it is recommended that STORM or RAIN
be used by selecting the function mode when the weather is bad. For details of the function mode, see
Section "5.13 MODE FUNCTION" and “5.2.3 FUNCTION SETTING”.
However, these functions may make some targets invisible, particularly targets with higher speeds.