JRC JMA-9133-SA Instruction Manual page 340

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JMA-9100 Instruction Manual 6.TRUE AND FALSE ECHOES ON DISPLAY
CLUTTER
As shown in Table 6-2, the number of SS increases as the wind speed becomes
high and the waves become large. Table 6.2 reveals that detection probability
decreases from V (80 %) to NV (less than 50 %) as the number of SS increases.
Therefore, even if the sea state is calm and a target clearly appears on the radar
display, when the sea state becomes rough, target detection probability decreases
resulting in difficulty of target detection by the radar.
Table 6-3： Relation between Douglas sea state and average wind speed and significant
For example, in the case of a standard marine buoy, RCS of X band radar is 5 m2
as shown in Table 6-1. When observing such a target in the sea state (SS3) in
which significant wave height exceeds 1.2 meters, detection probability is M-NV,
as shown in Table 6-2, which indicates 50 % or less.
b. 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. The amount of attenuation depends on the
transmission frequency, antenna beam width, and the pulse length. Fig 6-4 and
Fig 6-5 show examples in which detection distance is reduced due to the influence
of precipitation. Because of this, a target, which clearly appeared up to 10 NM by
an X band radar (pulse width of 0.8μs) when it was not raining, may become
dimly visible up to 5 NM when the amount of rain becomes 4 millimeters per
hour. Furthermore, when comparing the X band radar with the S band radar,
target detection distance decreases less when an S band radar is used, which
means it is influenced less by precipitation.
Sea state Average
speed (kn)
0 <4
1 5-7
2 7-11
3 12-16
4 17-19
5 20-25
6 26-33
i. Significant wave height: an average of top N/3 higher waves when
the number of waves detected within a constant time duration is N
6－ 6
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6.3 SEA CLUTTER AND RAIN AND SNOW
wave height
wind Significant
height (m)
<0,2
0,6
0,9
1,2
2,0
3,0
4,0
wave
i