JRC JMA-5212-4/6 User Manual page 324

As shown in Table 6.3, 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 wave height
Sea state
Significant wave height: an average of top N/3 higher waves when the number
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.
[II] 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. Figure 6.3 and Figure 6.4 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 0.8 Ps) 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.
Average wind speed (kn)
0 <4
1 5-7
2 7-11
3 12-16
4 17-19
5 20-25
6 26-33
of waves detected within a constant time duration is N
Significant wave height (m)
<0.2
0.6
0.9
1.2
2.0
3.0
4.0
6 7