JRC JMA-5912-6 Instruction Manual page 311

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
0
1
2
3
4
5
6
Significant wave height: an average of top N/3 higher waves when the
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 μs) 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.
6.3 Sea Clutter and Rain and Snow Clutter
Average wind speed (kn)
<4
5-7
7-11
12-16
17-19
20-25
26-33
number of waves detected within a constant time
duration is N
6-6
Significant wave height (m)
<0.2
0.6
0.9
1.2
2.0
3.0
4.0
6