Sea Clutters - JRC JMA-610 Series Instruction Manual

5.3

SEA CLUTTERS

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.
[I] 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.
Accordingly, it is necessary to properly adjust the sea clutter rejection function. Table 5.1 shows the
relation between the sea state (SS) showing the size of waves generated by wind and the radar's detection
probability.
As shown in Table5.2, the number of SS increases as the wind speed becomes high and the waves become
large. Table 5.1 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 5.2 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
When observing 5m
detection probability is M-NV, as shown in Table5.1, which indicates 50 % or less.
Table 5.1 Sea state and probability of target detection
RCS SS1 to 2
2
1m V-M
2
5 m
2
10 m
(Probability to detect a target at a distance of 0.7 NM)
V: Detection probability of 80 %
M: Detection probability of 50 %
NV: Detection probability of less than 50 %
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
2
target in the sea state (SS3) in which significant wave height exceeds 1.2 meters,
SS2 to 3 SS3 to 4
M-NV
V V-M M-NV
V V
Significant wave height (m)
5-4
SS4 to 5
V V-M
<0.2
0.6
0.9
1.2
2.0
3.0
4.0