Section 7 True and False Echoes on Display 7-6
As shown in the following table, the number of SS increases as the wind speed becomes high and the
waves become large. The tables in the previous page show 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.
Relation between Douglas Sea State and Average Wind Speed and Significant Wave Height
Sea state Average wind speed (kn) Significant wave height (m)
0 <4 <0.2
1 5-7 0.6
2 7-11 0.9
3 12-16 1.2
4 17-19 2.0
5 20-25 3.0
6 26-33 4.0
Significant wave height: an average of top N/3 higher waves when the number of waves detected
within a constant time duration is N
For example, in the case of a standard marine buoy, RCS of X band radar is 5 m
2
as shown in the
Table “Relation between Type and Height of Target and Detection Range and RCS” on Page 7-4.
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 the Table on page 7-5, which indicates 50 % or less.
7.3.2 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. The amount of
attenuation depends on the transmission frequency, antenna beam width, and the pulse length. The
following 2 graphs show examples in which detection range 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
length of 0.8 µs) when it was not raining, may become dimly visible up to 5 NM when the amount of
rain becomes 4 mm/hr. Furthermore, when comparing the X band radar with the S band radar, target
detection range decreases less when an S band radar is used, which means it is influenced less by
precipitation.
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