T
-900 series Chapter 7 Principle of radar system
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7-2
7.2
Characteristics of radar radio wave
The radio wave propagates slightly along the ground (primarily line of sight). This characteristic varies
depending on the density of atmosphere but generally calculated according to the formula as shown
below, considering that the distance with radar sight D is about 6% longer than the distance with optical
sight.
h2
Radar radio wave
Sight line
Earth
D(NM)=2.22 X ( h1 + h2
h1
Figure 7-3 Characteristics of radar radio wave
h2 : Target height (m)
h1 : Antenna height (m)
Target with less reflection
The strength of the reflected waves from a target varies depending on not only the distance from the
target, its height or size, but also (radar absorbing) material and shapes. A target with a low degree of
reflection or low incident angle, such as FRP and wooden is not reflected well. Therefore, care should
be taken that a FRP ship, a wooden ship or an object such as sand, a sandbar and muddy cay poor
radar targets.
Since the distance from the coastline, etc. to your ship on a radar image to be seen as longer than that
from the actual coastline, more caution should be used navigating around such objects.
Seen
Not seen
3
1
HU
Coastline on radar imag e
Actual (not seen)
coastline
Figure 7-4 Example of targets hard to be reflected
Radar shadow
Since the radar radio wave propagates straightly, your ship’s chimney stack and mast close to the
antenna, a large ship or mountain can create blind spots for which the radar wave cannot penetrate. In
such cases, they may completely or partially hide targets and cast a long shadow.
Since the shadow your ship’s chimney stack or mast is known at the time of installation, proper
selection of the antenna location is required to reduce the shadow effect. Since targets less likely to be
in this shadow area than in open sea, an extra attention should be taken in shadow areas.
False image
An image that does not actually exist (false image) at sea may appear on the display.
The phenomenon that causes false echoes are categorized and be described as follows:
