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3-1
3. RADAR OBSERVATION
For example, if the height of the antenna
above the waterline is 9 meters and the
height of the target is 16 meters, the maxi-
mum radar range is;
R
max
= 2.2 x ( 9 + 16 ) = 2.2 x (3 + 4) = 15.4 nm
It should be noted that the detection range
is reduced by precipitation (which absorbs
the radar signal).
X-band and S-band
In fair weather, the above equation does
not give a significant difference between
X- and S-band radars. However, in heavy
precipitation condition, an S-band radar
would have better detection than an X-
band radar.
Radar resolution
There are two important factors in radar
resolution (discrimination): bearing reso-
lution and range resolution.
Bearing resolution
Bearing resolution is the ability of the ra-
dar to display as separate pips the ech-
oes received from two targets which are
at the same range and close together. It is
proportional to the antenna length and re-
ciprocally proportional to the wavelength.
The length of the antenna radiator should
be chosen for a bearing resolution better
than 2.5° (IMO Resolution). This condition
is normally satisfied with a radiator of 1.2
m (4 ft) or longer in the X-band. The S-
band radar requires a radiator of about 12
feet (3.6 m) or longer.
Range resolution
Range resolution is the ability to display
as separate pips the echoes received from
two targets which are on the same bear-
ing and close to each other. This is deter-
3.1 General
Minimum and maximum ranges
Minimum range
The minimum range is defined by the
shortest distance at which, using a scale
of 1.5 or 0.75 nm, a target having an echo-
ing area of 10 m
2
is still shown separate
from the point representing the antenna
position.
It is mainly dependent on the pulselength,
antenna height, and signal processing
such as main bang suppression and digi-
tal quantization. It is a good practice to use
a shorter range scale as far as it gives fa-
vorable definition or clarity of picture. The
IMO Resolution A. 477 (XII) and IEC 936
require the minimum range to be less than
50 m. All FURUNO radars satisfy this re-
quirement.
Maximum range
The maximum detecting range of the ra-
dar, Rmax, varies considerably depending
on several factors such as the height of
the antenna above the waterline, the height
of the target above the sea, the size, shape
and material of the target, and the atmo-
spheric conditions.
Under normal atmospheric conditions, the
maximum range is equal to the radar hori-
zon or a little shorter. The radar horizon is
longer than the optical one by about 6%
because of the diffraction property of the
radar signal. The Rmax is given in the fol-
lowing equation.
R
max
= 2.2 x ( h1 + h2 )
where Rmax: radar horizon (nautical miles)
h1: antenna height (m)
h2 : target height (m)
Radar horizon
Optical horizon