2-1
RADAR OBSERVATION
For example, if the height of the scanner
above the waterline is 9 meters and the height
of the target is 16 meters, the maximum ra-
dar 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 precipita-
tion 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 resolution
and range resolution.
Bearing resolution
Bearing resolution is the ability of the radar
to display as separate pips the echoes re-
ceived from two targets which are at the same
range and close together. It is proportional
to the scanner length and reciprocally pro-
portional to the wavelength. The length of the
scanner radiator should be chosen for a bear-
ing resolution better than 2.5
°
(IMO Resolu-
tion). 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.
2.1 General
Minimum and maximum ranges
Minimum range
The minimum range is defined by the short-
est distance at which, using a scale of 1.5 or
0.75 nm, a target having an echoing area of
10 m
2
is still shown separate from the point
representing the scanner position.
It is mainly dependent on the pulselength,
scanner height, and signal processing such
as main bang suppression and digital quan-
tization. It is a good practice to use a shorter
range scale as far as it gives favorable defi-
nition or clarity of picture. The IMO Resolu-
tion A. 477 (XII) and IEC 936 require the
minimum range to be less than 50 m. All
FURUNO radars satisfy this requirement.
Maximum range
The maximum detecting range of the radar,
Rmax, varies considerably depending on sev-
eral factors such as the height of the scan-
ner above the waterline, the height of the
target above the sea, the size, shape and
material of the target, and the atmospheric
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% be-
cause of the diffraction property of the radar
signal. The Rmax is given in the following
equation.
R
max
= 2.2 x ( h1 + h2 )
where
Rmax: radar horizon (nautical miles)
h1:
scanner height (m)
h2:
target height (m)
Radar horizon
Optical horizon
Figure 2-1 Radar horizon
Summary of Contents for FR-1710
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