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-3.1-
3 RADAR OBSERVATION
3.1 General
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 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 quantization. It is a good practice to use a shorter range scale as far as it
gives favorable 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 requirement.
Maximum range
The maximum detecting range of the radar, Rmax, varies considerably depending on several
factors such as the height of the scanner 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 horizon 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 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
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 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 resolution and range
resolution.