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3-1
3. Radar Observation
3.1 General
3.1.1 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
echoing area of 10 m
2
is still shown separate from the point representing the antenna position.
It is mainly dependent on the pulse length, antenna height, and signal processing such as main bang reduction 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 MSC.192(79) requires the minimum range to be less than 40 m, respectively. This
series of 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 antenna 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.
where Rmax: radar horizon (nautical miles)
h1: antenna height (m)
h2 : target height (m)
For example, if the height of the antenna above the waterline is 9 meters and the height of the target is 16 meters, the
maximum radar range is;
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 equation on the previous page 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.
Bearing resolution
Bearing resolution is the ability of the radar to display as separate pips the echoes received from two targets that are
at the same range and close together. It is proportional to the antenna length and reciprocally proportional to the
wavelength. The length of the antenna radiator should be selected 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.