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2. RADAR, CHART RADAR OPERATION
2-65
2.42 Radar Observation
2.42.1 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 antenna position.
It is mainly dependent on the pulse length, antenna height, and signal processing such
as main bang suppression and digital quantization. It is a good practice to use a short-
er range scale as far as it gives favorable definition or clarity of picture. The IMO Res-
olution 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, R
max
, 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 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 R
max
is given in the follow-
ing equation.
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 differ-
ence 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.
R
max
= 2.2 x ( h1 + h2 )
w
here Rmax: radar horizon (NM)
h1: antenna height (m)
h2: target height (m)
Radar horizon
Optical horizon
R = 2.2 x ( 9 + 16 ) = 2.2 x (3 + 4) = 15.4 NM
max