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2-1
2. RADAR OBSERVATION
2.1 General
2.1.1 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 pulselength, antenna 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 MSC.64(67) Annex 4 (Shipborne radar) and A.820:
1995 (High Speed Craft Radar) require the minimum range to be less than 50 m
and 35 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.
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
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;
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).