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-3.1-
Chapter 3 RADAR OBSERVATION
3.1 General
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).