Furuno FR-2125V - Manual (Page 67)

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
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 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
Figure 2-1 Radar 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).