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15
3. RADAR OBSERVATION
3.1 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 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.
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 3-1 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).
Radar resolution
There are two important factors in radar resolu-
tion (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 which are at the same range
and close together. It is proportional to the
antenna length and reciprocally proportional to
the wavelength.
Range resolution
Range resolution is the ability to display as
separate pips the echoes received from two
targets which are on the same bearing and
close to each other. This is determined by
pulselength only. Practically, a 0.08
microsecond pulse offers the discrimination
better than 35 m as do so with all FURUNO
radars.
Test targets for determining the range and
bearing resolution are radar reflectors having an
echoing area of 10 m
2
.