Furuno FR-10, Operator'S Manual

Page: 95 / 171

3-1
3. HOW TO INTERPRET THE RA-
DAR DISPLAY
3.1 General
3.1.1 Minimum and maximum ranges
Minimum range
The minimum range is defined by the shortest distance at which, using a scale of
0.0625 or 0.125 nm, a target having an echoing area of 10 m
2
is shown separate from
the point representing the antenna position.
The minimum range depends on the pulse length, antenna height, and signal process-
ing (like main bang suppression and digital quantization). Use a shorter range scale
as far as it gives favorable definition or clarity of picture.
Maximum range
The maximum detection range, Rmax, varies depending on the height of the antenna,
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 or a little shorter
than the optical horizon. The radar horizon is longer than the optical one by approxi-
mately 6% because of the diffraction property of the radar signal. The Rmax is shown
in the following formula.
If the height of the antenna is 9 m and the height of the target is 16 m, the maximum
radar range is;
Note: The detection range is reduced by precipitation (which absorbs the radar sig-
nal).
R
max
= 2.2 x ( h1 + h2)
where R
max
: radar horizon (nautical miles)
h1: antenna height (m)
h2: target height (m)
Radar horizon
Optical horizon
R
max
= 2.2 x ( 9 + 16) = 2.2 x (3 + 4) = 15.4 nm