Furuno FR-1710 User Manual download pdf (Page 51)

2-1

RADAR OBSERVATION

For example, if the height of the scanner

above the waterline is 9 meters and the height

of the target is 16 meters, the maximum ra-

dar 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).

X-band and S-band

In fair weather, the above equation does not

give a significant difference between X- and

S-band radars. However, in heavy precipita-

tion condition, an S-band radar would have

better detection than an X-band radar.

Radar resolution

There are two important factors in radar

resolution (discrimination): bearing resolution

and range resolution.

Bearing resolution

Bearing resolution is the ability of the radar

to display as separate pips the echoes re-

ceived from two targets which are at the same

range and close together. It is proportional

to the scanner length and reciprocally pro-

portional to the wavelength. The length of the

scanner radiator should be chosen for a bear-

ing resolution better than 2.5° (IMO Resolu-

tion). This condition is normally satisfied with

a radiator of 1.2 m (4 ft) or longer in the X-

band. The S-band radar requires a radiator

of about 12 feet (3.6 m) or longer.

2.1 General

Minimum and maximum ranges

Minimum range

The minimum range is defined by the short-

est 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 scanner position.

It is mainly dependent on the pulselength,

scanner height, and signal processing such

as main bang suppression and digital quan-

tization. It is a good practice to use a shorter

range scale as far as it gives favorable defi-

nition or clarity of picture. The IMO Resolu-

tion 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 sev-

eral factors such as the height of the scan-

ner 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 hori-

zon or a little shorter. The radar horizon is

longer than the optical one by about 6% be-

cause 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: scanner height (m)

h2: target height (m)

Radar horizon

Optical horizon

Figure 2-1 Radar horizon

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