Bedienungsanleitung Furuno 1622 (Seite 23 von 35) (Englisch)

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2. RADAR OBSERVATION

2.1 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 m2 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.

Rmax= 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;

Rmax= 2.2 x

(

9 + 16

)

= 2.2 x

(

3 + 4

)

= 15.4 n

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. The length of the antenna

radiator should be chosen for a bearing

resolution better than 2.5 degrees.

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 m2.

Bearing accuracy

One of the most important features of the radar

is how accurately the bearing of a target can be

measured. The accuracy of bearing

measurement basically depends on the

narrowness of the radar beam. However, the

bearing is usually taken relative to the ship’s

heading, and thus, proper adjustment of the

heading line at installation is an important factor

in ensuring bearing accuracy. To minimize error

when measuring the bearing of a target, put the

target echo at the extreme position on the

screen by selecting a suitable range.

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