Furuno FR-8111 manual Minimum and Maximum Ranges, Side-lobe echoes, Minimum range, Maximum range

Side-lobe echoes

Small amounts of rf radiation on each side of the main beam are called “side-lobes.” If a target ex- ists where it can be detected by the side-lobes as well as the main-lobe, the side-lobe echoes may be represented on both sides of the true echo at the same range, as shown in Figure 2-6.

Side-lobes show usually only at short ranges and from strong targets. They can be reduced through careful reduction of the sensitivity or proper ad- justment of the A/C SEA.

2.5Minimum 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

Main-lobe

True echo

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

Side-lobe

Spurious target

Scanner

Figure 2-6 Side-lobe echoes

Blind and shadow sectors

Funnels, stacks, masts, or derricks in the path of antenna may reduce the intensity of the radar beam. Within the blind sector small targets at close range may not be detected while larger targets at much greater ranges may be detected. See Figure 2-7.

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 water- line, the height of the target above the sea, the size, shape and material of the target, and the at- mospheric conditions.

Under normal atmospheric conditions, the maxi- mum range is equal to the radar horizon or a little shorter. The radar horizon is longer than the opti- cal one by about 6% because of the diffraction property of the radar signal. The Rmax is given in the following equation.

Vessel taller than wharf

Wharf

Wharf

Blind sector (no echo)

Mast, etc. in path of radar beam

Size of blind sector depends on target size and range.

Rmax= 2.2 x (h1 + h2 )

where Rmax: radar horizon (nautical miles) h1: antenna height (m)

h2: target height (m)

Radar horizon

Optical horizon

For example, if the height of the antenna above the waterline is 9 meters and the height of the tar-

Figure 2-7 Blind and shadow sectors

get is 16 meters, the maximum radar range is;

Rmax= 2.2 x (9 + 16 ) = 2.2 x (3 + 4) = 15.4 nm

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