
Side-lobe echoes
Small amounts of rf radiation on each side of the main beam are called
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
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
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
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
2 – 3