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An antenna sited at 4 meters height gives a radar horizon of 4.4 nm. Only objects that project above the radar horizon will be detected by radar. Even radars that have impressive range specifications will only detect tall objects. Raising the height of the antenna has only a limited effect. An increase from 4 to 9 meters moves the radar horizon from 4.4 to 6.6 nm.
Layering in the atmosphere can refract radar energy and weather conditions can do the same thing. The result is to either increase or decrease the distance to the radar horizon.
Sub-refraction occurs when the temperature of the air is lower than that of the sea under it. Sub-refraction tends to decrease the radar horizon while super-refraction, which is associated with stable high pressure weather, does the opposite. A warm, moist body of air over cold water will cause the radar energy to refract more than usual, thereby following the surface of the earth. This causes the radar range to increase. An even greater range increase will occur when the radar energy is repeatedly internally reflected between an atmospheric layer and the sea surface ("ducting").
A radar overlay on an electronic chart of the Bornholm channel. The echo of some of the highest hills is visible somewhat inland on the mainland (Scania). The echo of the high part of the adjacent Bornholm is seen on the coastline
In order for the radar to register a target on a correct bearing its energy must be propagated in a narrow sector: the narrower the sector, the sharper the image. This sector is called the radar lobe. The energy requirements increase as the width of the lobe is decreased so smaller radars have to make a compromise. Really big radars equipped with long antennae can propagate within a sector subtending less than 1°, while a small radar on a leisure vessel, equipped with a short antenna, has a sector subtending about 6°. A broader lobe results in a poorer accuracy of bearing measurements. To enable the radar to produce an image even when a vessel is heeling, the vertical extent of the radar pulse is usually about 30°.
Common effects related to pulse width
2. PulsesA good target will reflect the leading edge of the pulse and continue to give an echo for the entire length of the pulse. A longer pulse contains more energy. The pulse length is automatically coupled to the range scale, although some sets have an optional manual override. There are three types of pulse length: short pulse (SP), medium pulse (MP) and long pulse (LP). At short ranges one wants to use short pulses in order to get a sharper image and to improve discrimination of targets that are in line.
Two targets in the same direction give one echo if the distance between them is less than half the pulse length, because the leading edge of the second echo will overlap with the trailing edge of the first echo. (Blue arrow = transmitted pulse. Red arrow = reflected pulse.)
When the pulse width is wider than the distance separating two targets then the two objects will return only one echo. This is a function of both pulse width and distance. At short range the pulse width is smaller and then the targets will give separate echoes.
These two boats give only one echo, located fine on the starboard side of the heading line
The frequensies used by marine radars are 2,9 - 3,1 GHz, (S-band) and 9,3 - 9,5 GHz (X-band). Approximate wavelength 10 cm and 3 cm respectively. The 3 cm radar are more sensitive than 10 cm. Which
Closing the coast. Piers and houses are good radar targets, as they are hard and have marry small angles that reflect the radar pulse
A racon (see picture on the left) is a special radar "lighthouse" that transmits signals on the radar frequency. On the screen this gives a special echo. The chart gives information about the specific character of the signal transmitted, expressed as short and long lines. Note that a racon signal may not register on every sweep of the antenna. Broad-band radar does not trigger racon signals.
A light buoy with built-in radar reflector> consisting of tetragonal depressions whose sides are oriented at 90 degrees to each other. This construction always reflects some of the pulse back to the receiver, no matter from what direction it arrived.
5. Target quality
Tall cylindrical objects, such as a chimney or a cistern, give a good echo because the radar lobe has a height component Although much of the radar pulse is reflected away, a small vertical area is always normal to the pulse.
Bigger ships are excellent radar targets as they have extensive hard surfaces with many angular structures, so they give echoes a long way off. In the picture on the right the echo of a vessel can be seen fine on the port side of the head line.
In tidal waters the radar image can vary considerably between high and low water. At low water large areas of drying sandbanks will give weak or no echoes as the smooth sloping surfaces reflect much energy away from the antenna.
At close range even small boats may give echoes, as shown in the pictures above
In the picture above the wake of a fast motorboat can be seen, as well as the after-glow of the echo. The extent of the after-glow can be tuned using the TRAIL (sometimes TAIL) button to adjust a time factor
Now turn up the GAIN (image amplification) strength. Now if not before an image will show on the screen. The correct GAIN setting depends on what the Navigator wants to observe. In confined waters a low GAIN setting will give sharp coastlines whereas a higher GAIN is useful in open water where long range is desirable. Using this GAIN setting when entering a channel will give too strong echoes from the shorelines and the image of the channel may become completely obliterated by the strength of the echo.
The GAIN control on this menu-controlled radar is activated with the button under the window and is regulated by the touch pad on the right
The upper left image shows a screen with a low GAIN setting. The image to the right is dominated by echoes of waves close to the ship. As the GAIN was increased the echo of a navigational mark became visible. This is seen on the chart image (to the left, red arrow). Another mark (green arrow on the chart) has not yet been displayed on the radar screen
The response of controls on radar sets from different manufacturers can be rather different. TUNE may require only 2 fine adjustment on one set, but may need large adjustments on another set. The same thing is true for GAIN, some sets give an image with a gain at its minimum setting, in other kits it should be set much higher.All radar installations have a device for changing the SCALE (RANGE). This is the distance at the sea surface, which corresponds to the distance from the center to the edge of the image. When navigating in confined waters, adjustments must be made frequently to detect important echoes. This includes changing the scale. Longer range for finding information near rudders and shorter range for checking the immediate surroundings of the vessel. A common mistake when swimming in confined waters is to use too long a short range - you have to switch back and forth!
|Information in the righthand upper corner; Plus/minus buttons for scale changes. Below this, pulse change button, etc..||
Information in the left-hand upper corner: mode = HEAD UP. Scale = 0.5 nm. Distance zetween the fixed rings = 0.1 nm. SP = SHORT PULSE
HEADLINE MARK (HM) or HEADLINE (HL) is a line from the center of the image in the direction that the bow of the boat is pointing. It is always visible, unless you turn it off with the button in manual mode. This is done when searching for a target that may be hidden by the HEADING LINE. This is normally only a need at long range when the echo of an approach lighthouse may be weak. The picture on the right shows a weak echo just touching the HEADING LINE. This would be clearer if the HEADING LINE was suppressed for a moment or two.