Leisure marine radar is quite unlike any of the other electronic navigation aids. Not only is it more expensive to buy and more demanding of electrical power, but it also requires more skill on the part of the operator to set it up and adjust it to suit prevailing conditions and to interpret the picture on its screen. In return it is the most versatile of all electronic aids.
The basic principle of yacht radar is similar to that of an echo sounder: it transmits pulses of energy and measures the time that elapses before the echo of each one returns. One major difference is that instead of using ultrasonic sound, radar uses extremely high frequency radio waves, called microwaves - in the order of 9.5 GHz (9500 MHz) and with a wavelength of about 3 cm. The other big difference is that instead of being transmitted downwards, like the ultrasonic clicks of echo sounders, radar microwave pulses are focused into a beam by a rotating aerial and transmitted horizontally through 360° around the boat.
So a boat radar is able to measure the range of a target from the time it takes a microwave pulse to make the out and back trip, and measures the target's bearing from the direction that the scanner is pointing. This information is used to build up a picture on the display - sometimes called a PPI or 'plan position indicator', because the overall effect is rather like a plan, or bird's-eye view, of the boat's surroundings.
All radars have seven main controls:
On/standby/transmit
The on/standby/transmit control is used to turn the set on. It will have to be left in its standby mode for at least a minute or two while the magnetron - the component that actually generates the microwaves - warms up, so on most modern sets this warm-up period is indicated by a count-down timer on the screen. Once the warm-up is completed, switching to transmit mode turns the transmitter on.
Brilliance
The brilliance control determines the brightness of the picture exactly like the corresponding control on a domestic television set, and should be adjusted to give a clear but not dazzling image. On radars with a liquid crystal display, the brilliance control has to be used in conjunction with the contrast setting; the two are interdependent, and their adjustment depends on the angle from which you are looking at the screen.
Gain
Gain refers to the amount of amplification applied to the returning echo. In some ways it is easy to confuse the effect of the gain control with that of brilliance, because turning it up makes weak contacts look bigger, brighter and more consistent. The two are not interchangeable, however: brilliance is adjusted to make the picture clearer or more comfortable to look at; whereas the setting of the gain control can determine whether some contacts appear at all. As a rule, the gain should be turned up until the screen is filled with a background speckle, then turned down until the speckle just disappears, but it may need to be readjusted each time the radar's operating range is changed.
Range
The range control, as its name suggests, is used to adjust the operating range of the set, typically in about eight steps from one eighth or one quarter of a mile, to between 16 and 48 miles. The range to use depends on the job in hand: short ranges (between half a mile and 4 miles) are generally of most use for pilotage; medium ranges (4, 6 or 8 miles) for collision avoidance and long ranges (8 to 24 miles) for coastal and offshore navigation. Ranges in excess of 24 miles are of very little practical use for small boat radars.
Tuning
The tuning control, like its counterpart on a domestic radio, is used to adjust the receiver to give the best possible reception of incoming signals. Returning echoes are very weak indeed, so a precise match between the transmitter and receiver is of paramount importance. The radar's tuning control offers very fine adjustment, to allow for small variations in the transmitting frequency caused mainly by variations of temperature. To tune a radar, start by setting the brilliance to a comfortable level, adjusting the gain until the background speckle just disappears, and selecting a medium range. Choose a weak contact somewhere near the edge of the screen and concentrate on that, while adjusting the gain control in small steps - allowing at least two seconds between each step - until the chosen contact is as spacious, bright and consistent as possible.
physical objects that reflect radar waves - - best opportunity to appear on the screen as "contacts" Setting these first five checks on the words "targets" for the director. Two other controls are used to refine or clarify the image, removing unwanted contact or disorder.
Sea Clutter Control
This sometimes is to win the STC or sweep, and is used to remove the clutter of echoes caused by the waves that can not otherwiseform a bright circle or star-burst pattern in the centre of the screen.
Under normal conditions - with the sea clutter control turned right down - the radar may be receiving echoes from targets at a variety of different ranges, but with much weaker echoes from very distant targets than from targets close at hand. This means that the echoes which return very soon after each pulse has been transmitted need much less amplification than those which are received later.
The sea clutter control works by reducing the amplification of early returns even more, while leaving the later levels of amplification intact. On the screen this has the effect of obliterating weak contacts close to the boat, allowing stronger contacts to show up more clearly. If it is overdone, however, the sea clutter control is quite capable of suppressing the amplification to such an extent that even the strongest contacts - such as land - are obliterated at ranges up to several miles from the boat, so it should be used with considerable caution and always as little as is necessary.
Rain clutter
This control is sometimes known as FTC or differentiation and, as its name suggests, is used to remove the clutter caused by meteorological effects such as rain, snow or hail. A heavy rain shower can be quite an effective reflector of radar pulses, but it does not reflect them in the same way as a solid object. Instead of returning an echo which is a crisp copy of the transmitted pulse, rain echoes are weaker but more drawn out. On the screen this produces a large but relatively diffuse contact, often described as looking like a smudge or 'cotton wool'.
The rain clutter control acts by ignoring all but the leading edge of each returning echo. This effectively reduces the energy received from rain echoes to such an extent that they do not appear as a contact at all. Almost inevitably though, it reduces the energy received from real targets. The drawn out echoes produced by gently sloping coastlines such as beaches or mudflats are particularly badly affected, so the rain clutter control, like the sea clutter control, should only be used when necessary.
Interpreting the picture
The first time one looks at a boat radar screen, it often comes as something of a disappointment: the picture may look crude and blobby, and bits of the coastline may be missing, making it difficult to relate what appears on the screen to the chart of the same area. A boat radar is definitely not 'an allseeing eye' and interpreting the picture calls for practice, and a slightly deeper understanding of how the radar works.
To begin with, it may help to visualize the stream of microwave pulses leaving the radar scanner as being like the beam of a searchlight. In order to produce an echo, a target has to be 'illuminated' by the radar beam. Some materials, such as GRP, which are opaque to light are transparent to radar waves. But something such as a steel funnel in the way of the radar beam can block radar waves just as effectively as it blocks light, to cause a shadow zone which can never be illuminated. The obvious solution to this problem is to make sure that the radar scanner is mounted higher than any large metal objects on the boat. Land has a very similar effect, though without the easy cure. Bays or river entrances will be hidden from the radar by surrounding headlands just as they are hidden from the naked eye. This is the main reason why there are gaps in the radar picture of the coastline.
The biggest obstruction of all is the earth itself. There is nothing unfamiliar about the idea of things being invisible because they are 'below the horizon', nor that hills can be seen at longer ranges than low lying ground or the shoreline itself because they are tall enough to be 'above the horizon'. The same effect appears on radar: at long ranges hills may appear to be isolated islands and the true coastline may not show up at all. Microwaves bend very slightly to follow the curvature of the earth, so the radar horizon is about five per cent more distant than the visual one.
Once a target has been illuminated by the radar beam, its ability to produce an echo depends on its material, size, shape and to some extent on its surface texture. Some materials (such as GRP) are almost transparent to the microwaves. Others (such as wood) absorb microwaves. This is why yachtsmen should never assume that their GRP or wooden vessels will be 'seen' by a ship's radar. Some materials, most significantly metal, rock and water, are good reflectors of microwaves.
The effect of size is fairly obvious: in general a large target can reflect more of the radar energy than a small one, so it stands a better chance of appearing as a contact on the radar screen. The effect of size, however, is masked to some extent by the effect of shape. Spherical or cylindrical objects are poor reflectors because they scatter radar energy, instead of reflecting it back the way it came. Flat surfaces, on the other hand, can be very good reflectors indeed, because if they happen to be positioned exactly at right angles to the approaching radar beam the effect is very much like a mirror, directing the radar energy straight back to the antenna. At any other angle, however, a flat surface is likely to send the echo off in the wrong direction. The most reliable all around reflectors tend to be those with uneven surfaces, because although some of the radar energy may be scattered the rough surface almost guarantees that at least some of it will be returned.
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