PRACTICALITIES BY RYA (TIM BARTLETT)
Using radar effectively could be the difference between staying safe in a foggy shipping channel or approaching a night anchorage without anxiety, so getting the basics of its operation right is important.
Radar works just like an echo sounder: it transmits short pulses of energy, and listens for the echoes produced when the pulses are reflected from something in their path.
Of course, there are significant differences between a radar and an echo sounder. One is that radar sweeps its beam of pulses around the horizon like the beam of a lighthouse, instead of sending them all downwards. Another is that it uses super-high frequency radio waves – microwaves – rather than sound waves.
Microwaves travel much faster than sound waves, so everything connected with radar happens very
quickly. At its longest range setting, for instance, a typulses per second, each lasting less than a millionth of a second. At short ranges, the pulses are even shorter, and there are many more of them.
The result is that ever, rotation of the scanner gathers information about hundreds of targets, from
thousands of echoes:
– Range information from the time taken for each pulse to travel out and back.
– Bearing information from the direction the antenna was pointing when each echo was received.
It then processes data to convert it to a form that we can understand. Almost invariably, in marine radar, this is in the form of a Plan Position Indicator (PPI), in which the position of ships, buoys, boats, and coastlines is shown in a map-like presentation. It's important to appreciate, though, that the PPI isn't really a map: it's a diagram showing the range and bearing of echoes.
A a few years ago, radar sets bristled with knobs and buttons. Now, knobs and buttons are out, and menu systems and soft keys are in. Somewhere on the front panel or hidden in the menu system, though, all radars have the same main controls. Six of these are concerned with getting a picture and four with improving it. Others deal with more advanced functions, including the measurement of ranges and hearings.
It's pretty obvious that if you don't switch it on, the radar won't work, but it's still a good idea to check with the instruction manual, because turning it off can sometimes be tricky!
2. Standby – transmit
A radar doesn't burst into life as soon as you switch the power on, most sets go through a self-test routine, like a desk-top computer, followed by a warm-up process that is intended to protect the transmitter from sudden temperature changes. There's usually an on-screen timer to show you how the warm-up process is going, but it won't transmit until you operate the transmit control.
3. Brilliance and contrast
Brilliance is like the brightness control on a TV; it doesn't affect what is in the
Liquid Crystal Displays (LCDs) have one extra control called contrast, which can turn the screen completely black or completely white. The ?right? setting is somewhere between the two, but it varies
depending on your angle of view. Again, the ?right? setting is whatever you feel most comfortable with.
Gain is like the squelch control on a VHF: it regulates the radar's sensitivity. If it is too low (Fig 147), weak echoes won't appear, but if it is too high (Fig 148), the whole screen will be filled with a snowstorm of speckles. A lot of books tell you to adjust the gain “to leave a light background speckle”, which was fine for old-fashioned “radial scan” radars. On most modern radars, though, it's best to turn it down until they just fade. Ignore the cluster of particularly stubborn speckles around the centre of the picture: they are a different symptom, with a different cure.
It's pretty obvious that the range control changes the scale of the radar well. In particular, at long range, the radar transmits longer pulses and waits longer between pulses than at short range. The result is that you may get a slightly better the job.
We all know that a radio can only receive signals if it is tuned to match the transmitter, but it still seems odd that a radar needs to be tuned to listen for echoes of its own transmissions, unless you appreciate that the echoes are very, very weak indeed, and that the frequency produced by the magnetron (transmitter) inevitably varies slightly as it warms up and cools down.
Fine tuning is the key to a good radar effect, until the contact is as big, bright, and consistent as possible.
Improving the picture
Those six controls are the ones that determine whether you will get a picture at all, and how good it will be. Some of them may be automated, but it's still worth knowing how to adjust them manually. It's like photography: an automatic camera will take competent pictures, but a good photographer will take better ones by setting up his camera by hand.
7. Sea clutter
The sea clutter control removes the mass of speckles around the centre of the screen, caused by radar waves reflecting from the sea (Fig 150). It's not a particularly subtle control: it works by reducing the gain applied to echoes at short ranges. That makes it quite capable of wiping out real targets as well as clutter so you mustn't overdo it (Fig 151) Treat it like hot chilli – if in doubt, leave it out!
8. Rain clutter
The rain clutter control is much more subtle and innocuous. It removes clutter caused by rain and heavy
cloud by ignoring everything except the leading edge of the returning echoes. This weakens the drawn-out returns from rain storms so that they become invisible, but leaves the crisper returns from ships and cliffs unscathed. Even so, it can still weaken the echoes from some real targets, especially from gently sloping coastlines, so it's best switched off unless you really need it.
9. Interference rejection
The IR control removes the pattern of large speckles or streaks that would otherwise appear when your radar 10. Echo stretch or expansion
Echo stretch is almost self-explanatory: it artificially lengthens each contact to make small contacts more noticeable. Unfortunately, it also makes coastlines less distinct, so although it's useful for collision avoidance, it's best switched off for pilotage.
11. Display mode
Until recently, it was fair to say that your own boat was always at the centre of the screen, pointing straight upwards. Because you're fixed at the centre of the screen, the rest of the world seems to move around you, so this type of display is called Relative Motion and because you are heading straight up the screen, it's called Head-Up.
For some jobs, this is fine. It's good for collision avoidance, because it's easy to relate what is on the screen to what you see around you: anything on the left-hand side of the screen is on the port side of the boat and so on. The drawback is that it can be difficult to relate the radar picture to the chart.
Most radars sold in the past few years, however, can be interfaced to a compass, and can then turn their pictures round to put north at the top. This North-Up mode is good for pilotage, but also has the important advantage that it stabilises the radar picture to stop it swinging when the boat yaws.
There's nothing special about north: it is just as easy to put any other direction at the top. One particularly useful direction is your course (that's the direction you are intending to steer, as opposed to your heading, which is the direction the bows happen to be pointing!). The advantage of Course-Up is that it produces a stabilised picture, but with the “left-to-port, right-to–starboard” simplicity of Head-Up. One thing to be careful of, when using a Course-Up is that you must remember to switch it off and on again when you alter course, otherwise the radar will stay locked-on to your old course,
Understanding the picture
At first sight, a radar picture seems just a mass of blobs. The general shape of coastlines are usually easy to pick out, but some of the details, along with ships, boats, and buoys are harder to identify. To understand what the radar is seeing, we need to remind ourselves that the radar picture isn't a map: it’s a diagram showing the range and bearing of echoes.
Shadows and horizons
To produce an echo, an object must receive a radar pulse. It can't do that if there's something in the way!
Common obstructions are things like headlands, or large ships, which block the radar's view of things beyond them. Radar can see in the dark or in fog, but it's no better at seeing round corners than you or I!
The biggest obstruction of all is the Earth itself. Like us, radar can see tall objects at great distances because they stand up above the horizon, and it gains a bit more because the scanner is usually higher than eye level. It gains another little bit because radar waves bend very slightly to follow the curve of the earth, but its range is still limited by a horizon.
It's easy to calculate the distance to your radar's horizon: it's about 2.2 x ?H where H is the height of the scanner in metres and the answer is in miles. So if your scanner is 4m above the water, its horizon is about four and a half miles away. If it's 9m high, its horizon is six and a half miles away. That may not sound very much, but remember that a big ship may well be over 25m high, which means its superstructure will be visible when it is 10 miles beyond the horizon.
We often talk about targets ?reflecting? radar waves, but that isn't really true. What happens is that some things absorb radar energy and re-radiate it. Others absorb them but don't re-radiate, while others don't absorb them at all, and are effectively transparent. To be a good reflector or re-radiator, the target needs to be made of something that conducts electricity.
Assuming the target is made of a suitable material, four other factors decide how strong an echo it will produce.
Size is pretty obvious. A big object can reflect more energy than a small one.
Shape and orientation are important because the energy is re-radiated at right angles to the surface. This means that a flat surface is a very good reflector, so long as it is at right angles to the approaching radar beam. If it isn't, it will bounce the echo away in the wrong direction. A curved surface is less efficient, because only a small part of it can be at 90 degrees to the beam. It's likely to be a reliable reflector, though, because if it's tilted or rotated, a different bit of the surface will be available to return an echo.
Texture is really a combination of shape and orientation: a rough surface – which in radar terms means one with lumps and bumps measured in centimetres rather than in microns – is almost certain to reflect some energy back in the right direction, whereas a smooth one will produce a better reflection but is more likely to send it the wrong way.
Perhaps the biggest source of confusion, particularly on small radars, is called beamwidth effect or bearing discrimination. Manufacturers try to produce scanners that focus the microwaves into as tight a beam as possible, but they are fighting the laws of physics: to get a narrow beam, you need a big scanner, so the narrowest you can get from an 46 cm radome is a fan shape about 13cm across. Even this is only a nominal measurement: some of the energy is scattered beyond the edges of the main beam (Fig 152).
This means that a good reflector will produce an echo before the scanner is actually pointing at it, and go on producing an echo even after the main beam has swept past. The radar can't ?know? this: it merely detects the echo and paints it on to the screen in the direction the antenna is pointing. The end result is that small targets appear on the screen as streaks, 5-10 degrees across, while coastlines appear smudgy, like a map drawn with a thick felt-tip pen instead of a fine ballpoint. The edges of narrow gaps, such as harbour entrances, look swollen, so the gap may disappear altogether until you get dose enough for the beam to pass straight through it, and a ship may appear to block the entrance completely.
Things like this aren't 'faults' any more than not being able to sail straight into the wind is a 'fault' on a sailing boat; they are just characteristics of the system. The way to deal with them is to be aware that they happen, and to get used to looking at the world with a radar's eye.
Measuring ranges and bearings
With only one or two rare exceptions, even the most basic small boat radars are equipped with facilities for measuring ranges and bearings, though the details of how they are activated and operated vary from one make and model to another.
The Electronic Bearing Line (EBL) appears as a line (often dotted or broken) that extends outwards from the centre of the radar.
To take a bearing of a small contact such as a buoy or another vessel, position the EBL so that it cuts through the centre of the contact that represents the target. To take a bearing of something like a headland, the effect of beam width needs to be considered by positioning the EBL so that it cuts through the headland rather than brushing its tip. The offset needs to be about half the radar's beam width.
In a head-up radar, the bearing shown is always relative to the boat's heading at that particular moment. A contact on the port bow, for instance, would appear on a bearing of 315 degrees, regardless of its compass bearing (Fig 153). A relative bearing can always be converted to a true bearing by adding the boat's heading, and subtracting 360 degrees if necessary. If, for instance, we were steering 200 degrees (Compass) at the moment a bearing was measured as 315 degrees (Relative), the Compass bearing of that particular target would be:
+ 200° (Compass)
It may then need to be corrected for variation and deviation in the usual way.
Beamwidth, steering errors and various other factors make radar bearings much less accurate and reliable than radar ranges.
The Variable Range Marker (VRM) appears as a circle, centred on the centre of the radar picture, whose radius can be varied by means of one of the controls on the radar's front panel. The range represented by the VIRM is shown in a data box, usually in one corner of the screen.
To measure the range of a target, expand the VIRM so that it just brushes the closest part of the contact.
So long as the set has been properly installed and adjusted according to the manufacturer's instructions, radar range measurements are generally accurate to about 1% of the range scale in use.
Cursors, like those on computers, and usually in the form of a 'cross-hair' or target marker are available on many radars. The range and bearing of the cursor from the centre of the screen are displayed in a data box, usually in one corner of the screen. While it is often convenient to have a single control to measure range and bearing simultaneously, many users prefer to use the VRM and ERL.
Floating VRMs and EBLs are an increasingly common feature. They work in much the same way as conventional VRMs and EBLs, but can be positioned anywhere on the screen, so they can be used to measure the range and bearing of one object from another, or as an electronic means of drawing on the screen.