Radar

Radar prices have tumbled over the past few years.  Technological improvements have come hand-in-hand with falling prices, making modern small craft radars more capable, yet easier to use than ever before.

The most obvious leap forward in small craft radar technology came in the early eighties with the development of the raster scan “daylight viewing” displays.  Until then, all radars had such dim pictures that in daylight they could only be seen by peering through a deep, light-tight cowl.

Raster scan changed all that. Instead of building up the picture in stages, in step with the rotating scanner, a raster display uses the same information to update a picture stored in its electronic memory.  Twenty five times a second, it flashes the latest update onto what is effectively a small television screen - generically called a cathode ray tube or CRT.

A second leap was the application of raster pictures to liquid crystal display (LCD’s).  LCD’s are slimmer, lighter and use less power than CRT’s and are less vulnerable to water.  A more recent innovation has been the introduction of colour liquid crystal displays that are suitable for viewing in bright daylight and the multi-purpose display screen allowing the user to select radar, chart plotter or fishfinder pictures depending on the sensors or antennas connected to the system.  Furthermore, with the enhancement of data and video interfacing using high speed bus technology, waterproof repeater displays for radars or chart plotters are now a reliable and readily available option, brining the necessary navigation information direct to the helmsman whether he is inside or out!

What makes a good picture?

The essence of a good picture is definition - how clearly it separates one blob from another.  It’s like the difference between a sketch map drawn in ballpoint and one drawn using a thick felt tip.  The felt tip loses definition hiding details which are clearly visible in the ball point version.

Three factors dominate radar’s definition:-

BEAMWIDTH, as the name suggests, is the width of the radar beam, and ranges from 7 degrees for the smallest sets down to less that a degree for a large ships’ set.  You can appreciate its significance if you imagine yourself looking for a narrow harbour entrance.  If the entrance is to show up as a gap, the radar beam has to pass straight through it with out being reflected from either side.  If it’s 0.1 wide, for instance, a 1 degree beam will pass straight through when you’re 6 miles away, but if the beam width is 6 degrees, you’ll need to be within a mile before it will show up.

PULSE LENGTH has a similar effect when it comes to discriminating between objects that are on the same bearing but slightly separated in range.  Short pulses give better definition, but unfortunately a “long” (up to about a microsecond!) pulse may be required to produce an echo from small or distant targets.  Manufacturers solve the problem by making radars that automatically change the pulse length to suit the range scale in use.

SPOT SIZE related to the graininess of the display.  All raster pictures are made up of a huge number of spots called pixels.  If the pixels are small enough, the human eye can’t distinguish them, so the picture can show smooth curves and fine detail.  Larger pixels show up as distinct rectangles, so curves show up as a series of steps, and fine details merge together.

Noise and clutter

Other factors affecting the clarity of radar picture can lumped together under the heading of CLUTTER – unwanted blips that don’t relate to anything you want to see, but which can obscure real contacts.  Manufacturers build extra controls into their radars, allowing their performance to be subtly adjusted to eliminate clutter, but unfortunately anything that eliminates clutter can also eliminate weak targets.  To overcome this, manufacturers have developed ever more sophisticated automatic controls to take over from the human operator.

NOISE – radio noise, not audible noise – is all around us, from sources as diverse as the sun, our on-board electrics, and even the radar itself.  It’s most obvious on radar when you turn the gain up too high, when it appears as a snowstorm of speckles that are the visual equivalent of the background hiss you get when you turn up the volume on a radio of tape player.  A high quality receiver will avoid picking up extraneous noise and won’t make much of a contribution itself: a lesser one will amplify everything, with the result that you have to turn the gain down to lose the noise, and risk losing weak contacts as well.

What am I paying for?

When you buy radar, what you’re paying for is a means of detecting solid objects, and of measuring their ranges and bearings.  As you move up the price range, one obvious change is that you’re likely to get a more powerful transmitter.  In most cases, this is matched by an increase in the set’s nominal range, but it’s important to appreciate your not really buying range.  The distance your radar can “see” is limited by the radar horizon, just as the distance you can see is limited by the visual horizon, so a 48-mile radar probably won’t detect a ship any further away than a 24 mile set.  The real virtue of power is that it enables the set to detect small targets without cluttering the screen with noise.

You’re also likely to get a bigger antenna.  This is important because beam width is largely determined by antenna size.  A good big one will always beat a good little one.

The other obvious change is the display: a bigger budget is likely to give you a bigger screen, a choice of CRT, colour or monochrome LCD, and a dramatic increase in the number of pixels.  But whether you need that extra quality depends on the size of your boat, the power available, and the use you make of it.  Bear in mind, though, that any radar needs to be properly installed and set up if you are to get the best out of it, especially if it is to be interfaced with other equipment such as a compass, GPS or chart plotter.

How radar works

A transmitter, inside the scanner unit, produces very short pulses of super high frequency radio waves (microwaves), which are focused into a narrow beam by the rotating antenna.  Pulses reflected back from solid objects such as land or other vessels are collected by the antenna, and passed to the receiver where they’re amplified and processed.

Part of that processing involves measuring the time between the pulse being transmitted and the returning echo being received.  Radio waves travel at a constant speed of 300 metres per microsecond, so that the time interval is directly related to the distance the pulse has travelled to and from the target.

Echoes are only received when the scanner is pointing straight at a target, so the direction in which the antenna is pointing when an echo is received corresponds the targets bearing.

The distance and bearing are then passed to the display, which uses them to build up a map like picture of a boat’s surroundings.

Hints and tips

If possible, try before you buy.  It’s much easier to appreciate picture quality by looking at it than from printed specifications.

Don’t be put off by the control panel or menu system.  A typical radar has only eight main controls - the rest will be for features and functions that are intended to make life easier.

Make sure it is properly installed, and that you know how to use it.

You are here: About marine electronics » Radar