NAVIGATION RADAR
The powerful tool that helped us win the Battle of Britain continues to keep us safe today – but only if it’s used properly. There’s more than a grain of truth in the old joke about ‘radar assisted collisions’, where the information was interpreted wrongly. The main problem is that there’s a tendency to rely on radar too completely, without fully understanding its limitations.
9
How radar works
The principles behind radar (the word comes from RAdio Detection And Ranging) are well known. A transmitter emits pulses of radio microwaves from a rotating scanner that focuses the beam both horizontally and vertically, rather like sweeping the horizon with a pair of binoculars. If the pulses hit a solid object within range, they are reflected back to the radar’s receiver. The time delay between transmission and reception is used to calculate the object’s distance, while the rotational position of the scanner tells in what direction it lies.
The radar picture is displayed on a screen, usually with your boat in the centre (though many modern radars allow offset displays). The most common display format is known as ‘head-up’, as if your bow was pointing towards the top of the screen. In this format, a target on the right-hand side of the screen would be on your starboard side, and one on the left would be to port. With input from an electronic compass, many offer ‘north-up’ – as if you were seeing the situation on a chart with north at the top – and even ‘course-up’ where your intended course is at the top. For collision avoidance, most mariners use head-up or course-up because what you see on the screen helps you visualise what’s going on around you.
Beam width
Probably the most important part of a radar system is the scanner. Small scanners sell well because they’re lighter and cheaper than large ones, but they come with serious limitations. A large scanner will focus the transmission beam width more narrowly, which enables it to pick out smaller details with greater precision. This is because the radar becomes aware of a target as soon as the beam starts to move over it, and continues to do so until it sweeps completely clear. For the whole time the target is somewhere within the beam, an echo will be returned. In Fig 9.6 a tiny target will appear much bigger on the screen and the gap in the sea wall won’t be noticed at all. On the other hand the narrower beam (Fig 9:7) will show the gap clearly.
RYA Seamanship for Sea Anglers 71
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