Heathrow air traffic control juggles 1,300 take-offs and landings each day, all thanks to radar (radio detection and ranging). So how does radar work – and how can it be beaten?
Radar is, in essence, echo detection. When you shout into a canyon, the sound spreads out, bouncing and scattering off hard rock surfaces. Some of this scattered sound bounces back at you which you hear as an echo.
Rather than sound waves, radar uses radio waves.
A radar dish sends out a pulse of radio waves then “listens” for some of that signal, known as the “radar signature”, to bounce back. Even though the reflection is a tiny fraction of the radio emission, an amplifier beefs it up. A big enough signal becomes a blip on a radar operator’s screen, even hundreds of kilometres away.
Calculating the speed and direction of a plane or ship is possible thanks to the Doppler effect. If the object is moving towards the detector, radio waves are slightly squashed. Similarly, if moving away, the waves are stretched.
(The same technology is today used in loads of other applications such as car auto-braking, missile defence systems and weather observations.)
Of course, radar can be beaten. Stealth technologies deflect radio waves in such a way that few, or none, are reflected in the source’s direction. While it sounds simple, actually putting this into practice proved extremely difficult.
A plane generally has a round body, or fuselage, and large flat surfaces on wings and tail. While this shape has aerodynamic benefits, it’s also perfect for bouncing back radio signals.
In the late 1970s, Lockheed’s secretive Skunk Works division in the US produced a radar-beating plane that looked like it flew straight out of a science fiction novel – the F-117 Nighthawk. It was 20 metres from tip to tail with a 13-metre wingspan, yet bounced back the radar signature of a bird.
Lots of small flat surfaces scattered radio signals away from their source. The plane also boasted radar-absorbing materials that are still top secret today.
Its modern counterpart, the F-22 raptor, has a radar signature smaller than a marble.