Radar

Last updated: August 6, 2007.

Imagine trying to land a jumbo jet the size of a large building on a short strip of tarmac, in the middle of a city, in the depth of the night, in thick fog. If you can't see where you're going, how can you hope to land safely? Airplane pilots get around this difficulty using radar, a way of "seeing" that uses high-frequency radio waves. Radar was originally developed to detect enemy aircraft during World War II, but it is now widely used in everything from police speed-detector guns to weather forecasting. Let's take a closer look at how it works!

Photo: This giant radar detector at Thule Air Base, Greenland is designed to detect incoming nuclear missiles. It's a key part of the US Ballistic Missile Early Warning System (BMEWS). Photo by Michael Tolzmann courtesy of US Airforce.

What is radar?

We can see objects in the world around us because light. (usually from the Sun) reflects off them into our eyes. If you want to walk at night, you can shine a torch in front to see where you're going. The light beam travels out from the torch, reflects off objects in front of you, and bounces into your eyes. Your brain instantly computes what this means: it tells you how far away objects are and makes your body move so you don't trip over things.

Radar works in much the same way. The word "radar" stands for radio detection and ranging—and that gives a pretty big clue as to what it does and how it works. Imagine an airplane flying at night through thick fog. The pilots can't see where they're going, so they use the radar to help them.

An airplane's radar is a bit like a torch that uses radio waves instead of light. The plane transmits an intermittent radar beam (so it sends a signal only part of the time) and, for the rest of the time, "listens" out for any reflections of that beam from nearby objects. If reflections are detected, the plane knows something is nearby—and it can use the time taken for the reflections to arrive to figure out how far away it is. In other words, radar is a bit like the echolocation system that "blind" bats use to see and fly in the dark.

Photo: This mobile radar truck can be towed to wherever it's needed. The antenna on top rotates so it can detect enemy airplanes or missiles coming from any direction. Photo by Shane A. Cuomo courtesy of US Airforce.

How radar works

Whether it's mounted on a plane, a ship, or anything else, a radar set needs the same basic set of components: something to generate radio waves, something to send them out into space, something to receive them, and some means of displaying information so the radar operator can quickly understand it.

The radar waves are generated by a piece of equipment called a magnetron. Radio waves are similar to light. They are made up of fluctuating patterns of electrical and magnetic energy, just like light waves, and they travel at the same speed—but their waves have much longer wavelengths and higher frequencies. The waves a magnetron produces are actually microwaves, similar to the ones generated by a microwave oven. The difference is that the magnetron in a radar has to send the waves many miles, instead of just a few inches, so it is much larger and more powerful.

Photo: A typical military radar screen, located in the flight tower at Eielson Air Force Base, Alaska. Photo by Christopher Griffin courtesy of US Air Force.

Once the radio waves have been generated, an antenna, working as a transmitter, hurls them into the air in front of it. The antenna is usually curved so it focuses the waves into a precise, narrow beam, but radar antennas also typically rotate so they can detect movements over a large area. The radio waves travel outward from the antenna at the speed of light (186,000 miles or 300,000 km per second) and keep going until they hit something. Then some of them bounce back toward the antenna in a beam of reflected radio waves also travelling at the speed of light. The speed of the waves is crucially important. If an enemy jet plane is approaching at over 3,000 km/h (2,000 mph), the radar beam needs to travel much faster than this to reach the plane, return to the transmitter, and trigger the alarm in time. That's no problem, because radio waves (and light) travel fast enough to go seven times around the world in a second! If an enemy plane is 160 km (100 miles) away, a radar beam can travel that distance and back in less than a thousandth of a second.

The antenna doubles up as a radar receiver as well as a transmitter. In fact, it alternates between the two jobs. Typically it transmits radio waves for a few thousandths of a second, then it listens for the reflections for anything up to several seconds before transmitting again. Any reflected radio waves picked up by the antenna are directed into a piece of electronic equipment that processes and displays them in a meaningful form on a television-like screen, watched all the time by a human operator. The receiving equipment filters out useless reflections from the ground, buildings, and so on, displaying only significant reflections on the screen itself. Using radar, an operator can see any nearby ships or planes, where they are, how quickly they're travelling, and where they're heading. Watching a radar screen is a bit like playing a video game—except that the spots on the screen represent real airplanes and ships and the slightest mistake could cost many people's lives.

There's one more important piece of equipment in the radar apparatus. It's called a duplexer and it makes the antenna swap back and forth between being a transmitter and a receiver. While the antenna is transmitting, it cannot receive—and vice-versa. Take a look at the diagram in the box below to see how all these parts of the radar system fit together.

What radar is used for

Photo (left): A scientist adjusts a radar dish to track weather balloons through the sky. Weather balloons, which measure atmospheric conditions, carry reflective targets underneath them to bounce radar signals back efficiently. Photo by courtesy of US Department of Energy.

Radar is still most familiar as a military technology. Radar antennas mounted at airports or other ground stations can be used to detect approaching enemy airplanes or missiles, for example. The United States has a very elaborate Ballistic Missile Early Warning System (BMEWS) to detect incoming missiles, with three major radar detector stations in Clear in Alaska, Thule in Greenland, and Fylingdales Moor in England. It's not just the military who use radar, however. Most civilian airplanes and larger boats and ships now have radar too as a general aid to navigation. Every major airport has a huge radar scanning dish to help air traffic controllers guide planes in and out, whatever the weather. Next time you head for an airport, look out for the rotating radar dish mounted on or near the control tower.

You may have seen police officers using radar guns by the roadside to detect people who are driving too fast. These are based on a slightly different technology called Doppler radar. You've probably noticed that a fire engine's siren seems to drop in pitch as it screams past. As the engine drives toward you, the sound waves from its siren arrive more often because the speed of the vehicle makes them travel a bit faster. When the engine drives away from you, the vehicle's speed works the opposite way—making the sound waves travel slower and arrive less often. So you hear quite a noticeable drop in the siren's pitch at the exact moment when it passes by. This is called the Doppler effect.

The same science is at work in a radar speed gun. When a police officer fires a radar beam at your car, the metal bodywork reflects the beam straight back. But the faster your car is travelling, the more it will change the frequency of the radio waves in the beam. Sensitive electronic equipment in the radar gun uses this information to calculate how fast your car is going.

Photo: Radar in action: Left: A Doppler radar unit scans the sky. Photo by courtesy of US Department of Energy. Right: A Gatso speed camera designed to make drivers keep to the speed limit. Photo taken at Think Tank, Birmingham, England by Explain that Stuff.

Radar has many scientific uses. Doppler radar is also used in weather forecasting to figure out how fast storms are moving and when they are likely to arrive in particular towns and cities. Effectively, the weather forecasters fire out radar beams into clouds and use the reflected beams to measure how quickly the rain is travelling and how fast it's falling. Scientists use a form of visible radar called lidar (light detection and ranging) to measure air pollution with lasers. Archaeologists and geologists point radar down into the ground to study the composition of the Earth and find buried deposits of historical interest.

One place radar isn't used is on board submarines. Electromagnetic waves don't travel readily through dense seawater (that's why it's dark in the deep ocean). Instead, submarines use a very similar system called SONAR (Sound Navigation And Ranging), which uses sound to "see" objects instead of radio waves.

Photo: A geologist moves a radar transmitter (mounted on a bike wheel) across the ground to study the composition of the Earth beneath. His partner in the pickup behind interprets the radar signals on an electronic display. This kind of ground-penetrating radar (GPR) is an example of geophysics. Photo by courtesy of US Department of Energy.

Countermeasures: how to avoid radar

Radar is extremely effective at spotting enemy aircraft and ships—so much so that military scientists had to develop some way around it! If you have a superb radar system, chances are your enemy has one too. If you can spot his airplanes, he can spot yours. So you really need airplanes that can somehow "hide" themselves inside the enemy's radar without being spotted. Stealth technology is designed to do just that. You may have seen the US airforce's sinister-looking B2 stealth bomber. Its sharp, angular lines and metal-coated windows are designed to scatter or absorb beams of radio waves so enemy radar operators cannot detect them. A stealth airplane is so effective at doing this that it shows up on a radar screen with no more energy than a small bird!

Photo: The unusual zig-zag shape at the back of this B2 stealth bomber is one of many features designed to scatter radio waves so the plane "disappears" on enemy radar screens. The rounded front wings and concealed engines and exhaust pipes also help to keep the plane invisible. Photo by courtesy of US Airforce.

A brief history of radar

Although many scientists contributed to the development of radar, best known among them was a Scottish physicist named Robert Watson-Watt (1892-1973). During World War I, Watson-Watt went to work for Britain's Meterological Office (the country's main weather forecasting organization) to help them use radio waves to detect approaching storms.

In the run up to World War II, Watson-Watt and his assistant Arnold Wilkins realized they could use the technology they were developing to detect approaching enemy aircraft. Once they'd proved the basic equipment could work, they constructed an elaborate network of ground-based radar detectors around the south and east of the British coastline. During the war, Britain's radar defences (known as "Chain Home") gave it a huge advantage over the German airforce and played an important part in the ultimate allied victory. A similar system was developed at the same time in the United States and even managed to detect the approach of Japanese airplanes over Pearl Harbor, in Hawaii, in December 1941—though no-one figured out the significance of so many approaching planes until it was too late.