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? Air traffic controllers, who help pilots land, get around this problem using radar, a way of “seeing” that uses high-frequency radio waves.
People originally developed radar to detect enemy aircraft during World War II, but now we use it widely in everything from police speed-detector guns to weather forecasting.
Let’s take a closer look at how it works!
What is radar?
We see objects around us because light (usually from the Sun) bounces off them and into our eyes. If you walk at night, you can shine a torch ahead to see your path.
The light beam shoots out from the torch, hits objects in front of you, and reflects back into your eyes. Your brain quickly figures out what this means: it tells you how far away things are and guides your body so you don’t trip.
Radar works in much the same way. The word “radar” stands for radio detection and ranging, and that gives a big clue about what it does and how it works. Picture an airplane flying at night through thick fog.
The pilots can’t see where they’re going, but they can talk with air traffic controllers on the ground who use radar to help them. Pilots don’t usually use radar as a “flight instrument” (something that helps them fly or navigate), but they do use it to track the weather.
An airplane’s radar is a bit like a torch that uses radio waves instead of light. The plane sends out an intermittent radar beam (so it transmits a signal only part of the time) and, for the rest of the time, “listens” for any reflections of that beam from nearby objects.
If the plane detects reflections, it knows something is nearby—and it can use the time it takes 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.
How does radar use radio?
Whether it’s on a plane, a ship, or any other vehicle, a radar set always needs the same basic parts: something to make radio waves, something to shoot them out, something to catch them when they bounce back, and a way to show the info so the operator gets it fast.
A magnetron makes the radio waves for radar. Radio waves act a lot like light waves: they move at the same speed, but their waves are much longer and their frequencies are much lower.
Light waves have wavelengths around 500 nanometers (500 billionths of a meter, about 100 to 200 times thinner than a human hair). Radar radio waves usually range from a few centimeters to about a meter, roughly the length of a finger to the length of your arm, or about a million times longer than light waves.
Light and radio waves both belong to the electromagnetic spectrum, so they both travel as fluctuating patterns of electrical and magnetic energy moving through the air.
The waves a magnetron makes are actually microwaves, like the ones in a microwave oven. The magnetron in a radar has to send waves many miles, not just a few inches, so it is much bigger and more powerful.
Once the radio waves are ready, an antenna acting as a transmitter shoots them out into the air ahead. The antenna is usually curved so it focuses the waves into a tight, narrow beam, and radar antennas also spin so they can scan movements over a wide area.
Radio waves shoot out from the antenna at the speed of light—186,000 miles (300,000 km) every second—and keep going until they hit something. Some of those waves bounce back toward the antenna as a reflected beam, also moving at light speed. That speed matters a lot.
If an enemy jet is closing in at over 3,000 km/h (2,000 mph), the radar beam has to move way faster than the plane. It needs to reach the plane, bounce back, and set off the alarm in time. No problem: radio waves (and light) are so fast they could circle the Earth seven times in one second. If a plane is 160 km (100 miles) away, the beam makes the round trip in less than a thousandth of a second.
The antenna does double duty as both transmitter and receiver. It switches between the two jobs. Usually, it sends out radio waves for a few thousandths of a second, then listens for reflections for up to several seconds before transmitting again.
Any reflected waves the antenna picks up go into electronic equipment that processes them and shows them in a useful way on a TV-like screen. A human operator watches that screen the whole time.
The receiving gear filters out useless reflections from the ground, buildings, and other clutter. It shows only the important reflections on the screen. With radar, an operator can see nearby ships or planes, where they are, how fast they’re moving, and where they’re headed.
Watching a radar screen feels a bit like playing a video game, except the dots on the screen are real airplanes and ships. One small mistake could cost many lives.
There’s one more key piece in the radar system: the duplexer. It makes the antenna switch back and forth between transmitting and receiving. While the antenna transmits, it can’t receive—and vice versa. Check out the diagram in the box below to see how all these parts fit together.
How does radar work?
Here’s a summary of how radar works:
- The magnetron makes high-frequency radio waves.
- The duplexer connects the magnetron to the antenna.
- The antenna works as a transmitter and sends a narrow beam of radio waves into the air.
- The radio waves hit an enemy airplane and bounce back.
- Between transmissions, the antenna picks up the reflected waves. The same antenna acts as both transmitter and receiver, switching between sending out radio waves and catching the ones that come back.
- The duplexer connects the antenna to the receiver unit.
- A computer inside the receiver unit processes the reflected waves and draws them on a TV screen.
- The enemy plane appears on the TV radar display along with any other nearby targets.
What is radar used for?
Radar still feels most familiar as a military tool. Ground stations and airports mount radar antennas to spot enemy planes or missiles as they approach.
Military radar and early warning
The United States runs a detailed Ballistic Missile Early Warning System (BMEWS) to catch incoming missiles. It relies on three major radar stations: Clear in Alaska, Thule in Greenland, and Fylingdales Moor in England.
Civilian radar in travel
Civilian users rely on radar too. Most commercial planes and large boats and ships carry radar. Every major airport operates a large radar dish that helps air traffic controllers guide planes in and out, no matter the weather. Next time you visit an airport, look for the rotating radar dish on or near the control tower.
Police speed checks and the Doppler effect
You may have seen police officers using radar guns by the roadside to catch drivers who go too fast. These use a related idea called Doppler radar. You have 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 get squeezed into a shorter distance, so they have a shorter wavelength and a higher frequency, which you hear as a higher pitch. When the engine drives away, the opposite happens.
The sound waves stretch out, the wavelength gets longer, the frequency drops, and the pitch falls. So you hear a clear drop in the siren’s pitch right when it passes you. This is the Doppler effect.
The same science powers a radar speed gun. When an officer fires a radar beam at your car, the metal body reflects the beam back. The faster your car moves, the more it shifts the frequency of the radio waves in that beam. Sensitive electronics in the radar gun use that shift to calculate your speed.
Scientific uses: weather, air, and the ground
Radar helps science in many ways. Weather forecasters use Doppler radar to figure out how fast storms move and when they will reach towns and cities. In effect, they send radar beams into clouds and read the reflected beams to measure how quickly rain travels and how fast it falls.
Scientists also 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 Earth’s composition and find buried deposits of historical interest.
Why submarines use sonar, not radar
One place radar does not work well is helping submarines navigate underwater. Electromagnetic waves do not travel easily through dense seawater, which is why the deep ocean stays dark.
Instead, submarines use a very similar system called SONAR (Sound Navigation And Ranging), which uses sound to “see” objects instead of radio waves. Submarines do carry radar systems for use on the ocean surface, such as when they enter or leave port.
Countermeasures: how can you avoid radar?
Radar works extremely well at spotting enemy aircraft and ships, so military scientists had to find a way around it. If you have a powerful radar system, your enemy probably has one too. If you can spot his airplanes, he can spot yours.
That means you need airplanes that can “hide” from the enemy’s radar so they don’t get detected. Stealth technology is built to do exactly that.
You may have seen the US Air Force’s B‑2 stealth bomber. Its sharp, angular shape and metal‑coated windows are designed to scatter or absorb radio waves so enemy radar operators cannot detect it.
A stealth airplane is so good at this that it shows up on a radar screen with no more energy than a small bird.
Who invented radar?
Radar started with a device called the Telemobiloskop (sometimes written in French style as Télémobiloscope). German electrical engineer Christian Hülsmeyer (1881–1957) invented it in 1904. After he heard about a sad ship collision, he found a way to use radio waves so ships could “see” each other when visibility was bad.
Many scientists helped build radar over time. The most famous was a Scottish physicist named Robert Watson-Watt (1892–1973). During World War I, Watson-Watt joined Britain’s Meteorological Office (the country’s main weather service) to help them use radio waves to spot storms moving in.
As World War II approached, Watson-Watt and his assistant Arnold Wilkins realized they could use their technology to detect enemy aircraft. Once they showed the basic equipment worked, they built a large network of ground-based radar stations along the south and east coasts of Britain.
During the war, Britain’s radar defenses (called Chain Home) gave the country a big advantage over the German air force. This system played an important role in the Allied victory.
The United States developed a similar system at the same time. It even detected Japanese planes approaching Pearl Harbor, Hawaii, in December 1941. But no one understood what so many incoming planes meant until it was too late.