by Chris Woodford. Last updated: October 24, 2017.
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 Air Force.
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 back 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 Air Force.
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 radio waves used by radar are produced by a piece of equipment called a magnetron. Radio waves are similar to light waves: they travel at the same speed—but their waves are much longer and have much lower frequencies. Light waves have wavelengths of about 500 nanometers (500 billionths of a meter, which is about 100–200 times thinner than a human hair), whereas the radio waves used by radar typically range from about a few centimeters to a meter—the length of a finger to the length of your arm—or roughly a million times longer than light waves.
Both light and radio waves are part of the electromagnetic spectrum, which means they're made up of fluctuating patterns of electrical and magnetic energy zapping through the air. 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 traveling 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 traveling, 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.