by Chris Woodford. Last updated: March 12, 2021.
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 can help pilots to land, 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, but they can communicate with air traffic controllers on the ground
who use radar to help them. Pilots themselves don't generally use radar as a "flight instrument"
(something that helps them fly or navigate), but they do use it for tracking the weather.
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 driven to
wherever it's needed. The antenna on top rotates so it can detect enemy
airplanes or missiles coming from any direction.
Photo by Nathanael Callon courtesy of
US Air Force.
How does radar use radio?
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
Photo: A modern digital radar screen, located at
Ellsworth Air Force Base, South Dakota, USA.
Photo by Corey Hook 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
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
miles) away, a radar beam can travel that distance and back in less
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.
What is radar used for?
Photo: 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.
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 are effectively
squeezed into a shorter distance, so they have a shorter wavelength
and a higher frequency—which we hear as a higher pitch.
When the engine drives away from you, it works the opposite
way—making the sound waves longer in wavelength, lower in
frequency, and lower in pitch. 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 traveling, 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: A Gatso speed camera designed to make drivers keep to the speed limit, invented by race car driver Maurice Gatsonides.
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
traveling 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. Archeologists and geologists point
radar down into the
ground to study the composition of the Earth and find buried deposits
of historical interest.
Photo: Radar in action: A Doppler radar unit scans the sky.
Photo by courtesy of US Department of Energy.
One place radar isn't used is to help submarines as they
navigate underwater. 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. Submarines do, however, have radar systems they can use while they're moving about
on the ocean surface (such as when they're entering and leaving port).
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 can you 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 air force'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
Photo by Bennie J. Davis III courtesy of
US Air Force.
Who invented radar?
Radar can be traced back to a device called a Telemobiloskop
(sometimes written French-style, Télémobiloscope), invented in 1904
by German electrical engineer Christian Hülsmeyer (1881–1957). After hearing
about a tragic collision between two ships, he figured out a way to use radio waves to help them
see one another when the visibility was poor.
Artwork: Radar before radar: Christian Hülsmeyer's Telemobiloskop predated
radar by over three decades, but was essentially the same concept. This artwork is based on a drawing from one of
Hülsmeyer's 1904 patents showing how transmitting and receiving apparatus mounted on one ship could be used to detect other ships nearby. The beams are "Hertzian Waves"—what we would now call radio waves—shooting out from gimbal-mounted apparatus that would always stay vertical despite the tossing movements of the sea.
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
Meteorological 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 defenses (known as Chain Home) gave it a huge advantage over
German air force 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