How radar works | Uses of radar (2024)

by Chris Woodford. Last updated: May 20, 2022.

Imagine trying to land a jumbo jet thesize of a large building on a short strip of tarmac, in the middle of a city, in the depth of thenight, in thick fog. If you can't see where you're going, how can youhope to land safely? Air traffic controllers, who can help pilots to land, get around this difficulty usingradar, a way of "seeing" that uses high-frequency radiowaves. Radar was originally developed to detect enemy aircraft duringWorld War II, but it is now widely used in everything from policespeed-detector guns to weather forecasting. Let's take a closer lookat how it works!

Photo: This giant radar detector at Thule AirBase, Greenland is designed to detect incoming nuclear missiles. It's akey 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 (usuallyfrom the Sun) reflects off them into our eyes. If you want to walk atnight, you can shine a torch in front to see where you'regoing. The light beam travels out from the torch, reflects off objectsin front of you, and bounces back into your eyes. Your brain instantlycomputes what this means: it tells you how far away objects are andmakes your body move so you don't trip over things.

Radar works in much the same way. The word "radar" stands for radio detectionand ranging—and thatgives a pretty big clue as to what it does and how it works. Imagine anairplane flying at night through thick fog. The pilots can't see wherethey'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 planetransmits an intermittent radar beam (so it sends a signal only part ofthe time) and, for the rest of the time, "listens" out for anyreflections of that beam from nearby objects. If reflections aredetected, the plane knows something is nearby—and it can use the timetaken for the reflections to arrive to figure out how far away it is.In other words, radar is a bit like the echolocation systemthat "blind" bats use to see and fly in the dark.

Photo: This mobile radar truck can be driven towherever it's needed. The antenna on top rotates so it can detect enemyairplanes 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 radarset needs the same basic set of components: something to generate radiowaves, something to send them out into space, something to receivethem, and some means of displaying information so the radar operatorcan 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 theirwaves are much longer and have much lower frequencies. Light waves have wavelengths of about500 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 electricaland magnetic energy zapping through the air.The waves a magnetron produces are actually microwaves, similar to the onesgenerated by a microwave oven. Thedifference is that the magnetron in a radar has to send the waves manymiles, instead of just a few inches, so it is much larger andmore powerful.

Photo: A modern SPN-43 air search digital radar screen, part of thesystem the US Navy uses for air traffic control onboard amphibious assault ships and aircraftcarriers. Photo by Gretchen M. Albrecht courtesy of US Navy andWikimedia Commons.

Once the radio waves have been generated, an antenna,working as a transmitter, hurls them intothe air in front of it. The antenna is usually curved so it focuses the waves into aprecise, narrow beam, but radar antennas also typically rotate so theycan detect movements over a large area. The radio waves travel outwardfrom the antenna at the speed of light (186,000 miles or 300,000 km persecond) and keep going until they hit something. Then some of thembounce back toward the antenna in a beam of reflected radio waves alsotraveling at the speed of light. The speed of the waves is cruciallyimportant. 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 theplane, return to the transmitter, and trigger the alarm in time. That'sno problem, because radio waves (and light) travel fast enough to goseven times around the world in a second! If an enemy plane is 160 km(100miles) away, a radar beam can travel that distance and back in lessthana thousandth of a second.

The antenna doubles up as a radar receiveras 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 secondsbefore transmitting again. Any reflected radio waves picked up by theantenna are directed into a piece of electronic equipmentthat processes and displays them in a meaningful form on a television-likescreen, watched all the time by a human operator. Thereceiving equipment filters out useless reflections from the ground,buildings, and so on, displaying only significant reflections on thescreen itself. Using radar, an operator can see any nearby ships orplanes, where they are, how quickly they're traveling, and wherethey're heading. Watching a radar screen is a bit like playing a videogame—except that the spots on the screen represent real airplanes andships and the slightest mistake could cost many people's lives.

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

What is radar used for?

Photo: A scientist adjusts a radar dish to trackweather balloons through the sky.Weather balloons, which measure atmospheric conditions, carryreflective targets underneath them to bounce radar signals backefficiently. Photo by courtesy of US Department of Energy (Flickr).

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

You may have seen police officers using radar guns by the roadsideto detect people who are driving too fast. These are based on aslightly 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 theengine drives toward you, the sound waves from its siren are effectivelysqueezed into a shorter distance, so they have a shorter wavelengthand a higher frequency—which we hear as a higher pitch.When the engine drives away from you, it works the oppositeway—making the sound waves longer in wavelength, lower infrequency, and lower in pitch. So you hear quite a noticeable drop in the siren's pitch at the exact moment when itpasses by. This is called the Doppler effect.

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.

The same science is at work in a radar speed gun. When a policeofficer fires a radar beam at your car, the metal bodywork reflects thebeam straight back. But the faster your car is traveling, the more it willchange the frequency of the radio waves in the beam. Sensitiveelectronic equipment in the radar gun uses this information tocalculate how fast your car is going.

Radar has many scientific uses. Doppler radar is also used inweather forecasting to figure out how fast storms are moving and whenthey are likely to arrive in particular towns and cities. Effectively,the weather forecasters fire out radar beams into clouds and use thereflected beams to measure how quickly the rain istraveling and how fast it's falling. Scientists use a form of visibleradar called lidar (light detection andranging) to measure air pollution with lasers. Archeologists and geologists pointradar down into theground to study the composition of the Earth and find buried depositsof historical interest.

Photo: A Doppler radar making up part of WSR-88D (NEXRAD), the comprehensive US weather surveillance network of around 160 such stations. Photo by Zachary Heal courtesy ofUS Air Force and DVIDS.

One place radar isn't used is to help submarines as theynavigate underwater. Electromagnetic waves don't travel readily through dense seawater (that's why it's darkin 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 abouton the ocean surface (such as when they're entering and leaving port).

Photo: NASA astronauts test ground-penetrating radar (GPR) for potential future moon missions on a simulated lunar-landing site. GPR can probe soil structure and locate buried objects without the need for excavation, which makes it very useful for archeological research.Photo by Sean Smith courtesy of NASA Langley andInternet Archive.

Countermeasures: how can you avoid radar?

Radar is extremely effective at spotting enemy aircraft and ships—somuch so that military scientists had to develop some way around it! Ifyou have a superb radar system, chances are your enemy has one too. Ifyou can spot his airplanes, he can spot yours. So you really needairplanes that can somehow "hide" themselves inside the enemy's radarwithout 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 toscatter or absorb beams of radio waves so enemy radar operators cannotdetect them. A stealth airplane is so effective at doing this that itshows up on a radar screen with no more energy than a small bird!

Photo: The unusual zig-zag shape at the back ofthis B2 stealth bomber is one of many features designed to scatterradio waves so the plane"disappears" on enemy radar screens. The rounded front wings andconcealed engines and exhaust pipes also help to keep the planeinvisible. 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 1904by German electrical engineer Christian Hülsmeyer (1881–1957). After hearingabout a tragic collision between two ships, he figured out a way to use radio waves to help themsee one another when the visibility was poor.

Artwork: Radar before radar: Christian Hülsmeyer's Telemobiloskop predatedradar 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'sMeteorological Office (the country's main weather forecastingorganization) 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 realizedthey could use the technology they were developing to detectapproaching enemy aircraft.Once they'd proved the basic equipment could work, they constructed anelaborate network of ground-based radar detectors around thesouth and east of the British coastline. During the war, Britain'sradar defenses (known as Chain Home) gave it a huge advantage overtheGerman air force and played an important part in the ultimate alliedvictory. A similar system was developed at the same time in the UnitedStates and even managed to detect the approach of Japanese airplanesover Pearl Harbor, in Hawaii, in December 1941—though no-one figuredout the significance of so many approaching planes until it was toolate.

Find out more

On this website

Antennas and transmitters
Lidar
Magnetrons
Microwave ovens
Radio
Television

Technical books

Principles of Modern Radar: Basic Principles,Principles of Modern Radar: Advanced Techniques, and Principles of Modern Radar: Radar Applications by Mark A. Richards et al. Institution of Engineering and Technology, 2010–2014. Together, these form a comprehensive, three-volume textbook.
Radar Principles for the Non-Specialist by John C. Toomay, Paul J. Hannen. Springer, 2012. A relatively accessible (but quite complex) overview and not suitable for ordinary, lay readers who don't have some scientific or mathematical background.
Radar Handbook by Merrill Ivan Skolnik. McGraw-Hill, 2007. Covers the theory and many applications of radar.
Understanding Radar Systems by Simon Kingsley and Shaun Quegan. SciTech, 1999. Theoretical principles and practical applications in peace and war.

Radar history

The Radar Pages: An interesting archive about radar, particularly its development in the UK during World War II.
Royal Radar Establishment 1958: An evocative, historic Pathe clip showing British scientists at work with radar equipment at the Royal Signal & Radar Establishment, Malvern, England in the late 1950s.
The Birth of British Radar by Colin Latham, Anne Stobbs. Radio Society of Great Britain, 2012. A detailed history of early radar development.
A Radar History of World War II: Technical and Military Imperatives by Louis Brown. Institute of Physics/Taylor & Francis, 1999. A very comprehensive technical history exploring how military needs drove the development of radar technology, mainly in Britain but also in Germany, Japan, and Russia.
[PDF] Tactical radars for ground surveillance by Thomas G. Bryant et al, Lincoln Laboratory Journal, Volume 12, Number 2, 2000. Explores the development of modern, military battleground radar from the 1960s onward.
[PDF] 98 Years of the Radar Principle: The Inventor Christian Hülsmeyer by Joachim Ender, 2002. A history of Hülsmeyer's Télémobiloscope, the forerunner of wartime radar. [Archived via the Wayback Machine.]

Articles

Letting robocars see around corners by Behrooz Rezvani. IEEE Spectrum, January 23, 2022. How creative use of radar bands can make self-driving cars safer.
How the Telemobiloskop Paved the Way for Modern Radar Systems by Joanna Goodrich. IEEE Spectrum, November 5, 2019. How Christian Hülsmeyer's Telemobiloskop spawned the radar we use today.
Radar sensor for the home which can 'see' through walls: BBC News, January 17, 2017. A breast-cancer detection system has been modified into a new kind of smart home monitor.
Metamaterial Radar Is Exactly What Delivery Drones Need by Evan Ackerman. IEEE Spectrum, November 8, 2016. Drones need accurate navigation equipment, but how can we make it small and light enough for them to carry?
Coffee-Can Radar: How to build a synthetic-aperture imaging system with tin cans and AA batteries by David Schneider. IEEE Spectrum, November 1, 2012. How to build your own object-tracking radar with everyday items and open-source software!
Feb. 26, 1935: Radar, the Invention That Saved Britain by Tony Long. Wired, February 26, 2008. A bried introduction to the work of Sir Robert Watson-Watt.
Veterans of Secret Unit Celebrate Their War Hero: Radar by Anahad O'Connor. The New York Times, September 10, 2002. Explores the North American contribution to the history of wartime radar.

Patents

Looking for more detail? This is a very small selection of the many patents covering some of the radar technologies discussed up above:

GB Patent 190425608A: Improvement in Hertzian-wave Projecting and Receiving Apparatus for Locating the Position of Distant Metal Objects by Christian Hülsmeyer, March 23, 1905. One of Hülsmeyer's early radar patents and the only one in English I've managed to find online. There's an earlier one GB13,170, granted September 22, 1904, titled "Hertzian-Wave Projecting and Receiving Apparatus Adapted to Indicate or Give Warning of the Presence of a Metallic Body, such as a Ship or a Train, in the Line of Projection of such Waves."
US Patent 1,639,667: Method for radio position finding by Richard H. Ranger, RCA Corp, August 23, 1927. An early navigational use of radar.
US Patent 3,831,173: Ground radar system by Robert Lerner, MIT, August 20, 1974. A description of geodar (ground echo detection and ranging), a type of ground-penetrating radar developed at MIT's Lincoln Laboratory in the mid-1960s for detecting tunnels during the Vietnam war.
European Patent EP0621573A1: Method and device for speed measurement by Tom Gatsonides, Gatsometer BV, October 26, 1994. Describes an improved version of Gatso speed radar measurements designed to frustrate drivers with radar detectors.
US Patent 6,091,355: Doppler radar speed measuring unit by Roland Kadotte, Jr., Speed Products, Inc., July 18, 2000. A typical Doppler speed gun.
US Patent 7,109,913: Airborne weather radar system and radar display by Steve Paramore et al, Rockwell Collins, September 19, 2006. A storm detection and warning system that can be used onboard aircraft.