Radar

The ability of radar to do so many tasks makes it useful for a wide variety of purposes. Pilots rely on radar to land their airplanes safely at busy airports. In bad weather, ship navigators use it to steer their ships clear of nearby vessels and dangerous objects. The United States, Canada, and many other countries use radar to guard against surprise attacks from enemy aircraft or missiles. Radar enables weather forecasters to keep track of approaching storms. Scientists use radar to investigate the upper atmosphere of the earth and also to study the other planets and their moons.

The word radar comes from radio detection and ranging. Almost every radar set works by sending radio waves toward an object and receiving the waves that are reflected from the object. The time it takes for the reflected waves to return indicates the object's range--how far away it is. The direction from which the reflected waves return tells the object's location.

Radar sets vary in size and shape, but they all have the same kinds of basic parts. Every set has a transmitter to produce radar waves and an antenna to send them out. In most types of radar, the same antenna collects the waves bounced back from an object. The reflected waves, commonly called echoes, are strengthened by a receiver so they can be seen on a display. The typical radar display resembles the picture tube of a television set. It shows the echoes as spots of light or as an image of the object observed.

Air traffic near large airports is extremely heavy. Specially trained air traffic controllers at all the world's major airports use radar to direct the continuous flow of incoming and outgoing planes. Radar shows the controllers the position of every plane in the air within at least 50 miles (80 kilometers) of the airport. This information enables them to prevent collisions by selecting the safest routes for pilots to follow. The controllers also depend on radar to enable them to direct landings from the ground when bad weather makes approach lights and runways difficult for pilots to see.

Most modern aircraft have various types of radar to aid pilots. For example, the radar altimeter shows how high a plane is flying and so helps pilots maintain the proper altitude. Another device, weather radar, detects nearby storms and thus enables pilots to change course to avoid rough weather whenever possible.

In ship navigation. Radar is widely used as a navigation aid on all kinds of boats and ships, from small pleasure craft to huge oil tankers. When visibility is poor, a ship's radar can spot other vessels, reefs, and icebergs in time to prevent an accident. When a ship is near shore, the navigator can determine the vessel's position by the radar echoes from special reflector buoys, islands, and other landmarks.

Harbor masters use radar to control ship traffic in crowded seaports. They follow the movements of all ships in a harbor on a radar display that provides a maplike picture of the harbor. By means of radio communication, harbor masters can guide ships into and out of a port safely in any weather.

Some United States Coast Guard stations keep track of vessels in their vicinity through radar observations. The Coast Guard also uses radar to search for ships that are reported missing.

In the military. Radar has a variety of military uses. The major uses include (1) air defense, (2) missile defense, (3) space surveillance, (4) intelligence gathering, (5) range instrumentation, and (6) weapon fire control.

Air defense requires long-range radar that can detect and track approaching enemy aircraft at great distances and so give the earliest possible warning. Vast networks of radar stations form the heart of most nations' air defense systems. The North Warning System, a network of radar stations across northern North America, protects the United States and Canada from attack from the north. The United States also has built over-the-horizon radar stations to detect attacks from the east, south, and west.

In addition to land radar stations, the United States and several other countries use aircraft equipped with radar for protection against surprise air attacks. Airborne radar can spot low-flying enemy bombers that may escape detection by ground-based radar.

Missile defense consists of radar networks like those used for early warning of hostile aircraft. But more powerful radar is needed to detect guided missiles because they fly faster and much higher than planes. The main radar network developed by the United States for missile defense is the Ballistic Missile Early Warning System (BMEWS). This system has installations at Clear, Alaska; Thule, Greenland; and Fylingdales Moor, England. Radar units at these places can spot long-range missiles up to 3,000 miles (4,800 kilometers) away.

Another major radar network is the Sea-Launched Ballistic Missile Detection System (SLBMDS). Its radar stations guard the east and west coasts of the United States against missiles launched from enemy submarines and surface vessels.

Space surveillance involves the use of extremely powerful radars to detect and track artificial satellites and other objects put into orbit around the earth. For this purpose, the United States and Canada operate a network called the United States Air Force (USAF) Spacetrack network. The network includes the three BMEWS installations and other facilities throughout the world. Each day, the USAF Spacetrack network provides more than 20,000 observations of hundreds of orbiting objects. Data from these observations can help identify reconnaissance satellites, which are used for spying.

Intelligence gathering. Radar is used to collect information about the preparations that other countries might be making for war. A mapping radar in a plane can produce detailed maps of the ground and show military installations and equipment. Other types of radar can obtain important information about another country's missile systems by monitoring its missiles during test firings.

Range instrumentation. Radar is often used at test ranges to check the performance of military equipment. For example, range instrumentation radars can accurately track the flight of a new missile. If the missile does not perform as well as expected, the tracking data might help the designer determine what went wrong.

Weapon fire control. Radar can locate objects so precisely that it is used to aim and fire many kinds of weapons. Radar controls the firing of antiaircraft guns on tanks and warships. It directs guided missiles from jet fighters and from land-based launching sites. In addition, planes with radar bombsights can drop bombs accurately on targets at night or in bad weather.

In controlling automobile speed and traffic. Police in many areas of the United States and Canada use radar to enforce speed laws by checking the speed of motor vehicles on streets and highways. Their portable radar sets can detect speeding vehicles up to 1/2 mile (0.8 kilometers) away.

In weather observation and forecasting. Radar has an important role in short-range forecasts of local weather conditions. Radar echoes can be detected from raindrops and ice particles in clouds up to about 250 miles (400 kilometers) away. In many cases, the intensity of these echoes reveals what type of storm is approaching. For example, strong echoes are produced by hailstones in a thunderstorm. Radar echoes also indicate the direction in which a storm is moving and its speed.

By analyzing radar observations, weather forecasters can predict when a storm will pass over a certain area. In many cases, they can give advance warning to communities in the path of a hurricane, tornado, or other violent storm. The United States National Weather Service operates hundreds of ground and airborne radar units that keep close track of such storms. Most major airports also have weather radar. If a severe storm is sighted along a particular flight path, air traffic is redirected to avoid it.

In scientific research. Scientists rely on radar in conducting various kinds of studies. They use high-powered radars to investigate the earth's upper atmosphere. At altitudes of 60 miles (100 kilometers) and higher, the air reflects radio waves. This air is in a part of the upper atmosphere known as the ionosphere. In this region, the sun's radiation is so strong that it breaks air molecules into electrically charged particles called electrons and ions. As a result, it can be studied by radar from the earth's surface. Radar observations help scientists determine the temperature of the upper atmosphere and the kinds of gases in the air. Radar observations also indicate how fast winds blow at such high altitudes during different periods of the day.

Radar equipment and techniques contribute much to the study of the solar system. Astronomers have made radar observations of the moon, the sun, and the planets closest to the earth. They have even collected radar echoes from several of Jupiter's largest satellites. Such radar observations provide extremely accurate measurements of the distances to these objects. They also show how rapidly the objects rotate. Astronomers have obtained detailed radar maps of the moon and Mars by recording radio waves bounced off their surfaces. By using the same technique, astronomers have succeeded in penetrating the thick clouds that surround Venus and discovered enormous mountains and valleylike features on its surface. See Telescope.

The study of bird migrations is another area of scientific research that has benefited from radar. Zoologists depend on radar to trace the flight patterns of birds that migrate at night or that are too small to be seen from the ground. Radar can also be used to measure and map currents on the ocean surface within about 45 miles (72 kilometers) from shore. Such information is useful for research in marine biology and for planning offshore oil-drilling projects.

In space travel. Radar is vital to the success of missions into outer space. The first step in such a mission is to launch a manned or unmanned spacecraft into orbit around the earth. During the launching, mission controllers use a system of ground-based radars and other radio equipment to track the vehicle. As soon as the spacecraft enters its orbit, the radars measure the orbit's size and shape. Computers take the measurements and calculate when the craft's remaining rocket engines should be fired and for how long to send the vehicle from earth orbit into outer space.

Spacecraft designed to land on the moon or on another planet carry landing radar. This instrument measures the height of the spacecraft above the landing site and the rate of descent. Such information is used to regulate the engines of the craft so that it lands at the correct speed. If the vehicle descends too fast, it will crash. If it lands too slowly, it will burn too much fuel. In addition, radar is used to select safe landing sites for spacecraft. For example, radar maps of the moon helped U.S. scientists choose landing areas where rugged rock formations would not damage the Apollo lunar modules.

A mission may call for a spacecraft to dock with another space vehicle. The astronauts in the spacecraft locate the other vehicle with radar. They then use the radar data to adjust the direction and speed of their own craft to perform the docking maneuver.

When the electromagnetic waves transmitted by a radar set strike an object, they are reflected. Some of the reflected waves return to the set along the same path on which they were sent. This reflection closely resembles what happens when a person shouts in a mountain valley and hears an echo from a nearby cliff. In this case, however, sound waves are reflected instead of radio waves or light waves.

The waves transmitted by radar have a definite frequency. The frequency of such a wave is measured in units called megahertz (MHz). One megahertz equals 1 million hertz (cycles per second). Radio waves have lower frequencies than light waves have. Most radars that transmit radio waves operate at frequencies of about 5 to 36,000 MHz. Optical radars operate at much higher frequencies. Some generate light waves with frequencies up to 1 billion MHz.

In many cases, radar sets designed for different purposes operate at different frequencies. Radars that transmit at lower frequencies are more effective than high-frequency radars in penetrating clouds, fog, and rain and so are widely used on planes and ships. On the other hand, high-frequency radars provide precise direction measurements with much smaller antennas than those used by lower frequency radars. An optical radar, for example, can produce an extremely narrow signal beam with an antenna only about 1/2 inch (1.3 centimeters) in diameter. Optical radars are especially useful for surveying rough terrain where distant points have to be measured between such objects as large rocks and trees. Over-the-horizon radars use relatively low-frequency radio waves between 3 and 30 MHz. These waves reflect from an upper layer of the atmosphere called the ionosphere and can reach beyond the horizon to detect ships and planes at great distances. Radars of the North Warning System use relatively high-frequency radio waves called microwaves, which pass through the ionosphere.

Radar sets also differ in how they transmit signals. On this basis, they are usually classified into two general types: (1) pulse radar and (2) continuous-wave radar. Pulse radar is the more common type.

Pulse radar sends out signals in powerful bursts, or pulses. These pulses last only a few millionths of a second. A pulse radar set has one antenna, which alternately transmits the pulses and receives their echoes.

The distance to an object is found by measuring the time it takes a radar wave to reach the object and return. Radar waves, like all other electromagnetic waves, travel at the speed of light--186,282 miles (299,792 kilometers) per second. Therefore, a radar wave that returns after two seconds would have traveled 372,564 miles (599,584 kilometers)--186,282 miles to the object and 186,282 miles back. A pulse radar set automatically converts the time required for the round trip into the distance to the object.

The antenna transmits the pulses of waves in a narrow beam, which enables the set to determine an object's direction. Only an object within the area of the beam can reflect the waves. Thus the direction from which the waves are reflected to the antenna indicates the location of the object.

Pulse radar can track an object by continuously transmitting signal pulses and by measuring the object's distance and direction at regular intervals. It also can be used to make radar maps from an airplane. Radar maps are produced by scanning a beam of pulses over an area and plotting the strength of the echoes from each direction. The echoes appear as images on the radar display and are recorded on photographic film. Such objects as buildings, bridges, and mountains produce especially bright images because they reflect strong echoes.

Continuous-wave radar sends out a continuous signal rather than short bursts. There are two kinds of continuous-wave radar. They are (1) Doppler radar and (2) frequency-modulated (FM) radar.

Doppler radar is used chiefly to make precise speed measurements. It works on the basis of the Doppler effect, which is a change in observed wave frequency caused by motion. Doppler radar transmits a continuous wave of a constant frequency and uses the same antenna for transmitting and receiving. When the outgoing wave strikes an object that is approaching the radar set, the wave is reflected at a higher frequency than the frequency at which it was sent out. When an object is moving away from the set, the wave is bounced back at a lower frequency. The faster an object moves in either direction, the greater the difference in frequency between the transmitted and reflected waves. By measuring the difference in frequency, Doppler radar determines the speed of the object observed.

Police use Doppler radar to detect speeding motorists. Military personnel often use it to measure the velocity of targets for directing weapon fire.

Frequency-modulated (FM) radar also transmits a continuous signal, but it rapidly increases or decreases the frequency of the signal at regular intervals. As a result, frequency-modulated radar, unlike Doppler radar, can determine the distance to a moving or stationary object. By the time a radar signal reaches an object and returns, the frequency of the transmitter has changed. The difference between the frequency of the echo and that of the transmitter is measured and converted into the distance to the object that produced the echo. The farther away an object is located, the greater the difference in frequency.

Frequency-modulated radar, like pulse radar, can be used for mapping and tracking. It also serves as an altimeter for airplanes.

Although radar sets differ in size, most have similar parts. These parts include (1) the oscillator, (2) the modulator, (3) the transmitter, (4) the duplexer, (5) the antenna, (6) the receiver, (7) the signal processor, (8) the display, and (9) the timer.

The oscillator is a device that generates a low-power electric signal of a constant frequency. The frequency of the oscillator determines the operating frequency of a radar set.

The modulator. In pulse radar, the modulator is an electronic switch that rapidly turns the transmitter on and off. It causes the transmitter to produce short bursts of waves. In frequency-modulated radar, the modulator varies the frequency of the continuously transmitted wave. Doppler radar has no modulator.

The transmitter serves as an amplifier. It takes the low-power electric signal generated by the oscillator and produces a high-power electromagnetic wave. For example, the transmitter of a pulse radar used in air traffic control may produce wave pulses with a peak power of several million watts.

The duplexer makes it possible to use one antenna for both transmitting and receiving. The duplexer routes the electromagnetic waves from the transmitter to the antenna and prevents them from flowing into the receiver. The powerful waves from the transmitter would damage the sensitive receiver if they flowed into it. After the waves have been released through the antenna, the duplexer connects the receiver to the antenna. This switching action enables the receiver to pick up incoming echoes.

The antenna sends out radar waves in a narrow beam. It also collects the reflected echoes. Because most modern radar units have a duplexer, they use the same antenna for transmitting and receiving.

The most common type of antenna consists of a horn attached to the front of a large reflecting dish called a reflector. The horn launches the radar waves, and the reflector focuses them into a narrow beam. The antenna rotates so that the beam sweeps around the radar station, scanning for objects in all directions.

Other types of antennas are used in radar sets that operate either at extremely low frequencies or at extremely high frequencies. Radars that transmit low-frequency radio waves have an antenna made of metal tubes or rods. Such antennas resemble the outdoor aerials of TV sets. Radars that operate at optical frequencies use a device called a laser, which generates an intense beam of light. In optical radars, lenses control the size of the transmitted laser beam and capture the light waves returning from the target.

The receiver takes the weak echoes collected by the antenna and greatly amplifies them. It is so sensitive that it can easily detect echoes of less power than a millionth of a millionth of a watt. The receiver also filters out much of the noise and other interference picked up by the antenna.

The signal processor. In most radar sets, the incoming waves from the receiver pass through a signal processor before going to the display. The signal processor performs different tasks in radars used for different purposes. In many radar units, it blocks out echoes from large, fixed objects and allows only echoes from small, moving targets to reach the display. By doing so, the signal processor enables the operator of a radar set to see an airplane, for example, even though the echoes from the plane arrive at the same time as much stronger echoes from a mountain. A computer serves as the signal processor in most modern radars.

The display presents radar operators with information obtained about an object. Some sets have a simple display. The portable Doppler radars used by police, for example, have a meter that indicates the speed of a car or truck. However, most radar sets have a more complex display that consists of a cathode-ray tube (CRT). A CRT is a type of vacuum tube with a fluorescent screen like that of a TV set (see Vacuum Tube). A CRT display can present radar data in several forms. The most common form is the Plan Position Indicator, generally called the PPI.

The PPI provides a circular, maplike picture of the area scanned by the radar beam. The center of the picture corresponds to the location of the radar set. The screen has a compass scale around its edge for direction readings. The screen might also have rings spreading out from the center of the picture to mark distance in miles or kilometers. Radar echoes appear as bright spots. The position of a spot with respect to the compass scale shows the direction of the object. The distance of the spot from the center indicates how far away the object is. The speed of a moving object can be determined by noting the time it takes a spot to cover a certain distance on the radar screen.

Other forms of a CRT display show the elevation of an object. Such types of presentation are used with radar sets designed to help direct aircraft landings.

The timer ensures the smooth, efficient operation of a radar set. This device automatically turns the other major parts of the radar set on and off at precisely the right time. The timer does so by sending control signals to the various parts of the system in the proper sequence.

Hertz's discovery promoted widespread efforts to find ways of using radio waves for communication. Some scientists realized that radio waves might also be used for detecting distant objects. However, little research could be done in this area until basic radio equipment was developed. Devices for sending and receiving radio signals over long distances became available by the early 1900's.

The first uses of radar. In 1925, two American physicists, Gregory Breit and Merle A. Tuve, bounced short radio pulses off the ionosphere. They determined the height of the ionosphere by measuring the time taken by the reflected signals to return. Many scientists consider this experiment to have been the first practical use of radar. The success of the experiment encouraged researchers in many countries to conduct further scientific studies of the ionosphere with similar equipment and techniques.

Scientists also began experimenting with radio echoes to detect airplanes and ships. Much early work in this area was done by Robert A. Watson-Watt, a Scottish engineer and physicist. In 1935, he and a team of British scientists refined the pulse techniques used in ionospheric studies to locate aircraft at distances up to about 17 miles (27 kilometers). During this time, researchers in France, Germany, and the United States also developed experimental radars that could detect planes and ships within a limited range. These early radars were unreliable and lacked the sensitivity needed for many tasks. But they provided information useful for military and navigational purposes.

The growing threat of a world war stimulated efforts to improve radar technology during the late 1930's. Before World War II began in September 1939, the British had built a chain of radar stations along the east and south coasts of England for defense against air and sea attacks. By 1940, the United States was producing pulse-type radar for tracking planes and for controlling antiaircraft guns. Germany also had similar kinds of radar by about the same time. Japan and the Soviet Union developed radar-warning systems a few years later.

Advances during World War II. The radar sets available at the beginning of the war proved extremely valuable for military operations. As a result, scientists were urged to develop even better equipment.

American and British radar experts cooperated closely during the war and produced important advances. The British were working to improve a special kind of vacuum tube called the magnetron. By late 1939, their version of the magnetron could generate pulses of microwave energy at high enough power levels to be used in radar systems. In 1940, the British turned it over to the Americans for further development and manufacturing.

The magnetron contributed greatly to the development of modern radar. This vacuum tube generates microwaves--that is, short radio waves with frequencies of more than 1,000 MHz. These high-frequency waves can be concentrated into narrow beams without the use of a huge radar antenna. Microwaves thus made it possible to design radar units small enough for aircraft, patrol boats, and mobile ground stations.

Before the war ended in 1945, British and American researchers had also developed methods of making enemy radar less effective. The Germans produced similar countermeasures. In one widely used method, planes on bombing missions dropped countless numbers of metal foil strips called chaff. Each strip reflected radio signals like a radar target. The bombers filled the air with so many strips that enemy radar operators had difficulty recognizing echoes from the planes.

In another countermeasure, planes and ships carried high-powered radio transmitters. These transmitters produced enough interference to prevent enemy radar from receiving echoes from the planes and ships. Engineers also designed equipment that received pulses from enemy radar and sent them back at an increased power level after a short pause. As a result, false targets appeared on the display of the enemy radar and drew attention from the real targets.

Continued progress. During the early 1950's, American scientists worked on a type of vacuum tube called the klystron. They succeeded in developing a high-powered klystron, which is well-suited for radars that require little variation in microwave frequency from one pulse to the next. Scientists later improved the klystron so that it could generate microwaves at extremely high power levels. This development helped increase the accuracy of radar. Scientists also worked to improve radar sensitivity. By the late 1960's, they had designed receivers that produced little internal noise, which interferes with the reception of faint echoes.

The rapid development of the electronic computer after World War II contributed much to radar technology. Computers make effective signal processors. They can analyze echoes efficiently at high speeds and present the information obtained in a form most useful to radar operators.

Radar also benefited from the invention of the transistor in 1947 and of related solid-state devices during the 1950's and 1960's. These devices enabled engineers to build lighter and more reliable radar sets. In addition, engineers used a solid-state device called a phase shifter to develop a new kind of radar. This radar, which is known as phased array, moves its signal beam electronically rather than by rotating an antenna. Phased array radars are especially useful in situations where the signal beam must be moved rapidly from one target to the next.

During the late 1960's, physicists perfected the laser. Their work resulted in the development of optical radars, which operate at the high frequencies of laser light. This type of radar requires an antenna only about the size of a thumbtack to send out an extremely narrow signal beam.

Radar in the future. Researchers today are seeking ways to reduce the size of microwave radars and to manufacture them at low cost. Pocket-sized radar units could be widely used as aids for blind people and as collision-warning devices in cars. Researchers have discovered that over-the-horizon radars can monitor weather over large areas of the ocean that could not be observed previously. These radars might be used to make weather predictions more accurate. In addition, microwave radars built into a single artificial satellite might one day track ship and aircraft traffic over most of the earth.

Contributor: G. D. Thome, Ph.D., Consulting Scientist, Raytheon Co.

Related articles include:

Electronics; Laser; Radio.

Why was the magnetron important in the development of radar?

How does pulse radar find the distance to an object?

What is a Plan Position Indicator?

What are some military uses of radar?

What is the special feature of phased array radar?

How does radar help weather forecasters?

What is Doppler radar? How is it used?

Why is radar an effective aid in ship navigation?

What is a duplexer? Why is it important?

Bowen, Edward G. Radar Days. Hilger, 1987.

Hitzeroth, Deborah. Radar. Lucent Bks., 1990.

Skolnik, Merrill I., ed. Radar Handbook. 2nd ed. McGraw, 1990.>

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