Radar
Radar
Radar (a word derived from RA dio D etection A nd R anging) is an electronic means of measuring distance and/or velocity of remote objects by sending out a radio signal and detecting echoes. In principle, it can pierce fog, darkness, or any atmospheric disturbance all the way to the horizon. Depending on its particular characteristics, a radar unit may show its users clouds, landmasses, or objects such as ships, airplanes, or spacecraft. Radar can measure distance or range to a target object, and aircraft can use radar to determine altitude. Speed detection is another common application. Radar can be used to land and guide aircraft, probe through ice or soil for geological information, and map the three-dimensional characteristics of oceans and landmasses precisely from orbit. Military applications include weapons detection (e.g., detection of missiles) and detection of enemy vessels (e.g., ships,planes, etc.).
Unlike water waves, electromagnetic waves do not require a medium to travel through. They can propagate through air, vacuum, and certain materials. Light waves, radio waves, microwaves, and radar waves are all examples of electromagnetic waves. Just as light reflects off of some surfaces and travels through others, radar waves bounce off some objects and travel through others.
Basic radar operation
The simplest mode of radar operation is range-finding, performed by time-of-flight calculation. The unit transmits a radar signal, i.e., sends radar waves out toward the target. The waves hit the target and are reflected back in the same way that water waves are reflected from the end of a bathtub. The returning wave is received by the radar unit, and the travel time is registered. Basic physics tells us that distance is equal to rate of travel multiplied by the time of travel. All electromagnetic waves travel at the same speed in a vacuum—the speed of light, which is 3.0× 108 m/s. This speed is reduced by some small amount when the waves are traveling in a medium such as air, but this can be calculated. If the radar system sends a pulse out toward a target and records the amount of time until the return pulse is received, the target distance can be determined by the simple equation d = ct /2, where d is distance to the target, c is the speed of light, and t is the time between transmission of the outgoing pulse and the detection of the echo.
A basic radar unit consists of: a frequency generator and timing control unit; a transmitter with a modulator to generate a signal; an antenna with a parabolic reflector to transmit the signal; a duplexer to switch between transmission and reception mode; an antenna to gather the reflected signal; a receiver to detect and amplify this return; and signal processing, data processing, and data display units. If the transmitter and receiver are connected to the same antenna or to antennas in the same location, the unit is called monostatic. If the transmitter and receiver antennas are in very different locations, the unit is known as bistatic. The frequency generator/timing unit is the master coordinator of the radar unit. In a monostatic system, the unit must switch between sending out a signal and listening for the return reflected from the target; the timing unit controls the duplexer that performs the switching. The transmitter generates a radio signal that is modulated, or varied, to form either a series of pulses or a continuously varying signal. This signal is reflected from the target, gathered by the antenna, and amplified and filtered by the receiver. the signal processing unit further cleans up the signal, and the data processing unit decodes it. Finally, the data is presented to the user on the display.
Before target range can be determined, the target must be detected, an operation more complicated than it would seem. Consider radar operation again. A pulse is transmitted in the direction that the antenna is facing. When it encounters a material that is different from the surrounding medium (e.g., fish in water or an airplane in the air), a portion of the pulse will be reflected back toward the receiver antenna. This antenna in turn collects only part of the reflected pulse and sends it to the receiver and the processing units where the most critical operations take place. Because only a small amount of the transmitted pulse is ever detected by the receiving antenna, the signal amplitude is dramatically reduced from its initial value. At the same time, spurious reflections from non-target surfaces or electronic noise from the radar system itself act to clutter up the signal, making it difficult to isolate. Various filtering and amplification operations help to increase the signal-to-noise ratio (SNR), making it easier to lock on to the actual signal.
If the noise is too high, the processing parameters incorrect, or the reflected signal amplitude too small, it is difficult for the system to determine whether a target exists or not. Real signals of very low amplitude can be swamped by interference, or “lost in the noise.” In military applications, interference can also be generated by reflections from friendly radar systems, or from enemy electronic countermeasures that make the radar system detect high levels of noise, false targets, or clones of the legitimate target. No matter what the source, interference and signal quality are serious concerns for radar system designers and operators.
Radar tracking systems
Radar systems can send out thousands of pulses per second. Using a rapid sequence of pulses, a radar system can not only determine the range of a target, but it can also track target motion. Ranging can be performed with an omnidirectional antenna, but target location and tracking require a more sophisticated system with knowledge of the antenna elevation (vertical) angle and azimuthal (horizontal) angle with respect to some fixed coordinate system. Land-based systems generally define true north as the azimuthal reference and the local horizontal as the elevation reference. The azimuthal reference for air and sea systems is the bow of the ship, but elevation reference varies depending on the pitch and roll stabilization of the ship or plane. When you are driving a car down the street, you might characterize other cars as to your left, to your right, or behind you; you define the location of the cars in terms of your own coordinate system. Similarly, when a radar system receives the reflection from a target, it checks the orientation of the receiving antenna with respect to the coordinate axes to determine the object location. Moreover, just as you can use a road-map to determine the absolute location of an object, so a radar system can be used to locate a target in terms of longitude and latitude. Multiple pulses are required to track the motion of a target. The pulses must be spaced far enough apart that a pulse can be sent out and return before the next pulse is sent, but this is quite feasible when you consider that a radar pulse can travel 100 mi (161 km), strike a target, and return in less than 1/1000 of a second.
Air Traffic Control uses radar to track and direct the courses of the many planes in civilian airspace. Civilian and military craft generally carry a beacon, or transponder, known as the Air Traffic Control Radar Beacon System (ATCRBS). An Air Traffic Control interrogator system sends a signal to the transponder that prompts it to reply with identification and altitude information. In this way, air traffic controllers can monitor the courses of planes in their region. A military version of the beacon, known as identification, Friend or Foe (IFF) uses coded signals to identify aircraft.
Doppler radar
A specialized type of radar uses the Doppler effect to detect the speed of a target. You have probably observed the Doppler effect hundreds of times without realizing it. The change in pitch as a vehicle approaches, then drives past you is an example of the Doppler frequency shift. The sound waves shift to a higher frequency as the vehicle comes toward you, raising the pitch, then as the vehicle pulls away the frequency of the sound is lowered, dropping the pitch. Doppler theory tells us that:
fd= 2 VR / c
where fd is the Doppler frequency shift, VR is the radial velocity of the target (i.e., velocity along the line-of-sight), and c is the speed of propagation of the radar pulse, known for pulses traveling in air. Doppler frequency shift is the difference between the frequency of the pulse transmitted to the target and the frequency of the return pulse. If this can be measured, then the radial speed, or speed along the line-of-sight can be determined. Note, however, that target velocity at right angles to the radar system line-of-sight does not cause Doppler shift. In such a case, the speed detector would register a target speed of zero. Similarly, if a target is moving at some angle to the direct line-of-sight, the system would only detect the radial component of its velocity. A cosine term can be added to the basic equation to account for non-radial motion. More sophisticated radar systems include this compensation, but typical law enforcement speed detectors do not, with the result that the measured velocity of the target is somewhat lower than the actual velocity.
A Doppler radar system consists of a continuously transmitting source, a mixer, and data and signal processing elements. The signal is sent out to the target continuously. When the return is received, it is “mixed” with a sample of the transmitted signal, and
KEY TERMS
Bistatic— A radar system with transmitting and receiving antennas in separate locations.
Duplexer— In a monostatic system, the device that switches system operation between transmit and receive mode.
Modulation— Variation, as in modulation of an electrical signal.
Monostatic— A radar system in which a single antenna both transmits and receives; a system in which transmitting and receiving antennas are at the same location.
Transponder— A beacon. In the case of an Air Traffic Control radar beacon system, a device that is capable of transmitting certain information when queried.
the frequency of the resultant output is the Doppler frequency shift caused by the radial velocity of the target. The Doppler shift is averaged over several samples and processed to yield target speed.
Effective operating range of a radar system is limited by antenna efficiency, transmitted power, the sensitivity of the detector, and the size of the target or energy it reflects. Reflection of electromagnetic waves from surfaces is fundamental to radar. All objects do not reflect radar waves equally well—the strength of the wave reflection depends on the size, shape, and composition of the object. Metal objects are the best reflectors, while wood and plastic produce weaker reflections. So-called stealth airplanes are based on this concept and are built from materials that produce a minimal reflection.
In recent years laser radar systems have been developed. Laser radar systems operate on essentially the same principle as conventional radar, but the significantly shorter wavelengths of visible light allow much higher resolution. Laser radar systems can be used for imaging and for measurement of reflectivity. They are used for vibration detection in automotive manufacturing and for mapping power lines. Because they are more difficult to detect than conventional radar systems, laser radar speed guns are increasingly being adopted by law enforcement agencies.
Radar has undergone considerable development since its introduction in the 1930s. It was used by the Magellan robotic spacecraft from 1989 to 1994 to map the surface of the planet Venus through its thick cloud layer and has been used, starting in 2004, to map the surface of Saturn’s giant moon Titan (larger than the planet Mercury) through its nearly opaque layer of organic smog. As of 2006, exploratory scientific radars were in orbit around Saturn and Mars. The sHARAD (shallow subsurface radar) unit aboard NASA’s Mars Reconnaissance orbiter, which began orbiting Mars in 2006, is designed to look for ice and water under the Martian soil to a depth of several hundred feet.
Resources
BOOKS
Fisher, David E. A Summer Bright and Terrible: Winston Churchill, Lord Dowding, Radar, and the Impossible Triumph of the Battle of Britain. Emeryville, CA: Shoemaker & Hoard, 2006.
Richards, Mark. Radar Signal Processing. New York: McGraw-Hill, 2005.
OTHER
NASA. “Mars Reconnaissance Orbiter: SHARAD (Shallow Subsurface Radar).” March 21, 2006. <http://mars.jpl.nasa.gov/mro/mission/sc_instru_sharad.html> (accessed November 15, 2006).
Kristin Lewotsky
Frank Lewotsky
Radar
Radar
Radar (RAdio Detection And Ranging) is an electronic detector system that measures distance or velocity by sending a signal out and receiving its return. It can pierce fog , darkness, or any atmospheric disturbance all the way to the horizon. Within its range, it can show an observer clouds , landmass, or objects such as ships, airplanes, or spacecraft. Radar can measure distance or range to a target object, and aircraft can use radar to determine altitude. Speed detection is another common application. Radar can be used to monitor atmospheric systems, to track storms, and to help predict the weather . Military applications include weapons ranging and direction, or control of guided missiles.
To understand radar, it is necessary to understand a bit about electromagnetic waves. Unlike water waves, electromagnetic waves do not require a medium to travel through. They can propagate through air, vacuum , and certain materials. Light waves, radio waves , microwaves, and radar waves are all examples of electro-magnetic waves. Just as light reflects off of some surfaces and travels through others, radar waves bounce off some objects and travel through others.
Basic radar operation
The simplest mode of radar operation is range-finding, performed by time-of-flight calculation. The unit transmits a radar signal, i.e., sends radar waves out toward the target. The waves hit the target and are reflected back in the same way that water waves are reflected from the end of a bathtub. The returning wave is received by the radar unit, and the travel time is registered. Basic physics tells us that distance is equal to rate of travel multiplied by the time of travel. Now all electromagnetic waves travel at the same speed in a vacuum—the speed of light, which is 3.0 × 108 m/s. This speed is reduced by some small amount when the waves are traveling in a medium such as air, but this can be calculated. If the radar system sends a pulse out toward a target and records the amount of time until the return pulse is received, the target distance can be determined by the simple equation d = vt, where d is distance, v is velocity, and t is time.
A basic radar unit consists of: a frequency generator and timing control unit; a transmitter with a modulator to generate a signal; an antenna with a parabolic reflector to transmit the signal; a duplexer to switch between transmission and reception mode; an antenna to gather the reflected signal; a receiver to detect and amplify this return; and signal processing, data processing, and data display units. If the transmitter and receiver are connected to the same antenna or to antennas in the same location, the unit is called monostatic. If the transmitter and receiver antennas are in very different locations, the unit is known as bistatic. The frequency generator/timing unit is the master coordinator of the radar unit. In a monostatic system, the unit must switch between sending out a signal and listening for the return reflected from the target; the timing unit controls the duplexer that performs the switching. The transmitter generates a radio signal that is modulated, or varied, to form either a series of pulses or a continuously varying signal. This signal is reflected from the target, gathered by the antenna, and amplified and filtered by the receiver. The signal processing unit further cleans up the signal, and the data processing unit decodes it. Finally, the data is presented to the user on the display.
Before target range can be determined, the target must be detected, an operation more complicated than it would seem. Consider radar operation again. A pulse is transmitted in the direction that the antenna is facing. When it encounters a material that is different from the surrounding medium (e.g., fish in water or an airplane in the air), a portion of the pulse will be reflected back toward the receiver antenna. This antenna in turn collects only part of the reflected pulse and sends it to the receiver and the processing units where the most critical operations take place. Because only a small amount of the transmitted pulse is ever detected by the receiving antenna, the signal amplitude is dramatically reduced from its initial value. At the same time, spurious reflections from non-target surfaces or electronic noise from the radar system itself act to clutter up the signal, making it difficult to isolate. Various filtering and amplification operations help to increase the signal-to-noise ratio (SNR), making it easier to lock on to the actual signal. If the noise is too high, the processing parameters incorrect, or the reflected signal amplitude too small, it is difficult for the system to determine whether a target exists or not. Real signals of very low amplitude can be swamped by interference , or "lost in the noise." In military applications, interference can also be generated by reflections from friendly radar systems, or from enemy electronic countermeasures that make the radar system detect high levels of noise, false targets, or clones of the legitimate target. No matter what the source, interference and signal quality are serious concerns for radar system designers and operators.
Radar tracking systems
Radar systems can send out thousands of pulses per second. Using a rapid sequence of pulses, a radar system can not only determine the range of a target, but it can also track target motion . Ranging can be performed with an omnidirectional antenna, but target location and tracking require a more sophisticated system with knowledge of the antenna elevation (vertical) angle and azimuthal (horizontal) angle with respect to some fixed coordinate system. Land-based systems generally define true north as the azimuthal reference and the local horizontal as the elevation reference. The azimuthal reference for air and sea systems is the bow of the ship, but elevation reference varies depending on the pitch and roll stabilization of the ship or plane. When you are driving a car down the street, you might characterize other cars as to your left, to your right, or behind you; you define the location of the cars in terms of your own coordinate system. Similarly, when a radar system receives the reflection from a target, it checks the orientation of the receiving antenna with respect to the coordinate axes to determine the object location. Moreover, just as you can use a roadmap to determine the absolute location of an object, so a radar system can be used to locate a target in terms of longitude and latitude. Multiple pulses are required to track the motion of a target. The pulses must be spaced far enough apart that a pulse can be sent out and return before the next pulse is sent, but this is quite feasible when you consider that a radar pulse can travel 100 mi (161 km), strike a target, and return in less than 1/1000 of a second.
Air Traffic Control uses radar to track and direct the courses of the many planes in civilian airspace. Civilian and military craft generally carry a beacon, or transponder, known as the Air Traffic Control Radar Beacon System (ATCRBS). An Air Traffic Control interrogator system sends a signal to the transponder that prompts it to reply with identification and altitude information. In this way, air traffic controllers can monitor the courses of planes in their region. A military version of the beacon, known as Identification, Friend or Foe (IFF) uses coded signals to identify aircraft.
Doppler radar
A specialized type of radar uses the Doppler effect to detect the speed of an target. You have probably observed the Doppler effect hundreds of times without realizing it. The change in pitch as a vehicle approaches, then drives past you is an example of the Doppler frequency shift. The sound waves shift to a higher frequency as the vehicle comes toward you, raising the pitch, then as the vehicle pulls away the frequency of the sound is lowered, dropping the pitch. Doppler theory tells us that
where fd is the Doppler frequency shift, Vr is the radial velocity of the target (i.e. velocity along the line-of-sight), and c is the speed of propagation of the radar pulse, known for pulses traveling in air. Doppler frequency shift is the difference between the frequency of the pulse transmitted to the target and the frequency of the return pulse. If this can be measured, then the radial speed, or speed along the line-of-sight can be determined. Note, however, that target velocity at right angles to the radar system line-of-sight does not cause Doppler shift. In such a case, the speed detector would register a target speed of zero . Similarly, if a target is moving at some angle to the direct line-of-sight, the system would only detect the radial component of its velocity. A cosine term can be added to the basic equation to account for non-radial motion. More sophisticated radar systems include this compensation, but typical law enforcement speed detectors do not, with the result that the measured velocity of the target is somewhat lower than the actual velocity.
A Doppler radar system consists of a continuously transmitting source, a mixer, and data and signal processing elements. The signal is sent out to the target continuously. When the return is received, it is "mixed" with a sample of the transmitted signal, and the frequency of the resultant output is the Doppler frequency shift caused by the radial velocity of the target. The Doppler shift is averaged over several samples and processed to yield target speed.
Effective operating range of a radar system is limited by antenna efficiency, transmitted power, the sensitivity of the detector, and the size of the target/energy it reflects. Reflection of electromagnetic waves from surfaces is fundamental to radar. All objects do not reflect radar waves equally well—the strength of the wave reflection depends on the size, shape, and composition of the object. Metal objects are the best reflectors, while wood and plastic produce weaker reflections. So-called stealth airplanes are based on this concept and are built from materials that produce a minimal reflection.
In recent years laser radar systems have been developed. Laser radar systems operate on essentially the same principle as conventional radar, but the significantly shorter wavelengths of visible light allow much higher resolution. Laser radar systems can be used for imaging and for measurement of reflectivity. They are used for vibration detection in automotive manufacturing and for mapping power lines. Because they are more difficult to detect than conventional radar systems, laser radar speed guns are increasingly being adopted by law enforcement agencies.
Radar has undergone considerable development since its introduction in the 1930s. It is a remarkably useful tool that touches our lives in a surprising number of ways, whether by the weather report that we listen to in the morning, or the guidance of the airplanes we ride in. It has given us a different way to see the world around us.
Resources
books
Blake, Bernard, ed. Jane's Radar and Electronic Warfare Systems. Alexandria, VA: Jane's Information Group Inc., 1992.
Edde, Byron. Radar: Principles, Technology, Applications. Englewood Cliffs, NJ: Prentice-Hall, 1993.
Kristin Lewotsky Frank Lewotsky
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Bistatic
—A radar system with transmitting and receiving antennas in separate locations.
- Duplexer
—In a monostatic system, the device that switches system operation between transmit and receive mode.
- Modulation
—Variation, as in modulation of an electrical signal.
- Monostatic
—A radar system in which a single antenna both transmits and receives; a system in which transmitting and receiving antennas are at the same location.
- Transponder
—A beacon. In the case of an Air Traffic Control radar beacon system, a device that is capable of transmitting certain information when queried.
RADAR
The Italian Guglielmo Marconi first demonstrated radio reflection for detection in the 1920s. In the United States, Gregory Breit and Merle A. Tuve discovered the principle of pulse ranging in 1925. Research and development was underway simultaneously in Germany, Great Britain, and the United States by the early 1930s. The Germans initially had better equipment aboard warships that began radar‐aided commerce raiding in September 1939. In 1937 the British began deploying the Chain Home early warning network along the Channel coast, which would provide the decisive advantage in the Battle of Britain. Early World War II radars used radio pulses of low frequency and long (a meter or more) wavelength, but these required large antennas, suitable only for large ships or ground stations and were imprecise compared to the next generation radars. With the invention in Great Britain of the cavity magnetron in 1940, however, much smaller sets employing centimeter wavelengths capable of much greater precision were possible. In 1940 Henry Tizard led a mission to the United States that successfully enlisted American industrial aid, and the Germans fell behind, never to regain parity. In the Pacific, the Japanese never even came close to it, and most Japanese radar systems were based on early ones captured from the British and Americans in 1942.
At sea, Allied naval radar was key in the defeat of the U‐boat threat in 1943, and radar‐directed naval gunfire was decisive in several sea battles, including the Battle of Leyte Gulf in October 1944, in which US battleships in the Surigao Straits using radar‐directed gunfire at night destroyed a Japanese fleet. In the air, the radar struggle between countermeasure and counter‐countermeasure was dynamic, deadly, and decisive. In July 1943 the Royal Air Force first used “window” (American term: “chaff”), small strips of reflective tinfoil, to negate German air defenses of Hamburg (Operation “Gomorrah”) in a raid that killed approximately 40,000 inhabitants. American bombers equipped with radar jamming transmitters (called “Carpet”) blocked German “Wurzburg” anti‐aircraft gun‐ laying radars and assisted in a deceptive spoof on the night of the Normandy landings. Offensively, American and British aircraft carried increasingly sophisticated navigational radars, such as the H2S and H2X (“Mickey”) sets that portrayed ground features with greater and greater detail and enabled bombing at night or through cloud cover. Night fighters equipped with small radar sets such as the German “Lichtenstein” hunted enemy aircraft in the darkness and located them entirely by radar. Specialized aircraft (“ferrets”) gathered radar intelligence while electronic warfare operators (“ravens”) waged an invisible but critical war in what was then called “the ether,” and might today be called “cyberspace.”
During the Cold War both the U.S. and Russians erected radar networks such as the Distant Early Warning or “DEW” line across Canada to warn of enemy aircraft. Strategic Air Command (SAC) warplans from the 1950s through the 1980s depended on radar to accurately navigate to and identify targets, and electronic countermeasures (ECM) such as radar jamming and chaff were the key to negating enemy defenses. Intercontinental ballistic missiles forced both sides in the 1960s to develop even more sophisticated radar nets such as the Ballistic Missile Early Warning System (BMEWS) to warn of missile attack. Perhaps the ultimate were radars devised to support anti‐ missile defenses, capable of not only detecting enemy missiles in space but also of tracking them for interception and destruction by defensive missiles. Radars belonging to the Space Detection and Tracking System (SPADATS) keep constant track of the thousands of objects orbiting the earth.
The air war over Vietnam was dominated by radar controlled air defenses, as North Vietnam successfully employed Russian radar‐guided surface‐to‐air missiles (SAMs) against American air operations. American countermeasures included not only traditional ECM, but also direct attacks on radar control systems. This technique, called “Wild Weasel”, had been tried in WW II, but not until the 1960s were detection and homing systems sufficiently advanced to be successful. Anti‐radar electronic warfare EW) was so important by the Persian Gulf War of 1991 that virtually no Coalition aerial attacks were mounted without EW support. Since the 1940s, designers have sought aircraft undetectable by enemy radars. This effort came to fruition with the F‐117 “Stealth Fighter” and B‐2 “Stealth Bomber”, both of which used Low Observable technology to make them almost invisible to enemy radars.
Modern military radars have become increasingly sophisticated, and those mounted in surveillance aircraft such as the Airborne Warning and Control System (“AWACS”) or Joint Surveillance and Tracking Radar System (“JSTARS”) provide virtually a three‐dimensional portrayal of a battlespace the size of a small country. Radar has also had an enormous effect in the civilian world. From radar astronomy, to traffic control, to weather and storm warning, to air and maritime navigation, radar has become an indispensable facet of modern life.
Bibliography
Alfred Price , Instruments of Darkness (1977).
Alfred Price , The History of US Electronic Warfare, Volumes I and II (1984, 1989).
Henry E. Guerlac , Radar in World War II (1987).
David Pritchard , The Radar War (1989).
Robert Buderi , The Invention That Changed the World (1996).
Alan Beyerchen , From Radio to Radar: Interwar Military Adaptation to Technological Change in Germany, the UK, and the US, in Alan R. Millet and Williamson Murray, editors, Military Innovation in the Interwar Period, (1996).
Daniel T. Kuehl
Radar
RADAR
RADAR, an acronym for "radio detection and ranging," is a method of locating distant targets by sending bursts of electromagnetic radiation and measuring their reflections. In the most common method, ultrashort radio waves are beamed toward the target by a scanning antenna. The resulting echoes are then displayed on a cathode-ray tube by means of a scanning signal synchronized with the antenna, so that the echo from each target appears as an illuminated dot, in the appropriate direction and at a proportional distance, on a map of the entire area being scanned. In other versions, continuous waves are used, and, in some, only moving targets are revealed (for example, in police sets used to detect speeding vehicles).
The science behind radar dates to the 1920s, when radio operators noticed perturbations caused by obstacles moving in a radio field. Such effects were familiar to both amateur and professional radio enthusiasts in many countries and were at first freely discussed in engineering journals. As the military significance of these observations dawned on researchers in government laboratories in the 1930s, such references grew rarer. Two American reports, in particular, helped shape the nascent science of radio detection: a 1933 report (by C. R. Englund and others in the Proceedings of the Institute of Radio Engineers) describing a systematic investigation of the interferences caused by overflying aircraft and a 1936 report (by C. W. Rice in the General Electric Review) on the uses of ultrahigh-frequency
equipment, among which was listed "radio-echo location for navigation."
The first innovations came from the commercial sector. Radio altimeters were developed to gauge the altitude of planes; experimental equipment intended to prevent collisions was installed on the French Line's giant ship Normandie, producing considerable publicity but only moderate success. Scientists, as well, found applications for these early forms of radar technology. They used radio detection to locate storms, measure the height of the ionosphere, and survey rugged terrain. Essential technologies evolved from these experiments, such as ultrahighfrequency (microwave) tubes, circuits, and antennas; cathode-ray (picture) display tubes; and wide-band receivers capable of amplifying and resolving extremely short pulses of one-millionth of one second (microsecond) or less.
As World War II approached, military laboratories in several countries rushed to develop systems capable of locating unseen enemy ships and aircraft. Such a capability, military planners knew, would provide enormous tactical advantages on sea and in the air. Six countries led the race—the United States, Great Britain, France, Germany, Italy, and Japan—but there were doubtless others, including Canada, the Netherlands, and the Soviet Union. Great Britain made the swiftest progress before the outbreak of the war. A team assembled by the engineer Robert Watson-Watt devised a system of radar stations and backup information-processing centers. This complex was partly in place when war broke out in September 1939 and was rapidly extended to cover most of the eastern and southern coasts of England. By the time of the air Battle of Britain a year later, the system was fully operational. The British radar system is credited with swinging the balance in the defenders' favor by enabling them to optimize their dwindling air reserves.
American military developments had started even earlier, in the early 1930s, and were carried on at fairly low priority at the Naval Research Laboratory under R. M. Page and at the army's Signal Corps laboratories under W. D. Hershberger. By the time the United States entered the war, radar had been installed on several capital warships and in a number of critical shore installations. Indeed, a radar post in the hills above Pearl Harbor spotted the Japanese attack in December 1941, but the backup system was not in place and the warning did not reach the main forces in time. American forces in the Pacific quickly corrected this situation, and radar played a significant role six months later in the pivotal victory over a Japanese naval force at Midway Island.
British researchers had not been idle in the meantime. Great Britain made a great step forward with the invention of a high-power magnetron, a vacuum tube that, by enabling the use of even shorter centimetric wavelengths, improved resolution and reduced the size of the equipment. Even before the attack on Pearl Harbor, a British delegation led by Sir Henry Tizard had brought a number of devices, including the centimetric magnetron, to the United States in an effort to enroll U.S. industry in the war effort, since British industry was already strained to full capacity. The resulting agreement was not entirely one-sided, since it placed some American developments at the Allies' disposal: for instance, the transmit-receive (TR) tube, a switching device that made it possible for a single antenna to be used alternately for radar transmission and reception. From then on until the end of the war, British and U.S. radar developments were joined, and the resulting equipment was largely interchangeable between the forces of the two nations.
The principal U.S. radar research laboratories were the Radiation Laboratory at the Massachusetts Institute of Technology (MIT), directed by Lee Du Bridge, where major contributions to the development of centimetric radar (including sophisticated airborne equipment) were made; and the smaller Radio Research Laboratory at Harvard University, directed by F. E. Terman, which specialized in electronic countermeasures (i.e., methods of rendering enemy's radar ineffective and overcoming its countermeasures). The MIT group produced an elaborate and detailed twenty-eight-volume series of books during the late 1940s that established a solid foundation for worldwide radar developments for several decades.
Wartime industrial advances gave U.S. manufacturers a head start over foreign competitors, notably in the defeated nations, where war-related industries remained shut down for several years. Postwar developments were enhanced by commercial demand—there was soon scarcely an airport or harbor any where that was not equipped with radar—and by the exigencies of the space age, including astrophysics. Many of the basic inventions of World War II remained fundamental to new developments, but additional refinements were introduced by researchers in many countries. Among them, the contributions of Americans were perhaps the most numerous and ensured that American-made radar equipment could compete in world markets despite high production costs.
BIBLIOGRAPHY
Buderi, Robert. The Invention that Changed the World. New York: Simon and Schuster, 1996.
Burns, Russell, ed. Radar Development to 1945. London: Institution of Electrical Engineers, 1988.
Fisher, David E. A Race on the Edge of Time. New York: McGrawHill, 1988.
Page, Robert M. The Origin of Radar. Garden City, N.Y.: Anchor Books, 1962.
CharlesSiisskind/a. r.
See alsoAir Defense ; Aircraft Industry ; Signal Corps, U.S. Army ; Weather Service, National .
RADAR
RADAR
█ LARRY GILMAN
RADAR—an acronym for RAdio Detection And Ranging— is the use of electromagnetic waves at sub-optical frequencies (i.e., less than about 1012 Hz) to sense objects at a distance. Hundreds of different RADAR systems have been designed for various purposes, military and other. RADAR systems are essential to the navigation and tracking of craft at sea and in the air, weather prediction, and scientific research of many kinds.
Principles. In basic RADAR, radio waves are transmitted from an antenna. These outgoing waves eventually bounce off some distant object and return an echo to the sender, where they are received, amplified, and processed electronically to yield an image showing the object's location. The waves sent out may be either short oscillatory bursts (pulses) or continuous sinusoidal waves. If a RADAR transmits pulses it is termed a pulse RADAR, whereas if it transmits a continuous sinusoidal wave it is termed a continuous-wave RADAR.
On closer examination, the RADAR process is seen to be more complex. For example, reflection of an echo by the object one wishes to sense is anything but straightforward. Upon leaving a transmitting antenna, a radio wave propagates in a widening beam at the speed of light (> 186,000 miles per hour [3 × 108 m/sec]); if it encounters an obstacle (i.e., a medium whose characteristic impedance differs from that of air and vacuum [> 377 Ω), it splits into two parts. One part passes into the obstacle and is (generally) absorbed, and the other is reflected. Where the reflected wave goes depends on the shape of the obstacle. Roundish or irregular obstacles tend to scatter energy through a wide angle, while flat or facet-like surfaces tend to send it off in a single direction, just as a flat mirror reflects light. If any part of the outgoing wave is reflected at 180° (which is not guaranteed) it will return to the transmitter. This returned or backscattered signal is usually detected by the same antenna that sent the outgoing pulse; this antenna alternates rapidly between transmitting pulses and listening for echoes, thus building a realtime picture of the reflecting targets in range of its beam. The energy the echoes receive is a small fraction of that in the pulses transmitted, so the strength of the transmitted pulse and the sensitivity of the receiver determines a RADAR's range. By systematically sweeping the direction in which its antenna is pointed, a RADAR system can scan a much larger volume of space than its beam can interrogate at any one moment; this is why many RADAR antennas, on ships or atop air-traffic control towers, are seen to rotate while in operation.
Radio waves are not the only form of energy that can be used to derive echoes from distant targets. Sound waves may also be used. Indeed, because radio waves are rapidly absorbed in water, sonar (SOund Navigation and Ranging) is essential to underwater operations of all sorts, including sea-floor mapping and anti-submarine warfare.
Applications. Since World War II RADAR has been deployed in many forms and has found a wide application in scientific, commercial, and military operations. RADAR signals have been bounced off targets ranging in size from dust specks to other planets. RADAR is essential to rocketry and early-warning detection of missiles, air traffic control, navigation at sea, automatic control of weapons such as antiaircraft guns, aircraft detection and tracking, mapping of the ground from the air, weather prediction, intruder detection, and numerous other tasks. Few craft, military or civilian, put to sea or take to the air without carrying some form of RADAR.
In recent decades, development of the basic RADAR principle—send pulse, listen for echo—has proceeded along a number of interesting paths. By exploiting the Doppler effect, which causes frequency shifts in echoes reflected from moving objects, modern RADARs can tell not only where an object is but what direction it is moving in and how quickly. The Doppler effect also allows for the precision mapping of landscapes from moving aircraft through the synthetic-aperture technique. Synthetic-aperture systems exploit the fact that stationary objects being swept by a RADAR beam projected from a moving source have, depending on their location, slightly different absolute velocities with respect to that source. By detecting these velocity differences using the Doppler effect, synthetic aperture type RADAR greatly permits the generation of high-resolution ground maps from small, airborne RADARs.
In many modern RADAR systems the need for a mechanically moving antenna has been obviated by phased arrays. A phased array consists of a large number of small, computer-controlled antennas termed elements. These elements, of which there are usually thousands, are crowded together to form a flat surface. In transmit mode, the elements are all instructed to emit a RADAR pulse at approximately the same time; the thousands of outbound waves produced by the elements merge into a single powerful wave as they spread outward. By timing, or phasing, the elements in the array so that, for example, elements along the left-hand edge of the array fire first while those farther to the right fire progressively later, the composite wave formed by the merging of the elements' lesser outputs can be steered in any desired direction within a wide cone (in this example, to the right). Beam steering can be accomplished by such a system millions of times more rapidly than would be possible with mechanical methods. Phased-array systems are used for a number of applications; including the 71.5-foot (21.8-m) tall AN/FPS-115 PAVE PAWS Early Warning RADAR Array Antennas, which provide early warning of ballistic-missile attack; shipboard systems such as the AN/SPY-1D, which is about 15 feet (3 m) across and is mounted flush with the upper hull of some warships; the Hughes AN/TPQ-37 Firefinder, a trailer-mounted system designed for tracking incoming artillery and missiles and calculating their point of origin; and many other real-world systems.
RADAR is a powerful weapon of war, but has its weaknesses. For example, numerous missiles have been developed to home in on the radio pulses emitted by RADARs, making it very dangerous to turn on a RADAR in a modern battlefield situation. Further, jamming and spoofing ("electronic warfare") have evolved rapidly alongside RADAR itself. For example, an aircraft that finds itself interrogated by a RADAR pulse can emit blasts of noise or false echoes, or request that a drone or other unit emit them, in order to confuse enemy RADAR. Finally, aircraft have been built that are "low observable," that is, which scatter very little energy back toward any RADAR that illuminates them. Low-observable or "stealth" aircraft are built of radio-absorbent materials and shaped to present little or no surface area perpendicular to RADAR pulses approaching from most angles (except directly above and directly below, the two least likely places for an enemy RADAR to be at any given moment). What RADAR they do reflect is deflected at low angles rather than returned to the RADAR transmitter. The U.S. B-2 bomber and F-117A and F-22 fighters are working examples of low-observable aircraft.
█ FURTHER READING:
BOOKS:
Edde, Byron. RADAR: Principles, Technology, Applications. Englewood Cliffs, NJ: PTR Hall, 1993.
Skolnik, Merrill I. Introduction to RADAR Systems. New York: McGraw Hills, 2001.
SEE ALSO
Stealth Technology
RADAR, Synthetic Aperture
Radar
RADAR.
BIBLIOGRAPHYRadar is a device that detects reflected radio signals to provide an electronic means of measuring distance and location of an object. The term radar originated in the United States during World War II as an acronym for radio detection and ranging.
The phenomenon of radio reflection was first observed in 1887–1888 by the German physicist Heinrich Hertz. This aspect of Hertz's experiments was largely forgotten by the end of the nineteenth century, when attention shifted to very long-wave radio waves, which Guglielmo Marconi had demonstrated could be used for radio.
Radio reflection was rediscovered in the early 1920s by radio hams using shortwave radio. In November 1923 American amateurs established two-way radio contact with a French ham operator in Nice. Although the cause of this phenomenon was not understood until 1925, these long ranges were made possible by the shortwave radio signals being reflected by the ionosphere, a previously little understood layer of the upper atmosphere. This discovery sparked immediate interest in short and ultrashort—also known as ultrahigh frequency (UHF)—wavelengths. In the late 1920s and early 1930s, a variety of new high-power shortwave receiver and transmitter radio valves, antenna systems, and related devices were introduced. Particularly important was the perfection by 1930 of a cathode ray tube (CRT), which could graphically display received signals.
As radio researchers turned their attention to shorter wavelengths, they began again to observe the reflective properties of these signals. The initial applications of radio reflection were in the measurement of distances. By early 1925 scientists used this phenomenon to demonstrate the existence of the ionosphere. Radio reflection was also utilized in devices used for geodetic surveying and in aircraft altimeters.
In the early 1930s scientists began observing interference to radio signals caused by passing aircraft. In 1930 radar research began at the United States Naval Research Laboratory. In 1933 Dr. Rudolph Kühnold, chief of the German navy's Signals Research division, began work on a radar system. Tests on radio location devices began in France in January 1934 and in the Soviet Union in July of that same year.
In January 1935 Robert Watson Watt proposed to the British government the development of a radio location system to detect and track approaching bombers. Watson Watt was authorized by the Air Ministry to assemble a team of radio researchers at the Bawdsey research laboratory. By the end of 1938 the Bawdsey researchers had developed the Chain Home radar system, providing early warning of a bomber attack at ranges up to sixty miles. At the same time, Air Marshal Sir Hugh Dowding, commander in chief of Fighter Command, and Sir Henry Tizard, who chaired the Committee for the Scientific Survey of Air Defence, directed a series of experiments that integrated information from the Chain radar stations into a comprehensive air defense system. During the summer of 1940, in the Battle of Britain, this air defense system made it possible for the Royal Air Force to defeat the Luftwaffe's efforts to win air superiority over Great Britain.
Prior to the war a group of Bawdsey scientists led by E. G. Bowen developed much smaller radar sets for use on aircraft and ships. Early airborne sets were hampered by the comparatively long wavelengths available. In February 1940, at the University of Birmingham, John Randall and Harry Boot produced the cavity magnetron, a revolutionary radio valve that could generate sufficient quantities of microwave radiation. By the end of 1941 the first microwave radar, the Royal Navy's Type 271, was introduced into service. The Type 271 soon proved instrumental in allowing escorts to detect German U-boats during Battle of the Atlantic.
In the summer and autumn of 1941, Tizard headed the British Technical Mission to North America. The mission resulted in a full Anglo-American exchange of information on radar, including the magnetron. This led to the establishment of the Radiation Laboratory at the Massachusetts Institute of Technology. Effectively mobilized, Allied scientists developed an extensive array of radar and radar countermeasures that proved decisive in campaigns as diverse as the Battle of the Atlantic and the strategic bomber offensives against both Germany and Japan.
Germany also developed a highly effective radar-based air defense system, which inflicted massive casualties on Allied bombers. The system only failed in early 1944 when American long-range escort fighters annihilated the German fighter force. None of the Axis powers emulated the Allies' use of microwave radar.
After the war radar was used to dramatically improve marine and air navigation and control systems. Direct postwar spin-offs of radar included medical imaging, microcircuitry, radar mapping, and radio astronomy.
See alsoAviation; Britain, Battle of; Warfare.
BIBLIOGRAPHY
Brown, Louis. A Radar History of World War II. Bristol, U.K., and Philadelphia, 1999.
Buderi, Robert. The Invention That Changed the World. New York, 1996.
Pritchard, David. The Radar War: Germany's Pioneering Achievement, 1904–45. Wellingborough, U.K., 1989.
Swords, Seán S. A Technical History of the Beginnings of Radar. London, 1994.
Zimmerman, David. The Great Naval Battle of Ottawa. Toronto, 1989.
———. Top Secret Exchange: The Tizard Mission and the Scientific War. Montreal and Stroud, U.K., 1996.
———. Britain's Shield: Radar and the Defeat of the Luftwaffe. Stroud, U.K., 2001.
David Zimmerman
Radar
Radar
Radar (a contraction of ra dio d etection a nd r anging) is an electronic system that measures the position, speed, or other characteristics of a far-off object by means of radio waves bounced off the surface of that object. It can pierce any atmospheric disturbance, such as a storm, all the way to the horizon. Within its range, radar can reveal clouds, a landmass, or objects such as ships, airplanes, or spacecraft. Radar can measure distance to a target object; for instance, aircraft use it to determine altitude. Radar is also used to monitor atmospheric systems, to track storms, and to help predict the weather. Military applications include weapons ranging (determining the distance from a weapon to a target) and direction in the control of guided missiles.
Basic radar operation
Light waves, radio waves, microwaves, and radar waves are all examples of electromagnetic waves. Unlike water waves, electromagnetic waves do not require a medium such as water or air to travel through. They can travel through a complete vacuum. Similar to light waves, radar waves bounce off some objects and travel through others.
The simplest mode of radar operation is range-finding, or determining how far away an object is. The radar unit sends radar waves out toward the target (radar systems can send out thousands of pulses per second). The waves hit the target and are reflected back. The returning wave is received by the radar unit, and the travel time is registered. According to basic principles of physics, distance is equal to the rate of travel (speed) multiplied by the time of travel. All electromagnetic waves travel at the same speed in a vacuum—the speed of light, which is 186,282 miles (299,727 kilometers) per second. This speed is reduced by a small amount when the waves are traveling through air, but this can be calculated.
Bats and dolphins are able to emit high-frequency sounds and orient (position) themselves by means of reflected sound waves. This ability is known as echolocation.
Radar and World War II
When the 1930s saw the possibility of a German air invasion, the English government accelerated its research into radar. A chain of radar stations was constructed that would throw an invisible net of radio waves over England. These waves could detect the approach and precise location of any aircraft.
When German dictator Adolf Hitler ordered a massive air strike against England in 1940, the radar shield worked. Although the German air force greatly outnumbered the British Royal Air Force, it was soundly beaten because the radar's eye could easily locate German planes, even in darkness and poor weather.
The military use of radar continued throughout World War II (1939–45). Compact transmitters were developed that could be mounted
on the underside of a plane to scan the ground far below for targets. Bombs and shells equipped with radar tracking systems were designed to "look" for their targets, exploding at just the right moment.
Other uses of radar
Radar devices began to trickle into everyday use soon after the end of the war. In 1947, a young engineer named John Barker attempted to use radar to regulate traffic lights. He noticed that a passing automobile would reflect a radio pulse, and that the speed of the vehicle could then be determined by examining the returning signal. Much to the dismay of speeders, Barker had devised the first radar speed-gun, now used by police worldwide.
Marine navigators, surveyors, meteorologists, and astronomers have also found uses for radar technology. A continuous-wave version called Doppler radar is often used to track storms and hurricanes. Probes launched into space have used radar to map the surfaces of other planets.
radar
radar
ra·dar / ˈrāˌdär/ • n. a system for detecting the presence, direction, distance, and speed of aircraft, ships, and other objects, by sending out pulses of high-frequency electromagnetic waves that are reflected off the object back to the source. ∎ an apparatus used for this.ORIGIN: 1940s: from ra(dio) d(etection) a(nd) r(anging).