Radar.
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Radar.
I
INTRODUCTION
Radar (Radio Detection And Ranging), remote detection system used to locate and identify objects. Radar signals bounce off objects in their path, and the radar system
detects the echoes of signals that return. Radar can determine a number of properties of a distant object, such as its distance, speed, direction of motion, and shape.
Radar can detect objects out of the range of sight and works in all weather conditions, making it a vital and versatile tool for many industries.
Radar has many uses, including aiding navigation in the sea and air, helping detect military forces, improving traffic safety, and providing scientific data. One of radar's
primary uses is air traffic control, both civilian and military. Large networks of ground-based radar systems help air traffic controllers keep track of aircraft and prevent
midair collisions. Commercial and military ships also use radar as a navigation aid to prevent collisions between ships and to alert ships of obstacles, especially in bad
weather conditions when visibility is poor. Military forces around the world use radar to detect aircraft and missiles, troop movement, and ships at sea, as well as to
target various types of weapons. Radar is a valuable tool for the police in catching speeding motorists. In the world of science, meteorologists use radar to observe and
forecast the weather (see Meteorology). Other scientists use radar for remote sensing applications, including mapping the surface of the earth from orbit, studying
asteroids, and investigating the surfaces of other planets and their moons (see Radar Astronomy).
II
HOW RADAR WORKS
Radar relies on sending and receiving electromagnetic radiation, usually in the form of radio waves (see Radio) or microwaves. Electromagnetic radiation is energy that
moves in waves at or near the speed of light. The characteristics of electromagnetic waves depend on their wavelength. Gamma rays and X rays have very short
wavelengths. Visible light is a tiny slice of the electromagnetic spectrum with wavelengths longer than X rays, but shorter than microwaves. Radar systems use longwavelength electromagnetic radiation in the microwave and radio ranges. Because of their long wavelengths, radio waves and microwaves tend to reflect better than
shorter wavelength radiation, which tends to scatter or be absorbed before it gets to the target. Radio waves at the long-wavelength end of the spectrum will even
reflect off of the atmosphere's ionosphere, a layer of electrically-charged particles in the earth's atmosphere.
A radar system starts by sending out electromagnetic radiation, called the signal. The signal bounces off objects in its path. When the radiation bounces back, part of
the signal returns to the radar system; this echo is called the return. The radar system detects the return and, depending on the sophistication of the system, simply
reports the detection or analyzes the signal for more information. Even though radio waves and microwaves reflect better than electromagnetic waves of other lengths,
only a tiny portion--about a billionth of a billionth--of the radar signal gets reflected back. Therefore, a radar system must be able to transmit high amounts of energy
in the signal and to detect tiny amounts of energy in the return.
A radar system is composed of four basic components: a transmitter, an antenna, a receiver, and a display. The transmitter produces the electrical signals in the correct
form for the type of radar system. The antenna sends these signals out as electromagnetic radiation. The antenna also collects incoming return signals and passes them
to the receiver, which analyzes the return and passes it to a display. The display enables human operators see the data.
All radar systems perform the same basic tasks, but the way systems carry out their tasks has some effect on the system's parts. A type of radar called pulse radar
sends out bursts of radar at regular intervals. Pulse radar requires a method of timing the bursts from its transmitter, so this part is more complicated than the
transmitter in other radar systems. Another type of radar called continuous-wave radar sends out a continuous signal. Continuous-wave radar gets much of its
information about the target from subtle changes in the return, or the echo of the signal. The receiver in continuous-wave radar is therefore more complicated than in
other systems.
A
Transmitter System
The system surrounding the transmitter is made up of three main elements: the oscillator, the modulator, and the transmitter itself. The transmitter supplies energy to
the antenna in the form of a high-energy electrical signal. The antenna then sends out electromagnetic radar waves as the signal passes through it.
A1
The Oscillator
The production of a radar signal begins with an oscillator, a device that produces a pure electrical signal at the desired frequency. Most radar systems use frequencies
that fall in the radio range (from a few million cycles per second--or Hertz--to several hundred million Hertz) or the microwave range (from several hundred million
Hertz to a several tens of billions Hertz). The oscillator must produce a precise and pure frequency to provide the radar system with an accurate reference when it
calculates the Doppler shift of the signal (for further discussion of the Doppler shift, see the Receiver section of this article below).
A2
The Modulator
The next stage of a radar system is the modulator, which rapidly varies, or modulates, the signal from the oscillator. In a simple pulse radar system the modulator
merely turns the signal on and off. The modulator should vary the signal, but not distort it. This requires careful design and engineering.
A3
The Transmitter
The radar system's transmitter increases the power of the oscillator signal. The transmitter amplifies the power from the level of about 1 watt to as much as 1
megawatt, or 1 million watts. Radar signals have such high power levels because so little of the original signal comes back in the return.
A4
The Antenna
After the transmitter amplifies the radar signal to the required level, it sends the signal to the antenna, usually a dish-shaped piece of metal. Electromagnetic waves at
the proper wavelength propagate out from the antenna as the electrical signal passes through it. Most radar antennas direct the radiation by reflecting it from a
parabolic, or concave shaped, metal dish. The output from the transmitter feeds into the focus of the dish. The focus is the point at which radio waves reflected from
the dish travel out from the surface of the dish in a single direction. Most antennas are steerable, meaning that they can move to point in different directions. This
enables a radar system to scan an area of space rather than always pointing in the same direction.
B
Reception Elements
A radar receiver detects and often analyzes the faint echoes produced when radar waves bounce off of distant objects and return to the radar system. The antenna
gathers the weak returning radar signals and converts them into an electric current. Because a radar antenna may both transmit and receive signals, the duplexer
determines whether the antenna is connected to the receiver or the transmitter. The receiver determines whether the signal should be reported and often does further
analysis before sending the results to the display. The display conveys the results to the human operator through a visual display or an audible signal.
B1
The Antenna
The receiver uses an antenna to gather the reflected radar signal. Often the receiver uses the same antenna as the transmitter. This is possible even in some
continuous-wave radar because the modulator in the transmitter system formats the outgoing signals in such a way that the receiver (described in following
paragraphs) can recognize the difference between outgoing and incoming signals.
B2
The Duplexer
The duplexer enables a radar system to transmit powerful signals and still receive very weak radar echoes. The duplexer acts as a gate between the antenna and the
receiver and transmitter. It keeps the intense signals from the transmitter from passing to the receiver and overloading it, and also ensures that weak signals coming in
from the antenna go to the receiver. A pulse radar duplexer connects the transmitter to the antenna only when a pulse is being emitted. Between pulses, the duplexer
disconnects the transmitter and connects the receiver to the antenna. If the receiver were connected to the antenna while the pulse was being transmitted, the high
power level of the pulse would damage the receiver's sensitive circuits. In continuous-wave radar the receivers and transmitters operate at the same time. These
systems have no duplexer. In this case, the receiver separates the signals by frequency alone. Because the receiver must listen for weak signals at the same time that
the transmitter is operating, high power continuous-wave radar systems use separate transmitting and receiving antennas.
B3
The Receiver
Most modern radar systems use digital equipment because this equipment can perform many complicated functions. In order to use digital equipment, radar systems
need analog-to-digital converters to change the received signal from an analog form to a digital form.
The incoming analog signal can have any value, from 0 to tens of millions, including fractional values such as ? . Digital information must have discrete values, in certain
regular steps, such as 0, 1, or 2, but nothing in between. A digital system might require the fraction ? to be rounded off to the decimal number 0.6666667, or 0.667,
or 0.7, or even 1. After the analog information has been translated into discrete intervals, digital numbers are usually expressed in binary form, or as series of 1s and
0s that represent numbers. The analog-to-digital converter measures the incoming analog signal many times each second and expresses each signal as a binary
number.
Once the signal is in digital form, the receiver can perform many complex functions on it. One of the most important functions for the receiver is Doppler filtering.
Signals that bounce off of moving objects come back with a slightly different wavelength because of an effect called the Doppler effect. The wavelength changes as
waves leave a moving object because the movement of the object causes each wave to leave from a slightly different position than the waves before it. If an object is
moving away from the observer, each successive wave will leave from slightly farther away, so the waves will be farther apart and the signal will have a longer
wavelength. If an object is moving toward the observer, each successive wave will leave from a position slightly closer than the one before it, so the waves will be closer
to each other and the signal will have a shorter wavelength. Doppler shifts occur in all kinds of waves, including radar waves, sound waves, and light waves. Doppler
filtering is the receiver's way of differentiating between multiple targets. Usually, targets move at different speeds, so each target will have a different Doppler shift.
Following Doppler filtering, the receiver performs other functions to maximize the strength of the return signal and to eliminate noise and other interfering signals.
B4
The Display
Displaying the results is the final step in converting the received radar signals into useful information. Early radar systems used a simple amplitude scope--a display of
received signal amplitude, or strength, as a function of distance from the antenna. In such a system, a spike in the signal strength appears at the place on the screen
that corresponds to the target's distance. A more useful and more modern display is the plan position indicator (PPI). The PPI displays the direction of the target in
relation to the radar system (relative to north)as an angle measured from the top of the display, while the distance to the target is represented as a distance from the
center of the display. Some radar systems that use PPI display the actual amplitude of the signal, while others process the signal before displaying it and display
possible targets as symbols. Some simple radar systems designed to look for the presence of an object and not the object's speed or distance notify the user with an
audible signal, such as a beep.
C
Radar Frequencies
Early radar systems were capable only of detecting targets and making a crude measurement of the distance to the target. As radar technology evolved, radar systems
could measure more and more properties. Modern technology allows radar systems to use higher frequencies, permitting better measurement of the target's direction
and location. Advanced radar can detect individual features of the target and show a detailed picture of the target instead of a single blurred object.
Most radar systems operate in frequencies ranging from the Very High Frequency (VHF) band, at about 150 MHz (150 million Hz), to the Extra High Frequency band,
which may go as high as 95 GHz (95 billion Hz). Specific ranges of frequencies work well for certain applications and not as well for others, so most radar systems are
specialized to do one type of tracking or detection. The frequency of the radar system is related to the resolution of the system. Resolution determines how close two
objects may be and still be distinguished by the radar, and how accurately the system can determine the target's position. Higher frequencies provide better resolution
than lower frequencies because the beam formed by the antenna is sharper. Tracking radar, which precisely locates objects and tracks their movement, needs higher
resolution and so uses higher frequencies. On the other hand, if a radar system is used to search large areas for targets, a narrow beam of high-frequency radar will be
less efficient. Because the high-power transmitters and large antennas that radar systems require are easier to build for lower frequencies, lower frequency radar
systems are more popular for radar that does not need particularly good resolution.
D
Clutter
Clutter is what radar users call radar signals that do not come from actual targets. Rain, snow, and the surface of the earth reflect energy, including radar waves. Such
echoes can produce signals that the radar system may mistake for actual targets. Clutter makes it difficult to locate targets, especially when the system is searching for
objects that are small and distant. Fortunately, most sources of clutter move slowly if at all, so their radar echoes produce little or no Doppler shift. Radar engineers
have developed several systems to take advantage of the difference in Doppler shifts between clutter and moving targets. Some radar systems use a moving target
indicator (MTI), which subtracts out every other radar return from the total signal. Because the signals from stationary objects will remain the same over time, the MTI
subtracts them from the total signal, and only signals from moving targets get past the receiver. Other radar systems actually measure the frequencies of all returning
signals. Frequencies with very low Doppler shifts are assumed to come from clutter. Those with substantial shifts are assumed to come from moving targets.
Clutter is actually a relative term, since the clutter for some systems could be the target for other systems. For example, a radar system that tracks airplanes considers
precipitation to be clutter, but precipitation is the target of weather radar. The plane-tracking radar would ignore the returns with large sizes and low Doppler shifts that
represent weather features, while the weather radar would ignore the small-sized, highly-Doppler-shifted returns that represent airplanes.
III
TYPES OF RADAR
All radar systems send out electromagnetic radiation in radio or microwave frequencies and use echoes of that radiation to detect objects, but different systems use
different methods of emitting and receiving radiation. Pulse radar sends out short bursts of radiation. Continuous wave radar sends out a constant signal. Synthetic
aperture radar and phased-array radar have special ways of positioning and pointing the antennas that improve resolution and accuracy. Secondary radar detects radar
signals that targets send out, instead of detecting echoes of radiation.
A
Simple Pulse Radar
Simple pulse radar is the simplest type of radar. In this system, the transmitter sends out short pulses of radio frequency energy. Between pulses, the radar receiver
detects echoes of radiation that objects reflect. Most pulse radar antennas rotate to scan a wide area. Simple pulse radar requires precise timing circuits in the duplexer
to prevent the transmitter from transmitting while the receiver is acquiring a signal from the antenna, and to keep the receiver from trying to read a signal from the
antenna while the transmitter is operating. Pulse radar is good at locating an object, but it is not very accurate at measuring an object's speed.
B
Continuous Wave Radar
Continuous-wave (CW) radar systems transmit a constant radar signal. The transmission is continuous, so, except in systems with very low power, the receiver cannot
use the same antenna as the transmitter because the radar emissions would interfere with the echoes that the receiver detects. CW systems can distinguish between
stationary clutter and moving targets by analyzing the Doppler shift of the signals, without having to use the precise timing circuits that separates the signal from the
return in pulse radar. Continuous wave radar systems are excellent at measuring the speed and direction of an object, but they are not as accurate as pulse radar at
measuring an object's position. Some systems combine pulse and CW radar to achieve both good range and velocity resolution. Such systems are called Pulse-Doppler
radar systems.
C
Synthetic Aperture Radar
Synthetic aperture radar (SAR) tracks targets on the ground from the air. The name comes from the fact that the system uses the movement of the airplane or satellite
carrying it to make the antenna seem much larger than it actually is. The ability of radar to distinguish between two closely spaced objects depends on the width of the
beam that the antenna sends out. The narrower the beam is, the better its resolution. Getting a narrow beam requires a big antenna. A SAR system is limited to a
relatively small antenna with a wide beam because it must fit on an aircraft or satellite. SAR systems are called synthetic aperture, however, because the antenna
appears to be bigger than it really is. This is because the moving aircraft or satellite allows the SAR system to repeatedly take measurements from different positions.
The receiver processes these signals to make it seem as though they came from a large stationary antenna instead of a small moving one. Synthetic aperture radar
resolution can be high enough to pick out individual objects as small as automobiles.
Typically, an aircraft or satellite equipped with SAR flies past the target object. In inverse synthetic aperture radar, the target moves past the radar antenna. Inverse
SAR can give results as good as normal SAR.
D
Phased-Array Radar
Most radar systems use a single large antenna that stays in one place, but can rotate on a base to change the direction of the radar beam. A phased-array radar
antenna actually comprises many small separate antennas, each of which can be rotated. The system combines the signals gathered from all the small antennas. The
receiver can change the way it combines the signals from the antennas to change the direction of the beam. A huge phased-array radar antenna can change its beam
direction electronically many times faster than any mechanical radar system can.
E
Secondary Radar
A radar system that sends out radar signals and reads the echoes that bounce back is a primary radar system. Secondary radar systems read coded radar signals that
the target emits in response to signals received, instead of signals that the target reflects. Air traffic control depends heavily on the use of secondary radar. Aircraft
carry small radar transmitters called beacons or transponders. Receivers at the air traffic control tower search for signals from the transponders. The transponder
signals not only tell controllers the location of the aircraft, but can also carry encoded information about the target. For example, the signal may contain a code that
indicates whether the aircraft is an ally, or it may contain encoded information from the aircraft's altimeter (altitude indicator).
IV
RADAR APPLICATIONS
Many industries depend on radar to carry out their work. Civilian aircraft and maritime industries use radar to avoid collisions and to keep track of aircraft and ship
positions. Military craft also use radar for collision avoidance, as well as for tracking military targets. Radar is important to meteorologists, who use it to track weather
patterns. Radar also has many other scientific applications.
A
Air-Traffic Control
Radar is a vital tool in avoiding midair aircraft collisions. The international air traffic control system uses both primary and secondary radar. A network of long-range
radar systems called Air Route Surveillance Radar (ARSR) tracks aircraft as they fly between airports. Airports use medium-range radar systems called Airport
Surveillance Radar to track aircraft more accurately while they are near the airport.
B
Maritime Navigation
Radar also helps ships navigate through dangerous waters and avoid collisions. Unlike air-traffic radar, with its centralized networks that monitor many craft, maritime
radar depends almost entirely on radar systems installed on individual vessels. These radar systems search the surface of the water for landmasses; navigation aids,
such as lighthouses and channel markers; and other vessels. For a ship's navigator, echoes from landmasses and other stationary objects are just as important as
those from moving objects. Consequently, marine radar systems do not include clutter removal circuits. Instead, ship-based radar depends on high-resolution distance
and direction measurements to differentiate between land, ships, and unwanted signals. Marine radar systems have become available at such low cost that many
pleasure craft are equipped with them, especially in regions where fog is common.
C
Military Defense and Attack
Historically, the military has played the leading role in the use and development of radar. The detection and interception of opposing military aircraft in air defense has
been the predominant military use of radar. The military also uses airborne radar to scan large battlefields for the presence of enemy forces and equipment and to pick
out precise targets for bombs and missiles.
C1
Air Defense
A typical surface-based air defense system relies upon several radar systems. First, a lower frequency radar with a high-powered transmitter and a large antenna
searches the airspace for all aircraft, both friend and foe. A secondary radar system reads the transponder signals sent by each aircraft to distinguish between allies and
enemies. After enemy aircraft are detected, operators track them more precisely by using high-frequency waves from special fire control radar systems. The air defense
system may attempt to shoot down threatening aircraft with gunfire or missiles, and radar sometimes guides both gunfire and missiles (see Guided Missiles).
Longer-range air defense systems use missiles with internal guidance. These systems track a target using data from a radar system on the missile. Such missile-borne
radar systems are called seekers. The seeker uses radar signals from the missile or radar signals from a transmitter on the ground to determine the position of the
target relative to the missile, then passes the information to the missile's guidance system.
The military uses surface-to-air systems for defense against ballistic missiles as well as aircraft (see Defense Systems). During the Cold War both the United States and
the Union of Soviet Socialist Republics (USSR) did a great deal of research into defense against intercontinental ballistic missiles (ICBMs) and submarine-launched
ballistic missiles (SLBMs). The United States and the USSR signed the Anti-Ballistic Missile (ABM) treaty in 1972. This treaty limited each of the superpowers to a single,
limited capability system. The U.S. system consisted of a low-frequency (UHF) phased-array radar around the perimeter of the country, another phased-array radar to
track incoming missiles more accurately, and several very high speed missiles to intercept the incoming ballistic missiles. The second radar guided the interceptor
missiles.
Airborne air defense systems incorporate the same functions as ground-based air defense, but special aircraft carry the large area search radar systems. This is
necessary because it is difficult for high-performance fighter aircraft to carry both large radar systems and weapons.
Modern warfare uses air-to-ground radar to detect targets on the ground and to monitor the movement of troops. Advanced Doppler techniques and synthetic aperture
radar have greatly increased the accuracy and usefulness of air-to-ground radar since their introduction in the 1960s and 1970s. Military forces around the world use
air-to-ground radar for weapon aiming and for battlefield surveillance. The United States used the Joint Surveillance and Tracking Radar System (JSTARS) in the Persian
Gulf War (1991), demonstrating modern radar's ability to provide information about enemy troop concentrations and movements during the day or night, regardless of
weather conditions.
C2
Countermeasures
The military uses several techniques to attempt to avoid detection by enemy radar. One common technique is jamming--that is, sending deceptive signals to the
enemy's radar system. During World War II (1939-1945), flyers under attack jammed enemy radar by dropping large clouds of chaff--small pieces of aluminum foil or
some other material that reflects radar well. "False" returns from the chaff hid the aircraft's exact location from the enemy's air defense radar. Modern jamming uses
sophisticated electronic systems that analyze enemy radar, then send out false radar echoes that mask the actual target echoes or deceive the radar about a target's
location.
Stealth technology is a collection of methods that reduce the radar echoes from aircraft and other radar targets (see Stealth Aircraft). Special paint can absorb radar
signals and sharp angles in the aircraft design can reflect radar signals in deceiving directions. Improvements in jamming and stealth technology force the continual
development of high-power transmitters, antennas good at detecting weak signals, and very sensitive receivers, as well as techniques for improved clutter rejection.
D
Traffic safety
Since the 1950s, police have used radar to detect motorists who are exceeding the speed limit. Most older police radar "guns" use Doppler technology to determine the
target vehicle's speed. Such systems were simple, but they sometimes produced false results. The radar beam of such systems was relatively wide, which meant that
stray radar signals could be detected by motorists with radar detectors. Newer police radar systems, developed in the 1980s and 1990s, use laser light to form a
narrow, highly selective radar beam. The narrow beam helps insure that the radar returns signals from a single, selected car and reduces the chance of false results.
Instead of relying on the Doppler effect to measure speed, these systems use pulse radar to measure the distance to the car many times, then calculate the speed by
dividing the change in distance by the change in time. Laser radar is also more reliable than normal radar for the detection of speeding motorists because its narrow
beam is more difficult to detect by motorists with radar detectors.
E
Meteorology
Meteorologists use radar to learn about the weather. Networks of radar systems installed across many countries throughout the world detect and display areas of rain,
snow, and other precipitation. Weather radar systems use Doppler radar to determine the speed of the wind within the storm. The radar signals bounce off of water
droplets or ice crystals in the atmosphere. Gaseous water vapor does not reflect radar waves as well as the liquid droplets of water or solid ice crystals, so radar returns
from rain or snow are stronger than that from clouds. Dust in the atmosphere also reflects radar, but the returns are only significant when the concentration of dust is
much higher than usual. The Terminal Doppler Weather Radar can detect small, localized, but hazardous wind conditions, especially if precipitation or a large amount of
dust accompanies the storm. Many airports use this advanced radar to make landing safer.
F
Scientific Applications
Scientists use radar in several space-related applications. The Spacetrack system is a cooperative effort of the United States, Canada, and the United Kingdom. It uses
data from several large surveillance and tracking radar systems (including the Ballistic Missile Early Warning System) to detect and track all objects in orbit around the
earth. This helps scientists and engineers keep an eye on space junk--abandoned satellites, discarded pieces of rockets, and other unused fragments of spacecraft that
could pose a threat to operating spacecraft. Other special-purpose radar systems track specific satellites that emit a beacon signal. One of the most important of these
systems is the Global Positioning System (GPS), operated by the U.S. Department of Defense. GPS provides highly accurate navigational data for the U.S. military and
for anyone who owns a GPS receiver.
During space flights, radar gives precise measurements of the distances between the spacecraft and other objects. In the U.S. Surveyor missions to the moon in the
1960s, radar measured the altitude of the probe above the moon's surface to help the probe control its descent. In the Apollo missions, which landed astronauts on the
moon during the 1960s and 1970s, radar measured the altitude of the Lunar Module, the part of the Apollo spacecraft that carried two astronauts from orbit around the
moon down to the moon's surface, above the surface of the moon. Apollo also used radar to measure the distance between the Lunar Module and the Command and
Service Module, the part of the spacecraft that remained in orbit around the moon.
Astronomers have used ground-based radar to observe the moon, some of the larger asteroids in our solar system, and a few of the planets and their moons. Radar
observations provide information about the orbit and surface features of the object.
The U.S. Magellan space probe mapped the surface of the planet Venus with radar from 1990 to 1994. Magellan's radar was able to penetrate the dense cloud layer of
the Venusian atmosphere and provide images of much better quality than radar measurements from Earth.
Many nations have used satellite-based radar to map portions of the earth's surface. Radar can show conditions on the surface of the earth and can help determine the
location of various resources such as oil, water for irrigation, and mineral deposits. In 1995 the Canadian Space Agency launched a satellite called RADARsat to provide
radar imagery to commercial, government, and scientific users.
V
HISTORY
Although British physicist James Clerk Maxwell predicted the existence of radio waves in the 1860s, it wasn't until the 1890s that British-born American inventor Elihu
Thomson and German physicist Heinrich Hertz independently confirmed their existence. Scientists soon realized that radio waves could bounce off of objects, and by
1904 Christian Hülsmeyer, a German inventor, had used radio waves in a collision avoidance device for ships. Hülsmeyer's system was only effective for a range of
about 1.5 km (about 1 mi). The first long-range radar systems were not developed until the 1920s. In 1922 Italian radio pioneer Guglielmo Marconi demonstrated a lowfrequency (60 MHz) radar system. In 1924 English physicist Edward Appleton and his graduate student from New Zealand, Miles Barnett, proved the existence of the
ionosphere, an electrically charged upper layer of the atmosphere, by reflecting radio waves off of it. Scientists at the U.S. Naval Research Laboratory in Washington,
D.C., became the first to use radar to detect aircraft in 1930.
A
Radar in World War II
None of the early demonstrations of radar generated much enthusiasm. The commercial and military value of radar did not become readily apparent until the mid1930s. Before World War II, the United States, France, and the United Kingdom were all carrying out radar research. Beginning in 1935, the British built a network of
ground-based aircraft detection radar, called Chain Home, under the direction of Sir Robert Watson-Watt. Chain Home was fully operational from 1938 until the end of
World War II in 1945 and was extremely instrumental in Britain's defense against German bombers.
The British recognized the value of radar with frequencies much higher than the radio waves used for most systems. A breakthrough in radar technology came in 1939
when two British scientists, physicist Henry Boot and biophysicist John Randall, developed the resonant-cavity magnetron. This device generates high-frequency radio
pulses with a large amount of power, and it made the development of microwave radar possible. Also in 1939, the Massachusetts Institute of Technology (MIT) Radiation
Laboratory was formed in Cambridge, Massachusetts, bringing together U.S. and British radar research. In March 1942 scientists demonstrated the detection of ships
from the air. This technology became the basis of antiship and antisubmarine radar for the U.S. Navy.
The U.S. Army operated air surveillance radar at the start of World War II. The army also used early forms of radar to direct antiaircraft guns. Initially the radar
systems were used to aim searchlights so the soldier aiming the gun could see where to fire, but the systems evolved into fire-control radar that aimed the guns
automatically.
B
Radar during the Cold War
With the end of World War II, interest in radar development declined. Some experiments continued, however; for instance, in 1946 the U.S. Army Signal Corps bounced
radar signals off of the moon, ushering in the field of radar astronomy. The growing hostility between the United States and the Union of Soviet Socialists Republics--the
so-called Cold War--renewed military interest in radar improvements. After the Soviets detonated their first atomic bomb in 1949, interest in radar development,
especially for air defense, surged. Major programs included the installation of the Distant Early Warning (DEW) network of long-range radar across the northern reaches
of North America to warn against bomber attacks. As the potential threat of attack by ICBMs increased, the United Kingdom, Greenland, and Alaska installed the Ballistic
Missile Early Warning System (BMEWS).
C
Modern Radar
Radar found many applications in civilian and military life and became more sophisticated and specialized for each application. The use of radar in air traffic control grew
quickly during the Cold War, especially with the jump in air traffic that occurred in the 1960s. Today almost all commercial and private aircraft have transponders.
Transponders send out radar signals encoded with information about an aircraft and its flight that other aircraft and air traffic controllers can use. American traffic
engineer John Barker discovered in 1947 that moving automobiles would reflect radar waves, which could be analyzed to determine the car's speed. Police began using
traffic radar in the 1950s, and the accuracy of traffic radar has increased markedly since the 1980s.
Doppler radar came into use in the 1960s and was first dedicated to weather forecasting in the 1970s. In the 1990s the United States had a nationwide network of
more than 130 Doppler radar stations to help meteorologists track weather patterns.
Earth-observing satellites such as those in the SEASAT program began to use radar to measure the topography of the earth in the late 1970s. The Magellan spacecraft
mapped most of the surface of the planet Venus in the 1990s. The Cassini spacecraft, scheduled to reach Saturn in 2004, carries radar instruments for studying the
surface of Saturn's moon Titan.
As radar continues to improve, so does the technology for evading radar. Stealth aircraft feature radar-absorbing coatings and deceptive shapes to reduce the
possibility of radar detection. The Lockheed F-117A, first flown in 1981, and the Northrop , first flown in 1989, are two of the latest additions to the U.S. stealth aircraft
fleet. In the area of civilian radar avoidance, companies are introducing increasingly sophisticated radar detectors, designed to warn motorists of police using traffic
radar.
Contributed By:
Robert E. Millett
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
Radar.
I
INTRODUCTION
Radar (Radio Detection And Ranging), remote detection system used to locate and identify objects. Radar signals bounce off objects in their path, and the radar system
detects the echoes of signals that return. Radar can determine a number of properties of a distant object, such as its distance, speed, direction of motion, and shape.
Radar can detect objects out of the range of sight and works in all weather conditions, making it a vital and versatile tool for many industries.
Radar has many uses, including aiding navigation in the sea and air, helping detect military forces, improving traffic safety, and providing scientific data. One of radar's
primary uses is air traffic control, both civilian and military. Large networks of ground-based radar systems help air traffic controllers keep track of aircraft and prevent
midair collisions. Commercial and military ships also use radar as a navigation aid to prevent collisions between ships and to alert ships of obstacles, especially in bad
weather conditions when visibility is poor. Military forces around the world use radar to detect aircraft and missiles, troop movement, and ships at sea, as well as to
target various types of weapons. Radar is a valuable tool for the police in catching speeding motorists. In the world of science, meteorologists use radar to observe and
forecast the weather (see Meteorology). Other scientists use radar for remote sensing applications, including mapping the surface of the earth from orbit, studying
asteroids, and investigating the surfaces of other planets and their moons (see Radar Astronomy).
II
HOW RADAR WORKS
Radar relies on sending and receiving electromagnetic radiation, usually in the form of radio waves (see Radio) or microwaves. Electromagnetic radiation is energy that
moves in waves at or near the speed of light. The characteristics of electromagnetic waves depend on their wavelength. Gamma rays and X rays have very short
wavelengths. Visible light is a tiny slice of the electromagnetic spectrum with wavelengths longer than X rays, but shorter than microwaves. Radar systems use longwavelength electromagnetic radiation in the microwave and radio ranges. Because of their long wavelengths, radio waves and microwaves tend to reflect better than
shorter wavelength radiation, which tends to scatter or be absorbed before it gets to the target. Radio waves at the long-wavelength end of the spectrum will even
reflect off of the atmosphere's ionosphere, a layer of electrically-charged particles in the earth's atmosphere.
A radar system starts by sending out electromagnetic radiation, called the signal. The signal bounces off objects in its path. When the radiation bounces back, part of
the signal returns to the radar system; this echo is called the return. The radar system detects the return and, depending on the sophistication of the system, simply
reports the detection or analyzes the signal for more information. Even though radio waves and microwaves reflect better than electromagnetic waves of other lengths,
only a tiny portion--about a billionth of a billionth--of the radar signal gets reflected back. Therefore, a radar system must be able to transmit high amounts of energy
in the signal and to detect tiny amounts of energy in the return.
A radar system is composed of four basic components: a transmitter, an antenna, a receiver, and a display. The transmitter produces the electrical signals in the correct
form for the type of radar system. The antenna sends these signals out as electromagnetic radiation. The antenna also collects incoming return signals and passes them
to the receiver, which analyzes the return and passes it to a display. The display enables human operators see the data.
All radar systems perform the same basic tasks, but the way systems carry out their tasks has some effect on the system's parts. A type of radar called pulse radar
sends out bursts of radar at regular intervals. Pulse radar requires a method of timing the bursts from its transmitter, so this part is more complicated than the
transmitter in other radar systems. Another type of radar called continuous-wave radar sends out a continuous signal. Continuous-wave radar gets much of its
information about the target from subtle changes in the return, or the echo of the signal. The receiver in continuous-wave radar is therefore more complicated than in
other systems.
A
Transmitter System
The system surrounding the transmitter is made up of three main elements: the oscillator, the modulator, and the transmitter itself. The transmitter supplies energy to
the antenna in the form of a high-energy electrical signal. The antenna then sends out electromagnetic radar waves as the signal passes through it.
A1
The Oscillator
The production of a radar signal begins with an oscillator, a device that produces a pure electrical signal at the desired frequency. Most radar systems use frequencies
that fall in the radio range (from a few million cycles per second--or Hertz--to several hundred million Hertz) or the microwave range (from several hundred million
Hertz to a several tens of billions Hertz). The oscillator must produce a precise and pure frequency to provide the radar system with an accurate reference when it
calculates the Doppler shift of the signal (for further discussion of the Doppler shift, see the Receiver section of this article below).
A2
The Modulator
The next stage of a radar system is the modulator, which rapidly varies, or modulates, the signal from the oscillator. In a simple pulse radar system the modulator
merely turns the signal on and off. The modulator should vary the signal, but not distort it. This requires careful design and engineering.
A3
The Transmitter
The radar system's transmitter increases the power of the oscillator signal. The transmitter amplifies the power from the level of about 1 watt to as much as 1
megawatt, or 1 million watts. Radar signals have such high power levels because so little of the original signal comes back in the return.
A4
The Antenna
After the transmitter amplifies the radar signal to the required level, it sends the signal to the antenna, usually a dish-shaped piece of metal. Electromagnetic waves at
the proper wavelength propagate out from the antenna as the electrical signal passes through it. Most radar antennas direct the radiation by reflecting it from a
parabolic, or concave shaped, metal dish. The output from the transmitter feeds into the focus of the dish. The focus is the point at which radio waves reflected from
the dish travel out from the surface of the dish in a single direction. Most antennas are steerable, meaning that they can move to point in different directions. This
enables a radar system to scan an area of space rather than always pointing in the same direction.
B
Reception Elements
A radar receiver detects and often analyzes the faint echoes produced when radar waves bounce off of distant objects and return to the radar system. The antenna
gathers the weak returning radar signals and converts them into an electric current. Because a radar antenna may both transmit and receive signals, the duplexer
determines whether the antenna is connected to the receiver or the transmitter. The receiver determines whether the signal should be reported and often does further
analysis before sending the results to the display. The display conveys the results to the human operator through a visual display or an audible signal.
B1
The Antenna
The receiver uses an antenna to gather the reflected radar signal. Often the receiver uses the same antenna as the transmitter. This is possible even in some
continuous-wave radar because the modulator in the transmitter system formats the outgoing signals in such a way that the receiver (described in following
paragraphs) can recognize the difference between outgoing and incoming signals.
B2
The Duplexer
The duplexer enables a radar system to transmit powerful signals and still receive very weak radar echoes. The duplexer acts as a gate between the antenna and the
receiver and transmitter. It keeps the intense signals from the transmitter from passing to the receiver and overloading it, and also ensures that weak signals coming in
from the antenna go to the receiver. A pulse radar duplexer connects the transmitter to the antenna only when a pulse is being emitted. Between pulses, the duplexer
disconnects the transmitter and connects the receiver to the antenna. If the receiver were connected to the antenna while the pulse was being transmitted, the high
power level of the pulse would damage the receiver's sensitive circuits. In continuous-wave radar the receivers and transmitters operate at the same time. These
systems have no duplexer. In this case, the receiver separates the signals by frequency alone. Because the receiver must listen for weak signals at the same time that
the transmitter is operating, high power continuous-wave radar systems use separate transmitting and receiving antennas.
B3
The Receiver
Most modern radar systems use digital equipment because this equipment can perform many complicated functions. In order to use digital equipment, radar systems
need analog-to-digital converters to change the received signal from an analog form to a digital form.
The incoming analog signal can have any value, from 0 to tens of millions, including fractional values such as ? . Digital information must have discrete values, in certain
regular steps, such as 0, 1, or 2, but nothing in between. A digital system might require the fraction ? to be rounded off to the decimal number 0.6666667, or 0.667,
or 0.7, or even 1. After the analog information has been translated into discrete intervals, digital numbers are usually expressed in binary form, or as series of 1s and
0s that represent numbers. The analog-to-digital converter measures the incoming analog signal many times each second and expresses each signal as a binary
number.
Once the signal is in digital form, the receiver can perform many complex functions on it. One of the most important functions for the receiver is Doppler filtering.
Signals that bounce off of moving objects come back with a slightly different wavelength because of an effect called the Doppler effect. The wavelength changes as
waves leave a moving object because the movement of the object causes each wave to leave from a slightly different position than the waves before it. If an object is
moving away from the observer, each successive wave will leave from slightly farther away, so the waves will be farther apart and the signal will have a longer
wavelength. If an object is moving toward the observer, each successive wave will leave from a position slightly closer than the one before it, so the waves will be closer
to each other and the signal will have a shorter wavelength. Doppler shifts occur in all kinds of waves, including radar waves, sound waves, and light waves. Doppler
filtering is the receiver's way of differentiating between multiple targets. Usually, targets move at different speeds, so each target will have a different Doppler shift.
Following Doppler filtering, the receiver performs other functions to maximize the strength of the return signal and to eliminate noise and other interfering signals.
B4
The Display
Displaying the results is the final step in converting the received radar signals into useful information. Early radar systems used a simple amplitude scope--a display of
received signal amplitude, or strength, as a function of distance from the antenna. In such a system, a spike in the signal strength appears at the place on the screen
that corresponds to the target's distance. A more useful and more modern display is the plan position indicator (PPI). The PPI displays the direction of the target in
relation to the radar system (relative to north)as an angle measured from the top of the display, while the distance to the target is represented as a distance from the
center of the display. Some radar systems that use PPI display the actual amplitude of the signal, while others process the signal before displaying it and display
possible targets as symbols. Some simple radar systems designed to look for the presence of an object and not the object's speed or distance notify the user with an
audible signal, such as a beep.
C
Radar Frequencies
Early radar systems were capable only of detecting targets and making a crude measurement of the distance to the target. As radar technology evolved, radar systems
could measure more and more properties. Modern technology allows radar systems to use higher frequencies, permitting better measurement of the target's direction
and location. Advanced radar can detect individual features of the target and show a detailed picture of the target instead of a single blurred object.
Most radar systems operate in frequencies ranging from the Very High Frequency (VHF) band, at about 150 MHz (150 million Hz), to the Extra High Frequency band,
which may go as high as 95 GHz (95 billion Hz). Specific ranges of frequencies work well for certain applications and not as well for others, so most radar systems are
specialized to do one type of tracking or detection. The frequency of the radar system is related to the resolution of the system. Resolution determines how close two
objects may be and still be distinguished by the radar, and how accurately the system can determine the target's position. Higher frequencies provide better resolution
than lower frequencies because the beam formed by the antenna is sharper. Tracking radar, which precisely locates objects and tracks their movement, needs higher
resolution and so uses higher frequencies. On the other hand, if a radar system is used to search large areas for targets, a narrow beam of high-frequency radar will be
less efficient. Because the high-power transmitters and large antennas that radar systems require are easier to build for lower frequencies, lower frequency radar
systems are more popular for radar that does not need particularly good resolution.
D
Clutter
Clutter is what radar users call radar signals that do not come from actual targets. Rain, snow, and the surface of the earth reflect energy, including radar waves. Such
echoes can produce signals that the radar system may mistake for actual targets. Clutter makes it difficult to locate targets, especially when the system is searching for
objects that are small and distant. Fortunately, most sources of clutter move slowly if at all, so their radar echoes produce little or no Doppler shift. Radar engineers
have developed several systems to take advantage of the difference in Doppler shifts between clutter and moving targets. Some radar systems use a moving target
indicator (MTI), which subtracts out every other radar return from the total signal. Because the signals from stationary objects will remain the same over time, the MTI
subtracts them from the total signal, and only signals from moving targets get past the receiver. Other radar systems actually measure the frequencies of all returning
signals. Frequencies with very low Doppler shifts are assumed to come from clutter. Those with substantial shifts are assumed to come from moving targets.
Clutter is actually a relative term, since the clutter for some systems could be the target for other systems. For example, a radar system that tracks airplanes considers
precipitation to be clutter, but precipitation is the target of weather radar. The plane-tracking radar would ignore the returns with large sizes and low Doppler shifts that
represent weather features, while the weather radar would ignore the small-sized, highly-Doppler-shifted returns that represent airplanes.
III
TYPES OF RADAR
All radar systems send out electromagnetic radiation in radio or microwave frequencies and use echoes of that radiation to detect objects, but different systems use
different methods of emitting and receiving radiation. Pulse radar sends out short bursts of radiation. Continuous wave radar sends out a constant signal. Synthetic
aperture radar and phased-array radar have special ways of positioning and pointing the antennas that improve resolution and accuracy. Secondary radar detects radar
signals that targets send out, instead of detecting echoes of radiation.
A
Simple Pulse Radar
Simple pulse radar is the simplest type of radar. In this system, the transmitter sends out short pulses of radio frequency energy. Between pulses, the radar receiver
detects echoes of radiation that objects reflect. Most pulse radar antennas rotate to scan a wide area. Simple pulse radar requires precise timing circuits in the duplexer
to prevent the transmitter from transmitting while the receiver is acquiring a signal from the antenna, and to keep the receiver from trying to read a signal from the
antenna while the transmitter is operating. Pulse radar is good at locating an object, but it is not very accurate at measuring an object's speed.
B
Continuous Wave Radar
Continuous-wave (CW) radar systems transmit a constant radar signal. The transmission is continuous, so, except in systems with very low power, the receiver cannot
use the same antenna as the transmitter because the radar emissions would interfere with the echoes that the receiver detects. CW systems can distinguish between
stationary clutter and moving targets by analyzing the Doppler shift of the signals, without having to use the precise timing circuits that separates the signal from the
return in pulse radar. Continuous wave radar systems are excellent at measuring the speed and direction of an object, but they are not as accurate as pulse radar at
measuring an object's position. Some systems combine pulse and CW radar to achieve both good range and velocity resolution. Such systems are called Pulse-Doppler
radar systems.
C
Synthetic Aperture Radar
Synthetic aperture radar (SAR) tracks targets on the ground from the air. The name comes from the fact that the system uses the movement of the airplane or satellite
carrying it to make the antenna seem much larger than it actually is. The ability of radar to distinguish between two closely spaced objects depends on the width of the
beam that the antenna sends out. The narrower the beam is, the better its resolution. Getting a narrow beam requires a big antenna. A SAR system is limited to a
relatively small antenna with a wide beam because it must fit on an aircraft or satellite. SAR systems are called synthetic aperture, however, because the antenna
appears to be bigger than it really is. This is because the moving aircraft or satellite allows the SAR system to repeatedly take measurements from different positions.
The receiver processes these signals to make it seem as though they came from a large stationary antenna instead of a small moving one. Synthetic aperture radar
resolution can be high enough to pick out individual objects as small as automobiles.
Typically, an aircraft or satellite equipped with SAR flies past the target object. In inverse synthetic aperture radar, the target moves past the radar antenna. Inverse
SAR can give results as good as normal SAR.
D
Phased-Array Radar
Most radar systems use a single large antenna that stays in one place, but can rotate on a base to change the direction of the radar beam. A phased-array radar
antenna actually comprises many small separate antennas, each of which can be rotated. The system combines the signals gathered from all the small antennas. The
receiver can change the way it combines the signals from the antennas to change the direction of the beam. A huge phased-array radar antenna can change its beam
direction electronically many times faster than any mechanical radar system can.
E
Secondary Radar
A radar system that sends out radar signals and reads the echoes that bounce back is a primary radar system. Secondary radar systems read coded radar signals that
the target emits in response to signals received, instead of signals that the target reflects. Air traffic control depends heavily on the use of secondary radar. Aircraft
carry small radar transmitters called beacons or transponders. Receivers at the air traffic control tower search for signals from the transponders. The transponder
signals not only tell controllers the location of the aircraft, but can also carry encoded information about the target. For example, the signal may contain a code that
indicates whether the aircraft is an ally, or it may contain encoded information from the aircraft's altimeter (altitude indicator).
IV
RADAR APPLICATIONS
Many industries depend on radar to carry out their work. Civilian aircraft and maritime industries use radar to avoid collisions and to keep track of aircraft and ship
positions. Military craft also use radar for collision avoidance, as well as for tracking military targets. Radar is important to meteorologists, who use it to track weather
patterns. Radar also has many other scientific applications.
A
Air-Traffic Control
Radar is a vital tool in avoiding midair aircraft collisions. The international air traffic control system uses both primary and secondary radar. A network of long-range
radar systems called Air Route Surveillance Radar (ARSR) tracks aircraft as they fly between airports. Airports use medium-range radar systems called Airport
Surveillance Radar to track aircraft more accurately while they are near the airport.
B
Maritime Navigation
Radar also helps ships navigate through dangerous waters and avoid collisions. Unlike air-traffic radar, with its centralized networks that monitor many craft, maritime
radar depends almost entirely on radar systems installed on individual vessels. These radar systems search the surface of the water for landmasses; navigation aids,
such as lighthouses and channel markers; and other vessels. For a ship's navigator, echoes from landmasses and other stationary objects are just as important as
those from moving objects. Consequently, marine radar systems do not include clutter removal circuits. Instead, ship-based radar depends on high-resolution distance
and direction measurements to differentiate between land, ships, and unwanted signals. Marine radar systems have become available at such low cost that many
pleasure craft are equipped with them, especially in regions where fog is common.
C
Military Defense and Attack
Historically, the military has played the leading role in the use and development of radar. The detection and interception of opposing military aircraft in air defense has
been the predominant military use of radar. The military also uses airborne radar to scan large battlefields for the presence of enemy forces and equipment and to pick
out precise targets for bombs and missiles.
C1
Air Defense
A typical surface-based air defense system relies upon several radar systems. First, a lower frequency radar with a high-powered transmitter and a large antenna
searches the airspace for all aircraft, both friend and foe. A secondary radar system reads the transponder signals sent by each aircraft to distinguish between allies and
enemies. After enemy aircraft are detected, operators track them more precisely by using high-frequency waves from special fire control radar systems. The air defense
system may attempt to shoot down threatening aircraft with gunfire or missiles, and radar sometimes guides both gunfire and missiles (see Guided Missiles).
Longer-range air defense systems use missiles with internal guidance. These systems track a target using data from a radar system on the missile. Such missile-borne
radar systems are called seekers. The seeker uses radar signals from the missile or radar signals from a transmitter on the ground to determine the position of the
target relative to the missile, then passes the information to the missile's guidance system.
The military uses surface-to-air systems for defense against ballistic missiles as well as aircraft (see Defense Systems). During the Cold War both the United States and
the Union of Soviet Socialist Republics (USSR) did a great deal of research into defense against intercontinental ballistic missiles (ICBMs) and submarine-launched
ballistic missiles (SLBMs). The United States and the USSR signed the Anti-Ballistic Missile (ABM) treaty in 1972. This treaty limited each of the superpowers to a single,
limited capability system. The U.S. system consisted of a low-frequency (UHF) phased-array radar around the perimeter of the country, another phased-array radar to
track incoming missiles more accurately, and several very high speed missiles to intercept the incoming ballistic missiles. The second radar guided the interceptor
missiles.
Airborne air defense systems incorporate the same functions as ground-based air defense, but special aircraft carry the large area search radar systems. This is
necessary because it is difficult for high-performance fighter aircraft to carry both large radar systems and weapons.
Modern warfare uses air-to-ground radar to detect targets on the ground and to monitor the movement of troops. Advanced Doppler techniques and synthetic aperture
radar have greatly increased the accuracy and usefulness of air-to-ground radar since their introduction in the 1960s and 1970s. Military forces around the world use
air-to-ground radar for weapon aiming and for battlefield surveillance. The United States used the Joint Surveillance and Tracking Radar System (JSTARS) in the Persian
Gulf War (1991), demonstrating modern radar's ability to provide information about enemy troop concentrations and movements during the day or night, regardless of
weather conditions.
C2
Countermeasures
The military uses several techniques to attempt to avoid detection by enemy radar. One common technique is jamming--that is, sending deceptive signals to the
enemy's radar system. During World War II (1939-1945), flyers under attack jammed enemy radar by dropping large clouds of chaff--small pieces of aluminum foil or
some other material that reflects radar well. "False" returns from the chaff hid the aircraft's exact location from the enemy's air defense radar. Modern jamming uses
sophisticated electronic systems that analyze enemy radar, then send out false radar echoes that mask the actual target echoes or deceive the radar about a target's
location.
Stealth technology is a collection of methods that reduce the radar echoes from aircraft and other radar targets (see Stealth Aircraft). Special paint can absorb radar
signals and sharp angles in the aircraft design can reflect radar signals in deceiving directions. Improvements in jamming and stealth technology force the continual
development of high-power transmitters, antennas good at detecting weak signals, and very sensitive receivers, as well as techniques for improved clutter rejection.
D
Traffic safety
Since the 1950s, police have used radar to detect motorists who are exceeding the speed limit. Most older police radar "guns" use Doppler technology to determine the
target vehicle's speed. Such systems were simple, but they sometimes produced false results. The radar beam of such systems was relatively wide, which meant that
stray radar signals could be detected by motorists with radar detectors. Newer police radar systems, developed in the 1980s and 1990s, use laser light to form a
narrow, highly selective radar beam. The narrow beam helps insure that the radar returns signals from a single, selected car and reduces the chance of false results.
Instead of relying on the Doppler effect to measure speed, these systems use pulse radar to measure the distance to the car many times, then calculate the speed by
dividing the change in distance by the change in time. Laser radar is also more reliable than normal radar for the detection of speeding motorists because its narrow
beam is more difficult to detect by motorists with radar detectors.
E
Meteorology
Meteorologists use radar to learn about the weather. Networks of radar systems installed across many countries throughout the world detect and display areas of rain,
snow, and other precipitation. Weather radar systems use Doppler radar to determine the speed of the wind within the storm. The radar signals bounce off of water
droplets or ice crystals in the atmosphere. Gaseous water vapor does not reflect radar waves as well as the liquid droplets of water or solid ice crystals, so radar returns
from rain or snow are stronger than that from clouds. Dust in the atmosphere also reflects radar, but the returns are only significant when the concentration of dust is
much higher than usual. The Terminal Doppler Weather Radar can detect small, localized, but hazardous wind conditions, especially if precipitation or a large amount of
dust accompanies the storm. Many airports use this advanced radar to make landing safer.
F
Scientific Applications
Scientists use radar in several space-related applications. The Spacetrack system is a cooperative effort of the United States, Canada, and the United Kingdom. It uses
data from several large surveillance and tracking radar systems (including the Ballistic Missile Early Warning System) to detect and track all objects in orbit around the
earth. This helps scientists and engineers keep an eye on space junk--abandoned satellites, discarded pieces of rockets, and other unused fragments of spacecraft that
could pose a threat to operating spacecraft. Other special-purpose radar systems track specific satellites that emit a beacon signal. One of the most important of these
systems is the Global Positioning System (GPS), operated by the U.S. Department of Defense. GPS provides highly accurate navigational data for the U.S. military and
for anyone who owns a GPS receiver.
During space flights, radar gives precise measurements of the distances between the spacecraft and other objects. In the U.S. Surveyor missions to the moon in the
1960s, radar measured the altitude of the probe above the moon's surface to help the probe control its descent. In the Apollo missions, which landed astronauts on the
moon during the 1960s and 1970s, radar measured the altitude of the Lunar Module, the part of the Apollo spacecraft that carried two astronauts from orbit around the
moon down to the moon's surface, above the surface of the moon. Apollo also used radar to measure the distance between the Lunar Module and the Command and
Service Module, the part of the spacecraft that remained in orbit around the moon.
Astronomers have used ground-based radar to observe the moon, some of the larger asteroids in our solar system, and a few of the planets and their moons. Radar
observations provide information about the orbit and surface features of the object.
The U.S. Magellan space probe mapped the surface of the planet Venus with radar from 1990 to 1994. Magellan's radar was able to penetrate the dense cloud layer of
the Venusian atmosphere and provide images of much better quality than radar measurements from Earth.
Many nations have used satellite-based radar to map portions of the earth's surface. Radar can show conditions on the surface of the earth and can help determine the
location of various resources such as oil, water for irrigation, and mineral deposits. In 1995 the Canadian Space Agency launched a satellite called RADARsat to provide
radar imagery to commercial, government, and scientific users.
V
HISTORY
Although British physicist James Clerk Maxwell predicted the existence of radio waves in the 1860s, it wasn't until the 1890s that British-born American inventor Elihu
Thomson and German physicist Heinrich Hertz independently confirmed their existence. Scientists soon realized that radio waves could bounce off of objects, and by
1904 Christian Hülsmeyer, a German inventor, had used radio waves in a collision avoidance device for ships. Hülsmeyer's system was only effective for a range of
about 1.5 km (about 1 mi). The first long-range radar systems were not developed until the 1920s. In 1922 Italian radio pioneer Guglielmo Marconi demonstrated a lowfrequency (60 MHz) radar system. In 1924 English physicist Edward Appleton and his graduate student from New Zealand, Miles Barnett, proved the existence of the
ionosphere, an electrically charged upper layer of the atmosphere, by reflecting radio waves off of it. Scientists at the U.S. Naval Research Laboratory in Washington,
D.C., became the first to use radar to detect aircraft in 1930.
A
Radar in World War II
None of the early demonstrations of radar generated much enthusiasm. The commercial and military value of radar did not become readily apparent until the mid1930s. Before World War II, the United States, France, and the United Kingdom were all carrying out radar research. Beginning in 1935, the British built a network of
ground-based aircraft detection radar, called Chain Home, under the direction of Sir Robert Watson-Watt. Chain Home was fully operational from 1938 until the end of
World War II in 1945 and was extremely instrumental in Britain's defense against German bombers.
The British recognized the value of radar with frequencies much higher than the radio waves used for most systems. A breakthrough in radar technology came in 1939
when two British scientists, physicist Henry Boot and biophysicist John Randall, developed the resonant-cavity magnetron. This device generates high-frequency radio
pulses with a large amount of power, and it made the development of microwave radar possible. Also in 1939, the Massachusetts Institute of Technology (MIT) Radiation
Laboratory was formed in Cambridge, Massachusetts, bringing together U.S. and British radar research. In March 1942 scientists demonstrated the detection of ships
from the air. This technology became the basis of antiship and antisubmarine radar for the U.S. Navy.
The U.S. Army operated air surveillance radar at the start of World War II. The army also used early forms of radar to direct antiaircraft guns. Initially the radar
systems were used to aim searchlights so the soldier aiming the gun could see where to fire, but the systems evolved into fire-control radar that aimed the guns
automatically.
B
Radar during the Cold War
With the end of World War II, interest in radar development declined. Some experiments continued, however; for instance, in 1946 the U.S. Army Signal Corps bounced
radar signals off of the moon, ushering in the field of radar astronomy. The growing hostility between the United States and the Union of Soviet Socialists Republics--the
so-called Cold War--renewed military interest in radar improvements. After the Soviets detonated their first atomic bomb in 1949, interest in radar development,
especially for air defense, surged. Major programs included the installation of the Distant Early Warning (DEW) network of long-range radar across the northern reaches
of North America to warn against bomber attacks. As the potential threat of attack by ICBMs increased, the United Kingdom, Greenland, and Alaska installed the Ballistic
Missile Early Warning System (BMEWS).
C
Modern Radar
Radar found many applications in civilian and military life and became more sophisticated and specialized for each application. The use of radar in air traffic control grew
quickly during the Cold War, especially with the jump in air traffic that occurred in the 1960s. Today almost all commercial and private aircraft have transponders.
Transponders send out radar signals encoded with information about an aircraft and its flight that other aircraft and air traffic controllers can use. American traffic
engineer John Barker discovered in 1947 that moving automobiles would reflect radar waves, which could be analyzed to determine the car's speed. Police began using
traffic radar in the 1950s, and the accuracy of traffic radar has increased markedly since the 1980s.
Doppler radar came into use in the 1960s and was first dedicated to weather forecasting in the 1970s. In the 1990s the United States had a nationwide network of
more than 130 Doppler radar stations to help meteorologists track weather patterns.
Earth-observing satellites such as those in the SEASAT program began to use radar to measure the topography of the earth in the late 1970s. The Magellan spacecraft
mapped most of the surface of the planet Venus in the 1990s. The Cassini spacecraft, scheduled to reach Saturn in 2004, carries radar instruments for studying the
surface of Saturn's moon Titan.
As radar continues to improve, so does the technology for evading radar. Stealth aircraft feature radar-absorbing coatings and deceptive shapes to reduce the
possibility of radar detection. The Lockheed F-117A, first flown in 1981, and the Northrop , first flown in 1989, are two of the latest additions to the U.S. stealth aircraft
fleet. In the area of civilian radar avoidance, companies are introducing increasingly sophisticated radar detectors, designed to warn motorists of police using traffic
radar.
Contributed By:
Robert E. Millett
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
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