Helicopter

I  INTRODUCTION

Helicopter in the City

Versatile and maneuverable, helicopters are used to make traffic reports, provide aerial coverage of breaking news stories, perform emergency rescues, and deliver business executives to meetings. Helicopters are particularly useful in cities because of their ability to negotiate crowded areas and land in restricted space.

Helicopter, aircraft that can take off and land vertically and can also hover motionless in the air. Helicopters are known as rotary-wing aircraft, as opposed to fixed-wing aircraft such as airplanes. A helicopter produces thrust by means of the blades of a main rotor as they rotate above the fuselage, or body, of the aircraft. As the blades rotate, an airflow is created over them, resulting in lift, which raises the helicopter skyward. The same rotor blades can be controlled to make the helicopter travel forward, backward, or sideways. Some helicopters can achieve forward flight at speeds of over 320 km/h (200 mph).

Helicopters vary in design, but all must provide a means of counteracting the torque, or rotational force, produced on the shaft that turns the main rotor. Otherwise, the main rotor would rotate in one direction while the fuselage would rotate in the other. One common solution is to use a tail rotor. A small tail rotor is mounted vertically at the rear of the helicopter and produces a side thrust, preventing the fuselage from rotating. By increasing or decreasing the thrust produced by the tail rotor, the pilot can steer the helicopter to the left or right. Another solution is to use tandem rotors, that is, two main rotors turning in opposite directions. Since each rotor cancels the torque produced by the other, a tandem-rotor helicopter does not need a tail rotor.

Helicopters have both advantages and disadvantages compared to fixed-wing aircraft. The helicopter’s ability to maneuver in and out of hard-to-reach areas and to hover efficiently for long periods of time makes it valuable for operating in places where airplanes cannot land. Helicopters can perform important military tasks such as ferrying troops directly into combat areas or quickly transporting wounded soldiers to hospitals. However, helicopters use more fuel than airplanes and cannot fly as fast. This is because the helicopter rotor must produce both lift, which raises the craft into the sky, and thrust, which enables it to move about. In an airplane, the wings create lift and the engine produces thrust. Despite its poor cruising performance, the helicopter is the obvious choice for tasks where vertical flight is necessary.

II  MANEUVERING A HELICOPTER

Helicopter Controls

The pedals and control sticks in a helicopter cockpit allow the pilot to control the direction, speed, and altitude of a helicopter. The controls move several different mechanisms on the main rotor and tail rotor to change the amount of lift and thrust generated by the rotors.

 

A pilot maneuvers a helicopter by changing the pitch, or angle, of the rotor blades as they rotate through the air. As the blades rotate, they create lift. When the pitch of a blade is increased, more lift is produced. By directing the lift, the helicopter can be propelled in different directions. Pilots use three different controls to maneuver helicopters: anti-torque pedals, a cyclic pitch stick, and a collective pitch stick.

The pilot’s feet control two anti-torque pedals, which are used to turn the helicopter to the left or right. The pedals control the pitch of the tail rotor blades, increasing or decreasing the thrust produced by that rotor. The tail rotor provides the sideways thrust needed to counteract the torque produced by the main rotor. When the thrust from the tail rotor balances the torque on the main rotor’s shaft, the helicopter points forward. However, when the right pedal is pushed, the pitch of the tail rotor blades decreases and the thrust is reduced. The torque from the main rotor shaft then turns the nose of the helicopter to the right. When the left pedal is pushed, the tail rotor thrust increases, and the nose turns to the left. Tandem-rotor helicopters, which use two main rotors instead of a main rotor and a tail rotor, turn by tilting the rotors in different directions.

The cyclic pitch stick moves a helicopter in a chosen direction by controlling the direction of the main rotor’s thrust. This stick affects the pitch of the rotor blades as they cycle through a rotation. Increasing the pitch of a blade at a particular point during its rotation increases the amount of lift at that point. By selecting where along the rotor’s path lift is increased, the pilot can tilt the helicopter forward, backward, or to either side.

The cyclic pitch stick changes rotor pitch through a device called a swashplate. This device consists of two circular plates that surround the rotor shaft. The upper plate rotates with the shaft and the rotor blades and rests on the lower plate, which is controlled by the cyclic pitch stick. Moving the cyclic pitch stick forward, for example, tilts the lower plate, which in turn tilts the upper plate controlling the rotor blades. The swashplate lowers the pitch of the blades as they pass the right side of the helicopter, momentarily decreasing lift and causing the blades to flap downward. The swashplate at the same time increases the pitch of the blades as they pass the left side of the helicopter, increasing lift and causing the blades to flap upward. The front of the helicopter then points lower than the rear, and so the helicopter moves forward. Pushing the cyclic pitch stick in any direction will tip the rotor blades accordingly, allowing the helicopter to travel in any direction. When the stick is centered, the helicopter hovers in midair.

The collective pitch stick is a lever that allows the helicopter to climb and descend vertically. It changes the pitch of all the main rotor blades equally, and performs much the same function as the pedals perform on the tail rotor. Pulling or pushing on the lever increases or decreases the thrust produced, varying the lift. Most collective pitch sticks also have a twist grip that changes the speed of the engine, in much the same way as the throttle of a motorcycle. Increasing rotor speed is another way to increase lift, but this is not normally done.

The engine of a helicopter powers a transmission system that turns the shaft of the main rotor blades. The tail rotor is driven by a gearbox powered by the main rotor as it spins. Piston engines, similar to those in small fixed-wing airplanes, power most small helicopters. Large commercial helicopters, and almost all military helicopters, use turboshaft engines. A turboshaft engine is similar to a turbojet engine used to power a jet aircraft. A turbojet engine is essentially a large cylindrical chamber open on both ends, with a rotating shaft inside. Fan blades on the rotating shaft draw in air from one end of the jet. Additional blades compress the air. Fuel is injected into the compressed air and then ignited, producing hot expanding gas that exits the other end of the jet. In a turbojet, the thrust from the exhaust gases propels the aircraft forward. In a helicopter turboshaft, the thrust powers a second shaft that turns the main rotor blades. Unlike jet airplanes, which use incoming air in forward flight to cool the engine, helicopters use cooling fans driven by the engine.

In the event of a power failure, a helicopter can land safely by going into autorotation, or unpowered rotation of the rotor blades. The rotors will continue to turn because the helicopter’s descent through the air produces an airflow over the blades and rotates them. When the rotor blades turn as the helicopter falls, they produce enough lift to allow the pilot to control the landing. Since the tail rotor gets power from the spinning of the main rotor, rather than directly from the engine itself, the tail rotor will continue to provide directional control. This safety feature allows the pilot to maintain a limited degree of control during an emergency landing.

III  HELICOPTER AERODYNAMICS

Helicopter in Flight

A pilot maneuvers a helicopter in flight by changing the amount of lift generated by the rotors. The controls allow the pilot to direct lift in specific places along the rotor path, moving the helicopter in the desired direction.

 

Helicopters experience unique forces when in flight, and the design of the rotor blades helps overcome problems created by these forces. When a helicopter is hovering, the speed at which the tips of the rotor blades move is constant. However, as soon as the helicopter starts moving forward, the tip speed begins to change as the blades rotate around the fuselage. The tip speed increases as the blade advances toward the nose in the direction of flight. This is because the speed of the helicopter is added to the speed of the tip. As the tip passes the nose of the helicopter and begins retreating around, the tip speed decreases, because the speed of the helicopter is subtracted from the tip speed. Since lift increases with airflow speed, the advancing blade will produce more lift than the retreating blade. Unless adjustments are made for this difference in lift, the helicopter will roll over.

To compensate for the unbalanced lift, Spanish aeronautical engineer Juan de la Cierva conceived the idea of the flexible, or articulated, rotor blade, in the 1920s. The articulated rotor blade is used today on all helicopters. Each main rotor blade is connected to the shaft by a flexible hinge. The hinges allow the rotor blades to rise and fall slightly as they rotate. This is called flapping, and it allows the advancing blade to rise slightly to avoid creating too much lift. The retreating blade, creating less lift, naturally flaps down so as to increase lift. Flapping allows the differences in lift caused by uneven rotor tip speed to cancel out, producing a stable ride. Many helicopters use mechanical hinges with lubricated bearings, but some use flexible straps made of a composite material in order to reduce the required maintenance.

Helicopters require different amounts of lift and thrust at different times during flight, because the aerodynamic forces acting on them change during hovering and acceleration. The power needed to overcome the aerodynamic drag, or wind resistance, of a helicopter increases as speed increases. There is also drag on the blades themselves as they pass through the air. And there is the power needed to produce lift, which decreases as the helicopter moves faster. These separate forces combine to require more power for flight as a helicopter takes off and hovers, but less power as it flies forward. However, as speed increases, eventually more power is needed. For example, a helicopter may require 1,100 horsepower to hover. But at a forward speed of 110 km/h (70 mph) the required power may drop to approximately 600 horsepower, since the helicopter is moving rather than hovering. But as speed increases, so does drag, and so at a speed of around 240 km/h (150 mph), as much as 1,200 horsepower may be required.

Helicopter speed in forward flight is also limited because of physical stresses on the blades at high speeds. As forward speeds approach 320 km/h (200 mph), the tip speed of the advancing rotor blade approaches the speed of sound, increasing vibration levels and required power. The Westland Lynx, a British military helicopter, holds the speed record for a helicopter. The Lynx achieved a speed of 401 km/h (249 mph) in 1986.

IV  USES FOR HELICOPTERS

UH-1 Iroquois

The Bell UH-1 Iroquois, commonly known as the Huey, is used for medical evacuation, troop and cargo transport, and air combat support. It was used extensively by American forces during the Vietnam War (1959-1975).

 

Helicopters come in many sizes and are designed for a variety of roles. One of the smallest and least expensive helicopters available is the two-seat Robinson R22, popular for flight training and aerial observation. The R22 has a gross weight of 620 kg (1,370 lb). Its two-bladed rotor has a diameter of 7.6 m (25.2 ft). The maximum forward speed of the R-22 is 180 km/h (112 mph), with a cruising speed of 153 km/h (95 mph). The largest helicopter is the 80-seat Russian Mi-26 military helicopter. Its eight-bladed rotor has a diameter of 32 m (105 ft) and supports a gross weight of 56,000 kg (123,450 lb). Its maximum speed is 295 km/h (183 mph) with a cruising speed of 254 km/h (158 mph).

Because of its ability to hover and to take off and land vertically, the helicopter performs many functions that a fixed-wing aircraft cannot. For civilian use, these include emergency medical services, search and rescue missions, police services, support of offshore oil operations, news and traffic reporting, and business travel. Thousands of lives have been saved by helicopters, which have rescued people from the tops of burning buildings, plucked them from trees surrounded by ravaging flood waters, or lifted them from the decks of sinking ships.

Helicopters also play an important role as military aircraft. Helicopters were first used in significant numbers during the Korean War (1950-1953), evacuating wounded soldiers from the battlefield to field hospitals. During the Vietnam War (1959-1975) helicopters also participated in combat missions. An attack helicopter can provide fire support to ground troops or serve as an antitank vehicle, capable of firing wire-guided or laser-guided missiles. Helicopters are frequently used to move troops quickly into and out of combat zones. Navies use helicopters equipped with sonar buoys to listen for enemy submarines. Other naval helicopters can rescue downed pilots from the sea or tow sleds that sweep for underwater mines.

V  HISTORY OF THE HELICOPTER

Sikorsky's VS-300 Helicopter

Aeronautical engineer Igor Sikorsky is shown in his 75-horsepower helicopter during its first successful, fully controlled vertical flight in 1939. Equipped with a variable pitch “windmill” rotor, the device went straight up for 9.5 m (30 ft), flew 62 m (200 ft) and came straight down. The two additional tiny rotors in the rear of the fuselage act as elevators and the third serves as the rudder.

 

Inventors and engineers perfected the design of the helicopter gradually, over many years. Original inspiration came from objects like an ancient Chinese top, which rose upward when spun rapidly. One of the earliest inventors to design a helicopter was Leonardo da Vinci. In one of his notebooks from 1480, he illustrated a model helicopter driven by a clockwork motor. His notes imply that the model flew, but, from his sketch, an antitorque device is not apparent.

One of the first mechanical devices actually to hover was the Bréguet-Richet Gyroplane No. 1, designed by French aircraft pioneer Louis Charles Bréguet. It first flew in France on September 29, 1907, piloted by one of Bréguet’s engineers. Lifted by four rotors 8 m (26 ft) in diameter, this fragile craft hovered about 0.6m (2 ft) off the ground for a minute while being restrained by four men holding on to the frame. Without a control system, it was far from being a practical helicopter, but it demonstrated that it was possible to lift a person vertically by means of powered rotors.

On November 13, 1907, Frenchman Paul Cornu became the first person in history to rise vertically in powered flight, completely unrestrained from any support. The Cornu helicopter used two rotors attached to each end of a skeletal frame and was powered by a 24-horsepower engine. Although Cornu achieved a historic first, the controls of his machine were completely inadequate, and the craft never developed into a practical helicopter.

Spanish engineer Juan de la Cierva paved the way for the development of a successful helicopter, but never built a helicopter himself. Cierva developed the autogiro, which resembles the helicopter but which uses an unpowered rotor. The rotor autorotates, or autogyrates, as the autogiro is pulled through the air by a separate propeller. The turning rotor provides lift much like an aircraft wing. In January 1923, Cierva successfully flew his C.4 autogiro, which incorporated articulated rotor blades. This allowed the blades to flap freely up and down in response to the unsteady aerodynamic forces that arise in forward flight. The articulated rotor was the technical breakthrough that led others to develop the successful helicopter. Cierva might have eventually done so himself, but he died in an airplane crash in December 1936.

Igor Sikorsky Piloting Helicopter

Igor Sikorsky built the first successful single main rotor helicopter, which he first flew in 1939. He was the first to include a tail rotor, the second set of blades at the rear of the helicopter, to stabilize the aircraft and make it easier to control.

Germany made rapid strides in helicopter development in the 1930s and 1940s. The FA-61, designed by Heinrich Focke, flew for the first time on June 26, 1936. The FA-61 was the first practical design for a maneuverable helicopter. In 1937, as a propaganda stunt for the Nazi regime, the renowned female pilot Hanna Reitsch flew the FA-61 inside the city of Berlin’s Deutschlandhalle sports arena. Another German helicopter, the FL-282 Kolibri, was used by the German navy during World War II (1939-1945). It could fly at 140 km/h (90 mph) and reach an altitude of 4,000 m (13,000 ft) with a payload of 360 kg (800 lb). It was the first helicopter design produced in quantity, but only a few became operational before the war ended.

Igor Sikorsky, a Russian-born American aeronautical engineer, flew the first successful single main rotor helicopter, the VS-300, in 1939. He flew the final version of his VS-300 helicopter in 1941. Unlike previous helicopter designs, the VS-300 was the first helicopter to use a tail rotor to counteract the torque of the main rotor. This represented a major accomplishment that has been copied by the majority of helicopter designs built since. Sikorsky’s research and development of the VS-300 led to the R4, the first American helicopter built in large quantities.

During the 1990s, aeronautical engineers applied radar-evading stealth technology to the design of certain military helicopters. The first helicopter to incorporate this technology was the U.S. Army’s RAH-66 Comanche, developed jointly by the Boeing Company and Sikorsky Aircraft Corporation. The fuselage is shaped to reduce the helicopter’s visibility to enemy radar, and weapons are carried internally to further reduce the helicopter’s detection by radar. The Comanche is also designed to radiate less heat than other helicopters in order to evade infrared (heat-seeing) detectors.

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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. 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.

II

HOW RADAR WORKS

Radar Dish

Radar antennas, such as this one at London's Heathrow airport, enable air-traffic controllers to safely and efficiently direct airplanes in flight. The shape of the dish is designed to focus radar waves into a beam that scatters off aircraft. The part of the beam that gets reflected is detected by the radar dish and gives important information about the airplane, such as its altitude, heading, and speed.

Radar relies on sending and receiving electromagnetic radiation, usually in the form of radio waves 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 long-wavelength 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.

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

Radar Screen

Radar displays indicate the presence and movement of objects out of the range of vision, which is particularly useful for navigators. Electronic equipment records the behavior of radio waves projected by the vessel; waves which do not encounter anything simply disperse, while waves bounced back reveal the shape and position of all objects in the region. The characteristic sweep of the radar display occurs because the area is continually reassessed for new information, and the screen is reprinted in response.

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.

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. 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. 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

Radar Gun

Radar guns are used to detect speeding motorists. Here, a gun transmits waves at a given frequency (shown in blue) toward an oncoming car. Reflected waves (shown in red) return to the gun at a different frequency, depending on how fast the car being tracked is moving. A device in the gun compares the transmission frequency to the received frequency to determine the speed of the car. In this case, the high frequency of the reflected waves indicates the motorist in the red car is speeding and is probably about to receive a ticket.

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

Doppler Radar Image

Doppler radar measures the speed and direction of the movement of clouds, in addition to cloud density. In this image of a thunderstorm over Oklahoma, Doppler radar shows a mesocyclone, a rotating mass of air that may signal that the formation of a tornado is imminent.

 

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

Global Positioning System (GPS)

The Navstar Global Positioning System (GPS) is a network of 24 satellites in orbit around the earth that provides users with information about their position and movement. A GPS receiver computes position information by comparing the time it takes for radar signals from three or four different GPS satellites to reach the receiver.

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 low-frequency (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 mid-1930s. 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.

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