stealth technology

The use of advanced design and specialized materials to make an aircraft difficult or even impossible to detect by radar. Stealth aircraft were used for the first time in the Persian Gulf War, where they were highly successful.The term is now used to refer more generally to something that remains hidden or unknown: “This is a Stealth proposal: no one knows what it means.”

Taking place in secret. It often refers to the position that startup companies take when developing a product they feel will be very competitive in the marketplace. They swear everyone to secrecy and keep a very low profile until they are ready to launch.

The term can also be used to refer to an unwanted task being performed by software. See polymorphic virus and malware.

Download Computer Desktop Encyclopedia to your iPhone/iTouch

Stealth technology has as its fundamental principle the prevention of detection by the enemy, and applies not only to aircraft as is commonly assumed, but also increasingly to naval vessels and to armoured vehicles, although in the latter cases, it is nascent technology upon which comment must be reserved. Stealth technology, therefore, does not simply mean the evasion by aircraft of radar through the reduction of radar signature. It also encompasses the reduction of an aircraft's visibility in other spectra, most notably acoustic, visual, and infra-red. Consequently the popular term ‘stealth technology’ would perhaps be better referred to as ‘low-observable technology’.

The only known operational equipment relying upon low observable technology is found in the inventory of the USAF. The Lockheed F-117 Nighthawk and the Northrop B-2 Spirit are both designed to penetrate sophisticated air-defence systems to attack key targets with precision-guided or nuclear weapons. Their utterly contrasting designs demonstrate that the principles of low observability may be obtained in a number of ways. The fundamental goal of stealth technology is to reduce the radar cross-section of an aircraft to an absolute minimum, so that it does not stand out on a radar screen. The term ‘radar cross-section’ refers to the amount of radar energy reflected from a body. The most obvious reflector on an aircraft is the flat plate surface, and the design of the F-117, so-called ‘faceting’, reduces such reflectors to an absolute minimum. The elimination of vertical control surfaces in the B-2 ‘flying wing’ design is not new; a flying wing was built by Northrop immediately after WW II. The novelty lies in the advance of computer technology which enabled far superior wing and fuselage algorithms to be developed than was possible when the F-117 was created, while both make extensive use of radar-absorbent materials.

Stealth technology provides its users with a number of tactical advantages. As well as allowing the penetration of heavily defended airspace, it enables single aircraft to carry out attacks in a manner impossible for conventional aircraft, which require a large number of support aircraft to conduct similar missions, including escort, defence suppression, and electronic warfare types. A conventional aircraft ‘package’ may employ up to 40 aircraft, while a stealth aircraft can conduct the mission by itself. This naturally requires the use of precision-guided weapons to ensure that the single aircraft has a high probability of success.

Stealth is not without its drawbacks. The cost of the technology is enormous, and it is hard to envisage that its eventual employment in maritime and land operations will ever be obtained at a price comparable to conventional technology. Nonetheless, precisely because of its elevated cost it does emphasize the economic power of the USA as well as its immense advantage in science and technology over the rest of the world.

— David Jordan

Stealth technology, also termed "low-observable" technology, is a set of techniques that render military vehicles, mostly aircraft, hard to observe. Because RADAR—an acronym for RAdio Detection And Ranging—is the primary detection technology for aircraft, most stealth technologies are directed at suppressing RADAR returns from aircraft, but stealth technology minimizes other "observables" as well, including energy emissions that of any kind that might be observed by an opponent. Stealth technology is deployed today on several types of aircraft and a few surface ships. Counter-stealth technologies are also under continuous development.

History of Stealth Technology

Development of stealth technology for aircraft began before World War I. Because RADAR had not been invented, visibility was the sole concern, and the goal was to create aircraft that were hard to see. In 1912, German designers produced a largely transparent monoplane; its wings and fuselage were covered by a transparent material derived from cellulose, the basis of movie film, rather than the opaque canvas standard in that era. Interior struts and other parts were painted with light colors to further reduce visibility. The plane was effectively invisible from the ground when flow at 900 ft (274 m) or higher, and faintly visible at lower altitudes. Several transparent German aircraft saw combat during World War I, and Soviet aircraft designers attempted the design of transparent aircraft in the 1930s.

With the invention of RADAR during World War II, stealth became both more needful and more feasible: more needful because RADAR was highly effective at detecting aircraft, and would soon be adapted to guiding antiaircraft missiles and gunnery at them, yet more feasible because to be RADAR-stealthy an aircraft did need to be not be completely transparent to radio waves; it could absorb or deflect them.

During World War II, Germany coated the snorkels of its submarines with RADAR-absorbent paint to make them less visible to RADARs carried by Allied antisubmarine aircraft. In 1945 the U.S. developed a RADAR-absorbent paint containing iron. It was capable of making an airplane less RADAR-reflective, but was heavy; several coats of the material, known as MX-410, could make an aircraft unwieldy or even too heavy to fly. However, stealth development continued throughout the postwar years. In the mid 1960s, the U.S. built a high-altitude reconnaissance aircraft, the Lockheed SR-71 Blackbird, that was extremely RADAR-stealthy for its day. The SR-71 included a number of stealth features, including special RADAR-absorbing structures along the edges of wings and tailfins, a cross-sectional design featuring few vertical surfaces that could reflect RADAR directly back toward a transmitter, and a coating termed "iron ball" that could be electronically manipulated to produce a variable, confusing RADAR reflection. The SR-71, flying at approximately 100,000 feet, was routinely able to penetrate Soviet airspace without being reliably tracked on RADAR.

Development of true stealth aircraft (i.e., those employing every available method to avoid detection by visible, RADAR, infrared, and acoustic means) continued, primarily in the U.S., throughout the 1960s and 1970s, and several stealth prototypes were flown in the early 1970s. Efforts to keep this research secret were successful; not until a press conference was held on August, 22, 1980, after expansion of the stealth program had given rise to numerous rumors and leaks, did the U.S. government officially admit the existence of stealth aircraft. Since then, much information about the two U.S. stealth combat aircraft, the B-2 bomber and the F-117 fighter (both discussed further below), has become publicly available.

Design for stealth requires the integration of many techniques and materials. The types of stealth that a maximally stealthy aircraft (or other vehicle) seeks to achieve can be categorized as visual, infrared, acoustic, and RADAR.

Visual stealth. Low visibility is desirable for all military aircraft and is essential for stealth aircraft. It is achieved by coloring the aircraft so that it tends to blend in with its environment. For instance, reconnaissance planes designed to operate at very high altitudes, where the sky is black, are painted black. (Black is also a low visibility color at night, at any altitude.) Conventional daytime fighter aircraft are painted a shade of blue known as "air-superiority blue-gray," to blend in with the sky. Stealth aircraft are flown at night for maximum visual stealth, and so are painted black or dark gray. Chameleon or "smart skin" technology that would enable an aircraft to change its appearance to mimic its background is being researched. Furthermore, glint (bright reflections from cockpit glass or other smooth surfaces) must be minimized for visual stealth; this is accomplished using special coatings.

Infrared stealth. Infrared radiation (i.e., electromagnetic waves in the. 72–1000 micron range of the spectrum) are emitted by all matter above absolute zero; hot materials, such as engine exhaust gases or wing surfaces heated by friction with the air, emit more infrared radiation than cooler materials. Heat-seeking missiles and other weapons zero in on the infrared glow of hot aircraft parts. Infrared stealth, therefore, requires that aircraft parts and emissions, particularly those associated with engines, be kept as cool as possible. Embedding jet engines inside the fuselage or wings is one basic design step toward infrared stealth. Other measures include extra shielding of hot parts, mixing of cool air with hot exhausts before emission; splitting of the exhaust stream by passing it through parallel baffles so that it mixes with cooler air more quickly; directing of hot exhausts upward, away from ground observers; and the application of special coatings to hot spots to absorb and diffuse heat over larger areas. Active countermeasures against infrared detection and tracking can be combined with passive stealth measures; these include infrared jamming (i.e., mounting of flickering infrared radiators near engine exhausts to confuse the tracking circuits of heat-seeking missiles) and the launching of infrared decoy flares. Combat helicopters, which travel at low altitudes and at low speeds, are particularly vulnerable to heat-seeking weapons and have been equipped with infrared jamming devices for several decades.

Acoustic stealth. Although sound moves too slowly to be an effective locating signal for antiaircraft weapons, for low-altitude flying it is still best to be inaudible to ground observers. Several ultra-quiet, low-altitude reconnaissance aircraft, such as Lockheed's QT-2 and YO-3A, have been developed since the 1960s. Aircraft of this type are ultralight, run on small internal combustion engines quieted by silencer-suppressor mufflers, and are driven by large, often wooden propellers. They make about as much sound as gliders and have very low infrared emissions as well because of their low energy consumption. The U.S. F-117 stealth fighter, which is designed to fly at high speed at very low altitudes, also incorporates acoustic-stealth measures, including sound-absorbent linings inside its engine intake and exhaust cowlings.

RADAR stealth. RADAR is the use of reflected electromagnetic waves in the microwave part of the spectrum to detect targets or map landscapes. RADAR first illuminates the target, that is, transmits a radio pulse in its direction. If any of this energy is reflected by the target, some of it may be collected by a receiving antenna. By comparing the delay times for various echoes, information about the geometry of the target can be derived and, if necessary, formed into an image. RADAR stealth or invisibility requires that a craft absorb incident RADAR pulses, actively cancel them by emitting inverse waveforms, deflect them away from receiving antennas, or all of the above. Absorption and deflection, treated below, are the most important prerequisites of RADAR stealth.

Absorption. Metallic surfaces reflect RADAR; therefore, stealth aircraft parts must either be coated with RADAR-absorbing materials or made out of them to begin with. The latter is preferable because an aircraft whose parts are intrinsically RADAR-absorbing derives aerodynamic as well as stealth function from them, whereas a RADAR-absorbent coating is, aerodynamically speaking, dead weight. The F-117 stealth aircraft is built mostly out of a RADAR-absorbent material termed Fibaloy, which consists of glass fibers embedded in plastic, and of carbon fibers, which are used mostly for hot spots like leading wing-edges and panels covering the jet engines. Thanks to the use of such materials, the airframe of the F-117 (i.e., the plane minus its electronic gear, weapons, and engines) is only about 10% metal. Both the B-2 stealth bomber and the F-117 reflect about as much RADAR as a hummingbird

Many RADAR-absorbent plastics, carbon-based materials, ceramics, and blends of these materials have been developed for use on stealth aircraft. Combining such materials with RADAR-absorbing surface geometry enhances stealth. For example, wing surfaces can be built on a metallic substrate that is shaped like a field of pyramids with the spaces between the pyramids filled by a RADAR-absorbent material. RADAR waves striking the surface zigzag inward between the pyramid walls, which increases absorption by lengthening signal path through the absorbent material. Another example of structural absorption is the placement of metal screens over the intake vents of jet engines. These screens—used, for example, on the F-117 stealth fighter—absorb RADAR waves exactly like the metal screens embedded in the doors of microwave ovens. It is important to prevent RADAR waves from entering jet intakes, which can act as resonant cavities (echo chambers) and so produce bright RADAR reflections.

The inherently high cost of RADAR-absorbent, airframe-worthy materials makes stealth aircraft expensive; each B-2 bomber costs approximately $2.2 billion, while each F-117 fighter costs approximately $45 million; the U.S. fields 21 B-2s and 54 F-117s. The Russian Academy of Sciences, however, according to a 1999 report by Jane's Defense Weekly, claims to have developed a low-budget RADAR-stealth technique, namely the cloaking of aircraft in ionized gas (plasma). Plasma absorbs radio waves, so it is theoretically possible to diminish the RADAR reflectivity of an otherwise non-stealthy aircraft by a factor of 100 or more by generating plasma at the nose and leading edges of an aircraft and allowing it flow backward over the fuselage and wings. The Russian system is supposedly lightweight (>220 lb [100 kg]) and retrofittable to existing aircraft, making it the stealth capability available at least cost to virtually any air force. A disadvantage of the plasma technique that it would probably make the aircraft glow in the visible part of the spectrum.

Deflection. Most RADARs are monostatic, that is, for reception they use either the same antenna as for sending or a separate receiving antenna colocated with the sending antenna; deflection therefore means reflecting RADAR pulses in any direction other than the one they came from. This in turn requires that stealth aircraft lack flat, vertical surfaces that could act as simple RADAR mirrors. RADAR can also be strongly reflected wherever three planar surfaces meet at a corner. Planes such as the B-52 bomber, which have many flat, vertical surfaces and RADAR-reflecting corners, are notorious for their RADAR-reflecting abilities; stealth aircraft, in contrast, tend to be highly angled and streamlined, presenting no flat surfaces at all to an observer that is not directly above or below them. The B-2 bomber, for example, is shaped like a boomerang.

A design dilemma for stealth aircraft is that they need not only to be invisible to RADAR but to use RADAR; inertial guidance, the Global Positioning System, and laser RADAR can all help aircraft navigate stealthily, but an aircraft needs conventional RADAR to track incoming missiles and hostile aircraft. Yet the transmission of RADAR pulses by a stealth aircraft wishing to avoid RADAR detection is self-contradictory. Furthermore, RADAR and radio antennas are inherently RADAR-reflecting.

At least two design solutions to this dilemma are available. One is to have moveable RADAR-absorbent covers over RADAR antennas that slip aside only when the RADAR must be used. The antenna is then vulnerable to detection only intermittently. Even short-term RADAR exposure is, however, dangerous; the only stealth aircraft known to be have been shot down in combat, an F-117 lost over Kosovo in 1999, is thought to have been tracked by RADAR during a brief interval while its bomb-bay doors were open. The disadvantage of sliding mechanical covers is that they may stick or otherwise malfunction, and must remain open for periods of time that are long by electronic standards. A better solution, presently being developed, is the plasma stealth antenna. A plasma stealth antenna is composed of parallel tubes made of glass, plastic, or ceramic that are filled with gas, much like fluorescent light bulbs. When each tube is energized, the gas in it becomes ionized, and can conduct current just like a metal wire. A number of such energized tubes in a flat, parallel array, wired for individual control (a "phased array"), can be used to send and receive RADAR signals across a wide range of angles without being physically rotated. When the tubes are not energized, they are transparent to RADAR, which can be absorbed by an appropriate backing. One advantage of such an array is that it can turn on and off very rapidly, and only act as a RADAR reflector during the electronically brief intervals when it is energized.

Stealthy Flying

Stealth technology is most effective when combined with other measures for avoiding detection. For example, the F-117 and B-2 are both designed to fly at night, the most obvious visual stealth measure. Further, the F-117 is designed to fly close to the ground (i.e., at less than 500 feet [152 m]). Normal ground-based RADAR cannot see oncoming targets until they are in a line of direct sight, which, for a fast, low-flying aircraft approaching through hilly terrain, may not occur until the aircraft is almost above the RADAR. Even down-looking RADARs carried on aircraft have more difficulty tracking craft that are flying near ground-level, mingling their reflections with the noisy pattern of echoes from the ground itself ("ground clutter"). The F-117 therefore can fly close to the ground, swerving under computer control to avoid obstacles such as hills or towers. This flight style is known as jinking, snaking, or terrain following. (An aircraft such as the B-2 is too large to perform the rapid maneuvers required for jinking, and so flies at higher altitudes.)

At the opposite extreme from jinking flight, ultra-high altitudes have also been used for stealth purposes. Reconnaissance aircraft deployed by the U.S. since the 1950s, including the U-2 and the SR-71, have set most of the altitude records for "air-breathing" craft (i.e., craft that do not, like rockets, carry their own oxygen). Such planes fly near the absolute limit of aerodynamic action; if they went any higher, there would be not be enough air to provide lift.

Counter-stealth. An aircraft cannot be made truly invisible. For example, no matter how cool the exhaust vents of an aircraft are kept, the same amount of heat is always liberated by burning a given amount of fuel, and this heat must be left behind the aircraft as a trail of warm air. Infrared-detecting devices might be devised that could image this heat trail as it formed, tracking a stealth aircraft.

Furthermore, every jet aircraft leaves swirls of air—vortices—in its wake. Doppler RADAR, which can image wind velocities, might pinpoint such disturbances if it could be made sufficiently high-resolution.

Other anti-stealth techniques could include the detection of aircraft-caused disturbances in the Earth's magnetic field (magnetic anomaly detection), networks of lowfrequency radio links to detect stealth aircraft by interruptions in transmission, the use of specially shaped RADAR pulses that resist absorption, and netted RADAR. Netted RADAR is the use of more than one receiver, and possibly more than one transmitter, in a network. Since stealth aircraft rely partly on deflecting RADAR pulses, receivers located off the line of pulse transmission might be able to detected deflected echoes. By illuminating a target area using multiple transmitters and linking multiple receivers into a coordinated network, it should be possible to greatly increase one's chances of detecting a stealthy target. No single receiver may record a strong or steady echo from any single transmitter, but the network as a whole might collect enough information to track a stealth target.

Stealth in wartime. Stealthy jet aircraft have been used for surveillance since the 1950s, but dedicated-design stealth warplanes were not used in combat prior to the first Gulf War (1991). In that war, F-117s—which first became operational in 1982—made some 1,300 sorties and were the only aircraft to bomb targets in downtown Baghdad. B-2 bombers were first used in combat in the Kosovo conflict in 1999, flying bombing sorties from Missouri to Yugoslavia (with midflight refueling over the Atlantic). F-117s were also used in the Kosovo conflict; one was shot down and two were damaged by enemy fire. The first overseas combat deployment of B-2 bombers occurred in 2003, during Operation Iraqi Freedom.

Stealth technology is also employed in U.S. cruise missiles such as the Tomahawk and the AGM-129A. The Tomahawk, a tactical weapon that can carry either nuclear or conventional warheads, has been deployed in four versions, including air-, sea-, and ground-launched types, and was used extensively in combat in both Gulf Wars and in Afghanistan in 2002. The AGM-129A is stealthier than the 1970s-vintage Tomahawk; it carries the W80 250-kiloton nuclear warhead and is designed to be fired from under the wings of the B-52H Stratofortress strategic bomber. The AGM-129A has not been used in combat.

Further Reading

Books

Jones, Joseph. Stealth Technology. Blue Ridge Summit, PA: TAB Books, 1994.

Periodicals

Hume, Andrew L., and Christopher Baker. "Netted RADAR Sensing," in Proceedings of the CIE International Conference on RADAR, (IEEE) 110–14, 2001.

Electronic

"Russians Offer Radical Stealth Device for Export." Jane's Defence Weekly. March 17, 1999. <http://www.aeronautics.ru/plasma04.htm> (March 20, 2003).

F-117 stealth attack plane

Stealth technology also known as LO technology (low observable technology) is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, in order to make them less visible (ideally invisible) to radar, infrared, sonar and other detection methods.

The concept of stealth is to operate or hide without giving enemy forces any indications as to the presence of friendly forces. This concept was first explored through camouflage by blending into the background visual clutter. As the potency of detection and interception technologies (radar, IRST, surface-to-air missiles etc.) have increased over time, so too has the extent to which the design and operation of military personnel and vehicles have been affected in response. Some military uniforms are treated with chemicals to reduce their infrared signature. A modern "stealth" vehicle will generally have been designed from the outset to have reduced or controlled signature. Varying degrees of stealth can be achieved. The exact level and nature of stealth embodied in a particular design is determined by the prediction of likely threat capabilities.

Contents 1 History of stealth technology 2 Stealth principles 2.1 Radar cross-section (RCS) reductions 2.1.1 Vehicle shape 2.1.2 Non-metallic airframe 2.1.3 Radar absorbing material 2.1.4 Radar stealth countermeasures and limitations 2.1.4.1 Low frequency radar 2.1.4.2 Multiple transmitters 2.2 Acoustics 2.3 Visibility 2.4 Infrared 2.5 Reducing radio frequency (RF) emissions 3 Measuring stealth 4 Stealth tactics 5 See also 6 References 6.1 Bibliography 6.2 Notes 7 External links

History of stealth technology

In England, irregular units of gamekeepers in the 17th century were the first to adopt drab colours (common in the 16th century Irish units) as a form of camouflage, following examples from the continent.

Yehudi lights were successfully employed in World War II by RAF Shorts Sunderland aircraft in attacks on U-boats. In 1945 a Grumman Avenger with Yehudi lights got within 3,000 yards (2,700 m) of a ship before being sighted. This ability was rendered obsolete by the radar of the time.

One of the earliest stealth aircraft seems to have been the Horten Ho 229 flying wing. This included carbon powder in the glue to absorb radio waves.^[1] However, it was never deployed in any quantity.

P.Ya. Ufimtsev published Method of Edge Waves in the Physical Theory of Diffraction, Soviet Radio, Moscow, 1962. In 1971 this book was translated into English with the same title by U.S. Air Force, Foreign Technology Division (National Air Intelligence Center ), Wright-Patterson AFB, OH, 1971. Technical Report AD 733203, Defense Technical Information Center of USA, Cameron Station, Alexandria, VA, 22304-6145, USA. This theory played a critical role in the design of American stealth-aircraft F-117 and B-2.^[2][3][4]

The F-117 project began with a model called "The Hopeless Diamond" (a wordplay on the Hope Diamond) in 1975 due to its bizarre appearance. In 1977 Lockheed produced two 60% scale models under the Have Blue contract. The Have Blue program was a stealth technology demonstrator that lasted from 1976 to 1979. The success of Have Blue lead the Air Force to create the Senior Trend^[5][6] program which developed the F-117.

Stealth principles

This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (March 2009)

Stealth technology (or LO for "low observability") is not a single technology. It is a combination of technologies that attempt to greatly reduce the distances at which a person or vehicle can be detected; in particular radar cross section reductions, but also acoustic, thermal, and other aspects:

Radar cross-section (RCS) reductions

Main article: Radar cross section

Almost since the invention of radar, various techniques have been tried to minimize detection. Rapid development of radar during WWII led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff.

The term "stealth" in reference to reduced radar signature aircraft became popular during the late eighties when the Lockheed Martin F-117 stealth fighter became widely known. The first large scale (and public) use of the F-117 was during the Gulf War in 1991. However, F-117A stealth fighters were used for the first time in combat during Operation Just Cause, the United States invasion of Panama in 1989.^[7] Increased awareness of stealth vehicles and the technologies behind them is prompting the development of techniques for detecting stealth vehicles, such as passive radar arrays and low-frequency radars. Many countries nevertheless continue to develop low-RCS vehicles because they offer advantages in detection range reduction and amplify the effectiveness of on-board systems against active radar guidance threats.^{[citation needed]}

Vehicle shape

The F-35 Lightning II offers better stealthy features (such as this landing gear door) than previous American fighters, such as the F-16 Fighting Falcon

The possibility of designing aircraft in such a manner as to reduce their radar cross-section was recognized in the late 1930s, when the first radar tracking systems were employed, and it has been known since at least the 1960s that aircraft shape makes a significant difference in detectability. The Avro Vulcan, a British bomber of the 1960s, had a remarkably small appearance on radar despite its large size, and occasionally disappeared from radar screens entirely. It is now known that it had a fortuitously stealthy shape apart from the vertical element of the tail. On the other hand, the Tupolev 95 Russian long range bomber (NATO reporting name 'Bear') appeared especially well on radar. It is now known that propellers and jet turbine blades produce a bright radar image; the Bear had four pairs of large (5.6 meter diameter) contra-rotating propellers.

Another important factor is the internal construction. Behind the skin of some aircraft are structures known as re-entrant triangles. Radar waves penetrating the skin of the aircraft get trapped in these structures, bouncing off the internal faces and losing energy. This approach was first used on the F-117.

The most efficient way to reflect radar waves back to the transmitting radar is with orthogonal metal plates, forming a corner reflector consisting of either a dihedral (two plates) or a trihedral (three orthogonal plates). This configuration occurs in the tail of a conventional aircraft, where the vertical and horizontal components of the tail are set at right angles. Stealth aircraft such as the F-117 use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them. A more radical approach is to eliminate the tail completely, as in the B-2 Spirit.

In addition to altering the tail, stealth design must bury the engines within the wing or fuselage, or in some cases where stealth is applied to an existing aircraft, install baffles in the air intakes, so that the turbine blades are not visible to radar. A stealthy shape must be devoid of complex bumps or protrusions of any kind; meaning that weapons, fuel tanks, and other stores must not be carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch is opened.

Planform alignment is also often used in stealth designs. Planform alignment involves using a small number of surface orientations in the shape of the structure. For example, on the F-22A Raptor, the leading edges of the wing and the tail surfaces are set at the same angle. Careful inspection shows that many small structures, such as the air intake bypass doors and the air refueling aperture, also use the same angles. The effect of planform alignment is to return a radar signal in a very specific direction away from the radar emitter rather than returning a diffuse signal detectable at many angles.

Stealth airframes sometimes display distinctive serrations on some exposed edges, such as the engine ports. The YF-23 has such serrations on the exhaust ports. This is another example in the use of re-entrant triangles and planform alignment, this time on the external airframe.

Shaping requirements have strong negative influence on the aircraft's aerodynamic properties. The F-117 has poor aerodynamics, is inherently unstable, and cannot be flown without a fly-by-wire control system.

HMS Helsingborg Stealth Ship

Ships have also adopted similar techniques. The Visby corvette was the first stealth ship to enter service, though the earlier Arleigh Burke class destroyer incorporated some signature-reduction features [1]. Other examples are the French La Fayette class frigate, the USS San Antonio amphibious transport dock, and most modern warship designs.

Similarly, coating the cockpit canopy with a thin film transparent conductor (vapor-deposited gold or indium tin oxide) helps to reduce the aircraft's radar profile because radar waves would normally enter the cockpit, bounce off something random (the inside of the cockpit has a complex shape, with the pilot's helmet itself providing a sizeable return), and possibly return to the radar, but the conductive coating creates a controlled shape that deflects the incoming radar waves away from the radar. The coating is thin enough that it has no adverse effect on the pilot's vision.

Non-metallic airframe

Dielectric composites are more transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites may also contain ferrites to optimize the dielectric and magnetic properties of the material for its application.

Radar absorbing material

Main article: Radar absorbent material

Radar absorbent material (RAM), often as paints, are used especially on the edges of metal surfaces. While the material and thickness of RAM coatings is classified, the material seeks to absorb radiated energy from a ground or air based radar station into the coating and convert it to heat rather than reflect it back.

Radar stealth countermeasures and limitations

Low frequency radar

Shaping does not offer stealth advantages against low-frequency radar. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. However, low-frequency radar is limited by lack of available frequencies which are heavily used by other systems, lack of accuracy given the long wavelength, and by the radar's size, making it difficult to transport. A long-wave radar may detect a target and roughly locate it, but not identify it, and the location information lacks sufficient weapon targeting accuracy. Noise poses another problem, but that can be efficiently addressed using modern computer technology; Chinese "Nantsin" radar and many older Soviet-made long-range radars were modified this way. It has been said that "there's nothing invisible in the radar frequency range below 2 GHz". [2]

Multiple transmitters

Much of the stealth comes from reflecting the transmissions in a different direction other than a direct return. Therefore detection can be better achieved if the sources are spaced from the receivers, known as bistatic radar, and proposals exist to use reflections from sources such as civilian radio transmitters, including cellular telephone radio towers.^[8]

Acoustics

Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles. Submarines have extensive usage of rubber mountings to isolate and avoid mechanical noises that could reveal locations to underwater passive sonar arrays.

Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy troops below. Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. The presence of supersonic and jet-powered stealth aircraft such as the SR-71 Blackbird indicates that acoustic signature is not always a major driver in aircraft design, although the Blackbird relied more on its extremely high speed and altitude.

Visibility

The simplest stealth technology is simply camouflage; the use of paint or other materials to color and break up the lines of the vehicle or person.

Most stealth aircraft use matte paint and dark colors, and operate only at night. Lately, interest on daylight Stealth (especially by the USAF) has emphasized the use of gray paint in disruptive schemes, and it is assumed that Yehudi lights could be used in the future to mask shadows in the airframe (in daylight, against the clear background of the sky, dark tones are easier to detect than light ones) or as a sort of active camouflage. The B-2 has wing tanks for a contrail-inhibiting chemical, alleged by some to be chlorofluorosulphonic acid[3], and mission planning also considers altitudes where the probability of their formation is minimized.

Infrared

An exhaust plume contributes a significant infrared signature. One means of reducing the IR signature is to have a non-circular tail pipe (a slit shape) in order to minimize the exhaust cross-sectional volume and maximize the mixing of the hot exhaust with cool ambient air. Often, cool air is deliberately injected into the exhaust flow to boost this process. Sometimes, the jet exhaust is vented above the wing surface in order to shield it from observers below, as in the B-2 Spirit, and the unstealthy A-10 Thunderbolt II. To achieve infrared stealth, the exhaust gas is cooled to the temperatures where the brightest wavelengths it radiates on are absorbed by atmospheric carbon dioxide and water vapor, dramatically reducing the infrared visibility of the exhaust plume. [4] Another way to reduce the exhaust temperature is to circulate coolant fluids such as fuel inside the exhaust pipe, where the fuel tanks serve as heat sinks cooled by the flow of air along the wings.

Reducing radio frequency (RF) emissions

In addition to reducing infrared and acoustic emissions, a stealth vehicle must avoid radiating any other detectable energy, such as from onboard radars, communications systems, or RF leakage from electronics enclosures. The F-117 uses passive infrared and low light level television sensor systems to aim its weapons and the F-22 Raptor has an advanced LPI radar which can illuminate enemy aircraft without triggering a radar warning receiver response.

Measuring stealth

The size of a target's image on radar is measured by the radar cross section or RCS, often represented by the symbol σ and expressed in square meters. This does not equal geometric area. A perfectly conducting sphere of projected cross sectional area 1 m² (ie a diameter of 1.13 m) will have an RCS of 1 m². Note that for radar wavelengths much less than the diameter of the sphere, RCS is independent of frequency. Conversely, a square flat plate of area 1 m² will have an RCS of σ = 4π A² / λ² (where A=area, λ=wavelength), or 13,982 m² at 10 GHz if the radar is perpendicular to the flat surface.^[9] At off-normal incident angles, energy is reflected away from the receiver, reducing the RCS. Modern stealth aircraft are said to have an RCS comparable with small birds or large insects, though this varies widely depending on aircraft and radar.^{[citation needed]}

If the RCS was directly related to the target's cross-sectional area, the only way to reduce it would be to make the physical profile smaller. Rather, by reflecting much of the radiation away or absorbing it altogether, the target achieves a smaller radar cross section.[5]

Stealth tactics

Stealthy strike aircraft such as the F-117, designed by Lockheed Martin's famous Skunk Works, are usually used against heavily defended enemy sites such as Command and Control centers or surface-to-air missile (SAM) batteries. Enemy radar will cover the airspace around these sites with overlapping coverage, making undetected entry by conventional aircraft nearly impossible. Stealthy aircraft can also be detected, but only at short ranges around the radars, so that for a stealthy aircraft there are substantial gaps in the radar coverage. Thus a stealthy aircraft flying an appropriate route can remain undetected by radar. Many ground-based radars exploit Doppler filter to improve sensitivity to objects having a radial velocity component with respect to the radar. Mission planners use their knowledge of the enemy radar locations and the RCS pattern of the aircraft to design a flight path that minimizes radial speed while presenting the lowest-RCS aspects of the aircraft to the threat radar. In order to be able to fly these "safe" routes, it is necessary to understand the enemy's radar coverage (see Electronic Intelligence). Airborne or mobile radar systems such as AWACS can complicate tactical strategy for stealth operation.

See also

Stealth aircraft B-2 Spirit bomber F-22 Raptor F-35 Lightning II F-117 Nighthawk Horten Ho 229 Military flying saucers Plasma stealth Pyotr Ufimtsev created much of the original theory behind radar stealth Radar

Stealth ship Braunschweig class corvette De Zeven Provinciën class frigate F125 class frigate Type 45 destroyer Formidable class frigate Hamina class missile boat La Fayette class frigate Sachsen class frigate Sea Shadow (IX-529) Shivalik class frigate Skjold class patrol boat Visby class corvette Milgems

References

Bibliography

• Doucet, Arnaud; Freitas, Nando de; Gordon, Neil (2001) [2001]. Sequential Monte Carlo Methods in Practice. Statistics for Engineering and Information Science (1st ed.). Berlin: Springer-Verlag. ISBN 978-0-387-95146-1. http://www.springer.com/statistics/book/978-0-387-95146-1. Retrieved on 2009-03-11. 
• Ufimtsev, Pyotr Ya., "Method of edge waves in the physical theory of diffraction," Moscow, Russia: Izd-vo. Sov. Radio [Soviet Radio Publishing], 1962, pages 1–243.
• Countering stealth
• How "stealth" is achieved on F-117A
• United States Patent No.6,297,762. October 2, 2001. Electronic countermeasures system (Apparatus for detecting the difference in phase between received signals at two spaced antennas and for then retransmitting equal amplitude antiphase signals from the two spaced antennas is disclosed.)

Notes

This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2008)

1. ^ Myhra 2009, p. 11.
2. ^ Browne, M.W. "Two rival designers led the way to stealthy warplanes", New York Times, Sci. Times Sec., May 14, 1991.
3. ^ Browne, M.W. "Lockheed credits Soviet theory in design of F-117", Aviation Week Space Technology p. 27, December 1991.
4. ^ Rich, Ben and L. Janos, Skunk Works, Little Brown, Boston, 1994.
5. ^ F-117A Senior Trend
6. ^ "Senior Trend". Vectorsite.net, 1 April 2008.
7. ^ Crocker, H. W. III (2006). Don't Tread on Me. New York: Crown Forum. p. 382. ISBN 9781400053636. 
8. ^ MIT's "The Tech - online edition" article Detection of the B-2 Stealth Bomber And a Brief History on “Stealth” by Tao Yue published November 30, 2001 in (Volume 121 >> Issue 63)
9. ^ Knott, Eugene; Shaeffer, John, and Tuley, Michael (1993). Radar Cross Section, 2nd ed. Artech House, Inc.. pp. 231. ISBN 0-89006-618-3. 

External links

• Grant R, The Radar Game - Understanding Stealth and Aircraft Survivability, Iris Independent Research
• The Advent, Evolution and New Horizons of United States Stealth Aircraft
• What is Stealth Technology?
• Stealth design used for military aircraft?
• Stealth Satellites
• Stealth in strike warfare
• Stealth technolgy
• The Paradigm Shift in Air Superiority (Stealth)
• Stealth Technology
• USAF Stealth Fighters briefing
• Pyotr Ufimstev biography
• Advancement of Plasma antennas in stealth

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)