Stealth technology in aircraft full report
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31-01-2010, 01:07 PM

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Stealth aircraft are aircraft that use stealth technology to make it harder to be detected by radar and other means than conventional aircraft by employing a combination of features to reduce visibility in the visual, audio, infrared and radio frequency (RF) spectrum. Well known examples include the United States' F-117 Nighthawk (1980s-2008), the B-2 Spirit "Stealth Bomber," and the F-22 Raptor. While no aircraft is totally invisible to radar, stealth aircraft limit current conventional radar's abilities to detect or track them effectively enough to prevent an attack. Stealth is accomplished by using a complex design philosophy to reduce the ability of an opponent's sensors to detect, track and attack an aircraft. Modern stealth aircraft first became possible when a mathematician working for Lockheed Aircraft during the 1970s adopted a mathematical model developed by Petr Ufimtsev, a Russian scientist, to develop a computer program called Echo 1. Echo made it possible to predict the radar signature an aircraft made with flat panels, called facets. In 1975, engineers at Lockheed Skunk Works found that an airplane made with faceted surfaces could have a very low radar signature because the surfaces would radiate almost all of the radar energy away from the receiver. Reduced radar cross section is only one of five factors that designers addressed to create a truly stealthy design. Designers also addressed making the aircraft less visible to the naked eye, controlling radio transmissions, and noise abatement.

The first combat use of stealth aircraft was in December 1989 during Operation Just Cause in Panama. On December 20, 1989 two USAF F-117s bombed a Panamanian defence Force Barracks in Rio Hato, Panama. In 1991, F-117s were tasked with attacking the most heavily fortified targets in Iraq and were the only jets allowed to operate inside Baghdad's city limits

"Stealth", a buzzword common in defence circles since the early 80s, only became a mainstream reference in the nineties, after the second Persian Gulf War in 1991.Night-enhanced images of the otherworldly-shapedF-117s taking off in the night and striking high-value targets with scarcely believable precision and seeming invulnerability to thick air defences were widely televised and etched in the memories of TV viewers worldwide. The subsequent exposure of stealth aircraft and their participation in numerous air operations in the 90s, in combination with the loss of at least one F-117 in Kosovo, has peeled off some of the mythical cloak surrounding stealth. However, a lot of misconceptions about the abilities and limitations of this technology still remain, even amongst people in posts of high professional responsibility. It is therefore useful to take a broad look at how stealth works, what it can and what it cannot do. This article will examine strictly the application of stealth in air assets. Different technologies and strategies for stealth are the province of land, naval and underwater forces. First of all, although it is common to discuss the principles of stealth technology (also referred to as VLO or Very Low Observables technology) only as relevant to a narrow band of the electromagnetic spectrum (radar emissions), stealth as a design practice applies a wide range of signatures. Ben Rich, the leader of the Lockheed team that designed the F-117, has stated: "A stealth aircraft has to be stealthy in six disciplines: radar, infrared,

visual, acoustic, smoke and contrail. If you don't do that, you flunk the course." That said, not all disciplines are equally important when discussing any given platform category. Underwater warfare will naturally hand dominance to the acoustic spectrum (though onacoustic sensors can and do exist). Land combat will emphasize visual, infra-red and acoustic signatures. Radar and (to a lesser extent) infrared bands dominate the scene of airspace surveillance, and so they have to be given higher priority when thinking the applications in air warfare. Before discussing the various techniques of reducing the radar and infrared signature, it is useful to understand the principles of radar reflectivity and how they can be exploited when one starts thinking about aiming for stealth in earnest.


All radar systems, from an AWACS to police speed radar, work in the same principle: A certain amount of electromagnetic energy is transmitted through a directional antenna, which focuses it into a conical beam. When a reflective target (in radar engineering terms, anything observed by radar is a potential target) blocks part of the beam, that part of the beam is reflected in many different directions, or "scattered." If the scattering is fairly random, as is usually the case, some energy will be reflected in the direction of the radar antenna. Most radar transmits this energy in pulses, thousands of them every second. In the gaps between the pulse transmissions, the radar becomes a receiver, and the gaps are carefully chosen to be just long enough for the signal to make its way to the target and back at the speed of light1. The time interval between the transmission and reception of the pulse gives the range from the radar to the target. The radar antenna moves at a pre-determined regular rate, so the time at which the target moves in and out of the beam can be tied to the position of the antenna, giving the target's bearing from the location of the radar. This process has been considerably developed and refined in the 6+ decades since the first workable radars were deployed. However, it is still true that radar does not "see" things in the way that the human eye does. Humans see in a world which is saturated with visible light, so that almost every square inch of it reflects some light toward us at all times; the radar only "sees" the energy that is reflected toward it. The radar can detect a target ONLY when its antenna captures enough

energy to rise above the electronic noise that is invariably present in The receiver. (Typically, there is a definite signal-to-noise threshold associated with a positive detection). All the variables in the transmission-scattering-reflection sequence affect the maximum range at which this can happen. These variables include: ¾ The strength of the outgoing signal ¾ the width of the beam ¾ the size of the antenna ¾ the reflectivity, or RCS, of the target. The radar beam, it is important to remember, is a cone. The greater the range, the greater the area illuminated by the radar, and the smaller the proportion of the energy which will be scattered by a target with a given RCS. The same effect results in the scattered energy returning to the radar. Therefore, at a longer range, the already-reduced energy hitting the target is scattered over a wider area and less of it will be captured by the antenna. The eventual amount of energy received back by the antenna, even at the best of circumstances, is a very small fraction of the original outgoing pulse. Increasing the power of the radar will increase its range (a longtime Soviet/Russian favourite), but the benefits are limited by the fact that much of the extra radiated energy is simply wasted on empty space. Greater power canal so mean more noise in the system. An antenna of larger aperture is helpful, because it can produce a narrower, more intense outgoing beam and intercepts

more returned energy. The limit is the physical size of the antenna, which is important on any mobile or transportable radar and critical on an airborne system.


RCS is the one single variable that is out of the radar designer's control. The relationship of RCS to the detection range is not in direct proportion, because of the aforementioned conical beam and radial scattering effects. Detection range is in proportion to the fourth root of RCS. For example, if given radar has a range of 100 miles against a target with an RCS of 10 square meters, its range will be eighty-five miles against a target of half the reflectivity (5 square meters). A 1m2 RCS translates into a fiftyfive-mile detection range. Thus, a ninety percent reduction in reflectivity equals a forty-five percent reduction in detection range; hardly a very inspiring feature. A very large reduction in RCS, not 1/10 but 1/1000, is essential to have a tactically significant effect (e.g. an 82% range reduction at 1/1000). What makes stealth possible and worth the effort is that such tremendous reductions in target RCS (entire orders of magnitude) are achievable, and the reason that they are achievable is that conventional non-stealthy aircraft are almost ideal radar targets. Searching for an aircraft with radar can be compared to searching with a flashlight for a tiny model airplane suspended somewhere in a pitch-black concert hall, hung with matte-black drapes. How hard it will be to find the model depends on many things other than its size. If the model aircraft is white in colour, it may be picked out easily. If it is highly polished, it will glint; the observer will see patches of light on its surface that seems almost as bright as the flashlight. The glints will be particularly strong if the model has flat surfaces which are angled at ninety degrees to the source

of the light. Other targets may have completely different characteristics. A flat mirror might Seem likely to be highly visible, but unless its surface makes two right angles to the beam (that is to say, it is "normal" to the beam); it will reflect all the light away from an observer. A bowling ball does the opposite; it always reflects the same amount of light, regardless of its attitude. To the radar wave, most synthetic surfaces, like the skin of an aircraft, are mirror-like. A conventional aircraft has a complex external shape, full of curves, flat panels and edges. While its shape agrees with the laws of aerodynamics and the principles of engineering, it is entirely random in terms of the way it scatters radar energy. As the airplane moves (rapidly, relative to a radar which is pulsing energy toward it), it throws off a constantly changing, scintillating pattern of concentrated reflections. The measurement called RCS was originally developed by radar engineers, as they tried to measure the performance of their creations against a common reference point. RCS is determined by first measuring, or calculating, the amount of radar energy reflected from a target toward an observer. RCS is based on the size of a reflective sphere (the optical equivalent would be a spherical mirror) that would return the same amount of energy. The project and implimentationed area of the sphere, or the area of a disk of the same diameter, is the RCS number itself. The most important point to be made about RCS is that a small, efficient reflector (such as a flat plate, normal to the radar beam) can reflect as much energy as a very large sphere, and will have a

very large RCS. A 10x10cm square plate, for example, has an actual physical area of 0.01 square meters. Its RCS however, when it is normal to the radar beam, is 1 square meter, or 100 times as large as its physical area. Composite or complex shapes can be even worse. Reflective surfaces at ninety degrees to one another (as, for example, the tail-mounted horizontal and vertical stabilizers of numerous aircraft) can turn a radar signal through two right angles and fire it back to the receiver in full intensity. Many modern aircraft are full of such reflectors, and the resulting RCS figures are almost staggering. Viewed from the side, a typical fighter, such as the F-15, may have a project and implimentationed area of 25 square meters. Because of the aircraft's design, however, the broadside RCS may be sixteen times as large, at 400 square meters, or the size of a very large house. Typical frontal-aspect RCS figures for modern aircraft run around 3-10 square meters for fighters, and up to 1,000 square meters for a bomber such as the B-52 or a transport aircraft like the Boeing 747.

There are two broad aspects of RCS minimization techniques. One falls under the effort to shape the airframe, and covers the geometric design considerations that are taken into account when aiming for a low RCS. The other principle is referred to as radarabsorbent materials and is concerned with the materials that help to reduce the reflectivity of the airframe, as well as the structures that will support these materials and integrate them into the airframe (often referred to as Radar-absorbent structures. These two axes are of course not taken in isolation during the design; trade-offs often have to be made between them.

The stealth designer's mission starts with the same words as the physician's Hippocratic Oath: "First, does no harm." There are certain popular design features that are incompatible with low RCS: ¾ Engines in external pods or hung on pylons, such as those of the B-52, provide many excellent retro-reflectors. Their firststage compressor blades are also prime reflectors on their own2. ¾ Vertical stabilizers and slab-sided bodies (particularly when combined with the unavoidable horizontal wings) are ruled out. ¾ External stores are a strong no-no, as they create multiple hard-to-control reflections on their own. The designers can, however, take advantage of the fact that the most threatening radar beams will illuminate his aircraft from a point that is much more distant horizontally than vertically. Most radar waves will impinge on the target from a narrow range of shallow angles. If as much as possible of the surface of the aircraft is highly oblique to those angles, the RCS will be low because most of the energy will be scattered. This can be accomplished by blending the airplane's bulky body into the wing. Aircraft shaping is useful over a wide range of radar frequencies but over a limited range of aspect angles. The forward cone is of greatest interest and hence, large returns can be shifted out of this sector into the broadside directions.


Engines produce strong radar reflections and have to be concealed in some way, while permitting air to reach the engine efficiently. This tends to demand a long, complex inlet system, which takes up a great deal of internal space. The prohibition on external stores puts further pressure on internal volume. There are a number of basic methods of using geometry to control the way the airframe will reflect and scatters a radar wave. One is to make the shape flat or rectilinear and at the same time oblique to the incoming waves, as already mentioned, so that reflection will never go toward the likely location of a receiver. This is the principle behind the "faceted" F-117A. Another trick, similar but antipodal to the first one in principle, is to shape the airframe in such way that, instead of having the reflected energy scatter in all directions (and thus a portion of it being always picked-up by the enemy radar), it will bounce back on a very limited number of directions, maybe only one or two. This means that an enemy radar will get only one strong reflection (a spike) when the spatial geometry is just perfect, but virtually no reflection at all in any other instance. Unless the radar beam makes two ninety-degree angles to one of the surfaces (which is unlikely, except at ex-tree look-down angles), the aircraft may remain undetectable. A good example is the frontal wing surface of the B-2. A radar which illuminates the B-2 from anywhere in the front quadrant would produce only two strong "glint" reflections, one from each wing, and these two spikes are impossible to generate concurrently. This method is extensively used in numerous stealthy and semi-stealthy


Aircraft in order to minimize RCS. It does have the drawback that, in order to make a useful difference, pretty much every straight line on the entire airframe has to be aligned in the direction of the few selected spikes, thus posing extra headaches for the design of everything from landing gear doors to access panels to stabilizers to fasteners etc. etc. (Pete West/AIR International) Another method is to use a compact, smoothly blended external geometry to achieve a continuously varying curvature. Most conventional aircraft have constant-radius curves for simplifying the design and manufacturing processes. However, a constant curve is an isotropic scatterer: It reflects energy equally in all directions, an effect which has been likened to the rear window of a Volkswagen Beetle car, gleaming in the sun regardless of the incoming angle. A varying curvature is similar to a sea-shell helix: The curves have an ever-changing circle radius, as though they are sections of a spiral rather than arcs of a circle, and thus do not reflect energy in the usual predictable way. Rather, they tend to absorb the energy as it scatters towards the interior of the curve itself (in a fashion similar to the manner in which hi-fi sound speakers absorb superfluous sound in their internal helix structures). This careful shaping technique can be observed in the over wing engine nacelles of the B-2, as well as the basic fuselage cross-section of the Rafale. This method, however, requires far greater predictive ability and enormously increased computational capacity over the much simpler faceting. It is thus barely surprising that the F-117, an aircraft almost completely based on faceting, has been operational since the early 80s while more

complex designs were significantly later in the pipeline. Eliminating the radar reflections of the cockpit also results in a useful RCS

reduction. Techniques here usually include the application of several absorbent layers on the canopy/windshield walls. This is applicable both on stealthy airframes and conventional assets like the F-16. The amount of precision engineering necessary for exploiting VLO geometry is often overlooked or underappreciated. During the F-117â„¢sfull-scale development phase one of the prototypes was suddenly found to have a much higher RCS than expected. After an inch-by-inch examination of the airframe, it was discovered that a single screw had not been tightened 100% into the fuselage and it was the culprit for the increased radar reflection.
Following is a summary list of shaping laws for VLO designs: ¾ Avoid flat or re-entrant surfaces likely to be vertical to the incoming radiation. This is one of the primary reasons for the highly-angled stabilizers on both the F-22 and the JSF. ¾ Bury the engines, with air intakes and exhausts located over the from the major illuminating radar threat. Use a screen over the air intake, together with gauzes, vanes and deflectors within the diffuser duct. This is aptly demonstrated by general placement of the engines on theF-117, and in particular their grill-type covers. ¾ Give the inlet duct an 'S' shape to hide the duct. The Euro fighter Typhoon follows this rule with its single inlet shape. ¾ Avoid variable geometry intakes to minimise reflections from the gaps and steps of the compression ramps and eliminate bypass doors by finding other methods to control intake

airflow. The Rafael has deliberately a fixed (though anything but simple) inlet system, and the EF-Typhoon also includes small moving lips on the inlet leading edge in order to deal with excess airflow Without the need for bypass doors. ¾ Carefully shape the inlet lips (including sharpness) and nozzles by sweeping to align with major surfaces. Various modern designs follow this paradigm including the B-2, F-22, JSF, F/A-18E, Rafael etc. ¾ Design and manufacture any internal structure within radartransparent skins to reduce reflections in given directions. The cumulative effect of the interior reflections could easily exceed the radar return from a metallic skin. ¾ Use RAM wherever appropriate (e.g. leading edges, bulkhead and black boxes within radar cavity, on the interior of the inlet and on metallic structure under radar-transparent skins). ¾ Use a very high quality of manufacture to avoid gaps, holes, etc., since attention n to detail is vital. ¾ Cover gun port, inlet and exhaust of auxiliary power unit (APU) when not in use. The covert gun port is probably going be a feature of both the F-22 and the Rafael. ¾ Long wavelengths are less affected by the small details of shape and absorbent structures. Though current stealth technology may frustrate modern air defence radars the same is not true of older long wavelength (lower frequency) radars that have been kept operational worldwide. Some countries were prompted to do this not because of low RCS aircraft but to avoid over-reliance on

one type of radar and to overlap many different types to make their air defence system more difficult to jam. However, all airborne targets detected by long range surveillance radars must eventually be passed over to fighters or SAM sites. These are equipped with high frequency tracking and targeting radars that can be defeated by proper shaping and RAM. How effectively surveillance radar systems could hand over to shorter wavelength sensors is questionable and is one of the main arguments for investing in stealthy designs.

The aircraft still keeps reflecting enough energy to be pickedup at a tactically dangerous range. What now The next step is to use certain special materials to further attenuate radar waves. The term Radar-absorbent materials (RAM) applies to a whole class of materials in different forms which redesigned specifically to do this. Radar-absorbing structure (RAS) involves building these materials into practical loadbearing structures and shapes for the target vessel (in this case, aircraft). All RAM and RAS work on the same basic principle. Radar signals are electromagnetic waves, and thus bounce efficiently off any conductive object. However, the electromagnetic characteristics of different objects and materials are not the same. One of the best demonstrations of this principle is the domestic microwave oven. The microwave oven is based on a magnetron tube, a radarwave generator which was invented during World War II and which made British and American radars decisively superior to their German counterparts. It is hardly a coincidence that one of the major US brands of microwave ovens is made by a division of Raytheon, a well-known manufacturer of radars and radar-related systems. The device was originally invented by radar engineers who had observed its effects. While some substances reflect radar waves efficiently, others do not. The difference lies in their molecular structure. Some materials, including many organic substances (such as food), include "free electrons" in their molecular chains. Electrical

Engineers call them "lossy." Radars, like radios and televisions, operate on a given wavelength; in the case of most radar, the wavelength is measured in gigahertz (GHz), or billions of cycles per second. When a radar transmitter illuminates an object with such characteristics, the free electrons are forced to oscillate back and forth at the frequency of the radar wave. But these particles have friction and inertia, however tiny, and the process is not one hundred percent efficient. The radar's energy is transformed into heat, and the chicken is cooked or (depending what modern folk myth is being repeated) the poodle explodes or your

underwear catches fire. These substances are "lossy dielectrics" because they are non-conductive. RAM has been available for years in many forms, and many of them are not even classified. Most such material consists of an active ingredient”a dielectric, such as carbon, or magnetic ferrites”which is moulded into a non-lossy dielectric matrix, usually a plastic of some kind. Lockheed developed a lossy plastic material for the A-12/SR-71, as well as the hypersonic D-21 drone. Loral has long provided a material that resembles a ferrite-loaded neoprene, which is used in the inlet ducts of the B-l. A ferrite-based paint known as "iron ball" is used on the U-2 and SR-71. Some basic limitations apply in some degree to all kinds of RAM: All of them absorb only a portion of the radar energy and reflect the rest. A given type of RAM is also most effective at a certain frequency and less so at others. Therefore, comprehensive spectrum coverage demands a combination of different materials, often bulky. The effectiveness of RAM varies with the angle of the incident radar wave. Generally, the thickness and weight of RAM increases with its effectiveness. This means that a large aircraft is generally easier to be fitted with a broad-coverage RAM collection than a smaller aircraft. This is one of the reasons that the B-2 is far stealthier than the F-117. Many types of RAM are sensitive to adverse weather condition. This was of particular headache to early B-2 airframes, which were deemed unsuitable for operations from foreign bases partially because of the material™s sensitivity to rain. Reportedly a new type of material has been installed more recently and the

problem has been rectified. RAS is more complicated, more recent in origin and more Classified. However, the essential principle seems to be a "defence in depth" against radar waves, to achieve a high degree of absorption over a wide bandwidth. Except in a case of dire need, nobody is going to cover an airplane with a thick, solid skin. One alternative means of providing the necessary depth is to use "honeycomb structure. Honeycomb is so called because it looks like the natural honeycomb. Its core is made of a light fibre material, such as Du Pont's Nomex, bonded together in such a way that it forms a flexible slab with hexagonal passages from front to back. Load-bearing skins, which can be relatively light and flexible, are then bonded to the front and back of the slab. The result is a panel across which you can drive a truck without breaking it, and an aircraft skin which needs no stiffeners or stringers. From the viewpoint of RAS, the advantage of honeycomb is depth without proportionate weight. A honeycomb RAS might consist of an outer skin of Kevlar/epoxy composite, which is transparent to radar, and an inner skin of reflective graphite/epoxy. The Nomex core, between them, would be treated with an absorbent agent, increasing in density from front to rear of the honeycomb. The front-face reflection of such an RAS would be minimal. As the radar wave encounters the thinly spread absorber on the outer edges of the core, a small part of its energy is absorbed and a small part scattered. As the wave proceeds through the core, it encounters more densely loaded core material which both absorbs and reflects

more energy. But before the reflected energy can reach free space again, the outermost layer of absorber once more attenuates it. It is an electromagnetic Roach Motel; radar waves check in, but they don't check out. A properly configured RAS layer can also reduce the radar reflection by passive cancellation. The way this works is that the external skin may reflect back part of the energy pulse (E1), but the rest will be redirected through refraction into the internal of the airframe and then bounced blackout against the exactly opposite phase (E2). Thus hopefully the two radar returns will cancel each other out. The problem with this method is that, in order to work, the distance that the internally-refracted radiation will travel (i.e. the depth of the under-skin layer) must be very precisely tuned to match the one-half of the radiationâ„¢s wavelength (in order to reverse the phase of the outgoing signal). This of course means that the method will work only against a very narrow frequency spectrum, and that it will be impractical against low-frequency (large wavelength) radar.
Another popular structure that follows the gradual absorption principle is extensively used on the leading and trailing edges of stealthy airframes. The idea is that the external skin is composed of a high-frequency ferrite absorber, while the interior begins with a low-absorption layer and thickens back into gradually deeper and more absorbent layers. This has an effect similar to the honeycomb structure, in trapping and successively absorbing an ever-growing amount of the energy.


A method of passive cancellation of the reflected radar signal was already discussed, together with its shortcomings. A far more flexible but also more complex approach is to actively replicate the incoming signal and reverse its phase in order to achieve the same effect. Since it involves active emissions, this technique is more appropriately classified as part of the active jamming effort, but is nevertheless noteworthy with regards to stealth because its net effect is the reduction (or even complete elimination)of the amplitude of the reflected signal, and thus the reduction of the targeted objectâ„¢s apparent RCS. Just how complicated it is to cancel a reflected radar signal can be reasoned from the fact that the original incoming signal from the radar will be reflected from many spots on the aircraft's body. Each spot will produce an individual reflection with its own unique amplitude and phase. The amplitude of the reflection would depend on many factors, such as incidence angle, particular type of material, geometrical form of a certain location on the aircraft's body that produced the reflection and some other factors. The phase shift will be dictated by the wavelength of the radar signal and the location (and geometrical form) of the particular spot that produced the reflection in question. The enemy radar does not, however, receive all of the reflected variations of the original signal as separate entities. It either selects the strongest return signal, or

averages several strongest reflections. This simplification can be used to the advantage of the aircraft, since it will only need two antennas to transmit a simulated return signal averaged over the length of the aircraft. The return signal, picked Up by the radar, would look somewhat chaotic, consisting of background noise and the main return spikes. These spikes are, presumably, the main targets of active cancellation (here again we see the importance of first shaping the aircraft to minimize and actively control the formed spikes). It is important to understand, however, that in case of a real-world effective system we are dealing with an immensely complicated issue. Something that can be popularly explained with a single wave sinusoidal signal will become progressively more complex in real-life situations. Active cancellation as a working method places strong emphasis on several things to happen properly: ¾ The aircraft has to have a system capable of analysing the incoming signal in real-time and replicating its characteristics faithfully enough to disguise itself as the true signal, before its phase is reversed. Analysing the signal on first contact is not enough; the enemy is likely to shift the emission characteristics of the radar equipment within its physical limits (PRF, signal frequency etc.) throughout the duration of the detection/tracking attempt. Likewise therefore, the analysis process has to be repeatedly performed as long as the aircraft remains within the detection envelope of the emitter.

¾ The phase-reversed signal must be transmitted with just enough power to match the real signal reflected back at the receiver. Careful power management is crucial here; a clever software algorithm in a modern radar system may try to check the signal strength difference between incoming spikes and reject those that seem a bit too powerful for the given situation. The purpose here is deception, not to flood the other guy™s scope with whitenoise static. ¾ The bearing of the incoming signal must be determined accurately so that the fake reflection will be reflected at the original transmitter and nowhere else. This also implies a very accurate laying of the onboard beamtransmitter for the fake signal, as well as rapid beamsteering for circumstances where the airframe™s attitude and velocity vector is rapidly changing(e.g. while manoeuvring to avoid enemy fire). This is easier said than done: it is hard enough to precisely locate (in both azimuth and elevation) the emitter in order to point the fake signal only there and nowhere else; let alone keeping the beam on-target while the aircraft is performing anything from routine subtle navigation course adjustments to gut-wrenching missile-avoidance manoeuvres. For this reason, only an electronic-scan array is practically suitable for emitting the fake signal. Despite this tall order of requirements, active cancellation offers several advantages compared to more conventional jamming techniques. Both barrage and deception jamming cannot avoid

tipping-off the enemy on something going-on; here, however, the element of surprise is fully retained for exploitation. A significantly less amount of transmission power is required, only enough to replicate the weak energy reflection back to the enemy emitter; thus the overall system can be light and compact enough to be fitted to aircraft hitherto unable to benefit from the existence of heavyweight jammers. This also means that other onboard avionics are significantly less hampered by RF-interference while active cancellation is in progress (those who recall the EWavionics interference troubles of aircraft such as the B-1, the EF111, the Su-27 or the EA-6 will certainly appreciate this). The Spectra integrate dew suite on the Rafael fighter is a prime example of active cancellation. All the elements described above are in place: sensitive and precise interferometers for passive detection & localization, powerful signal processors as part of the overall avionics suite, and conformal electronic-scan arrays dedicated to the transmission of EW signals. Combining a semistealthy airframe structure (treated with RAM in significant quantities) with various traditional forms of jamming plus active cancellation can result in an airborne weapons platform of vastly Lower RCS than one would expect from an otherwise ordinarylooking canard-delta aircraft.

There have been speculations that the Russians may be using this technique on their S-37 Berkut and possibly MiG 1.42 prototype fighters. It is also believed that the ZSR-63 defensive aids equipment installed on B-2 bombers may be using this technique. It is not clear whether the F-22and F-35 are going to employ active cancellation in their EW arsenal. Certainly the pieces are in place hardware-wise: An added bonus of the AESA radars fitted on both aircraft is that the operation of multiple RF beams in parallel(as opposed to the single beam of mechanical-scan and passive electronic-scan systems) enables the radar to scan, track and jam at the same time. It is however unknown if the relevant software is going to be in place to exploit this capability. Certainly the F-22 is more than capable of performing this function with its ultra-sensitive ALR-94 receivers and ample onboard processing power, in addition to the large AESA set. Whether the significantly smaller and thus volume/weightchallenged F-35 will be able to perform the function on its own hardware remains to be seen.

A more recent approach to the art of VLO is the employment of plasma fields. Plasma physics as a potential aerospace technological branch has been long under research, mainly for the purposes of space borne propulsion and thermal heating for endo/exo-atmosphericspacecraft3. The effect of plasma as an RFsignal inhibitor is well known for decades now, as the communications black-out that a space vehicle encounters during re-entry is caused by the shielding effects of plasma. This builds naturally in front of the spacecraft as it hits the Earth's atmosphere and compresses the air to high temperatures. According to JED, Russia is working to develop plasma-cloudgeneration technology for stealth applications and achieved highly promising results, reportedly reducing the RCS of an aircraft by a factor of 100. Russian research into plasma generation is spearheaded by a team of scientists led by Anatoliy Korotoyev, director of Keldysh Research Center. The institute has developed a plasma generator weighing only 100 kg, which could easily fit onboard a tactical aircraft. For the system to work there has to be an energy source on the aircraft that ionizes the surrounding air, probably at the leading surfaces. Since the resulting ions are in the boundary layer of the aircraft, they follow the airflow around the plane. But the system is not without drawbacks. First, the amount of power required is quite high, so it will likely only be activated when enemy radar is detected. The other is that the plasma also blocks

the radar of the aircraft being protected, necessitating holes in the plasma field to look through it. The plasma generator was tested first on flying models and then on actual aircraft. The new Su-27IB/Su-34 strike aircraft (known in export - certainly without the plasma generator - as the Su32FN) utilizes the system and is likely the first production combat aircraft with this critical technology. Work on plasma generation is not the purview of Russia alone, though. In the US, for example, research in this field is being conducted by Accurate Automation Corporation (Chattanooga, TN) and Old Dominion University (Norfolk, VA). French companies Dassault (Saint-Cloud, France) and Thales (Paris, France) are jointly working in the same area as well. “ (Michal Fiszer and Jerzy Gruszczynski) The US Navy has been experimenting (through third-party development) with a plasma stealth antenna developed for use on VLO vessels & aircraft. The system employs arrays of multiple U-shaped glass tubes filled with low-pressure gas (somewhat equivalent to fluorescent tubes). This antenna is energized and acts as a highly-directional, electronically steered transmitter/receiver in pretty much the same principles as an AESA system. When de-energized, the antenna is virtually transparent to hostile electromagnetic signals. One of the problems with such a system is its vulnerability to resonant signals at the tubes™ self-frequency

Passive IR detection devices rely on the fact that every atom of matter, including clouds and rain, continuously sends out electromagnetic radiation at an IR wavelength which corresponds to its temperature. It is necessary to think in terms of absolute (Kelvin) temperature. Even though a certain object may be regarded as cold, a snowflake for example at 0°C, on the absolute temperature scale it is at 273K. For aircraft detection, IR seekers look for contrasts between hot parts on the airframe such as jet pipes and surfaces subject to kinetic heating, and the background radiation. In designing IR detectors several things have to be considered: the range of wavelengths emitted by the target, the likely wavelength of the most intense radiation, the ways these wavelengths are affected by the atmosphere; and because the maximum contrast is desired, the character of the likely background radiation. Many IR devices operate in the 8-13 micron band since this is the most IR-transparent band in the atmosphere. In engine exhausts, carbon dioxide produces most of the IR signature at 4.2 microns, so modern IR sensors can 'see' at two different wavelengths, (medium: 3-5 microns and long:8-14 microns) to provide good target discrimination. The engine exhausts are the primary battlefield in the war against infrared detection. There are many types of infrared sensor in service, and their different capabilities are sometimes confused. The basic fact is that the atmosphere absorbs infrared energy. At a range of a few miles, a small infrared sensor can receive enough energy to produce TV-type image of the scene; at greater ranges,

this capability is much diminished. Most medium-to-long-range systems do not detect the Infrared emissions from the aircraft itself, but the radiation from the hot air and water vapour emitted by its engines the radiated IR energy is proportional to the fourth power of absolute temperature. With engine turbine entry temperatures (TETs) currently at around1, 900K and rising, the back end of a military aircraft is the greatest source of IR radiation. With afterburner on, it becomes more so. Moderate stagnation temperatures are inevitable on leading edges of a fighter's airframe due to kinetic heating at high Mach numbers. As the stealthiest of fighterâ„¢s increases so their missiles' exhaust plumes play a greater role in early detection. Lower visibility plumes will minimise detection of both launch platform and missile. The key to degrading the performance of IRST systems is to ensure that the exhaust dissipates as quickly as possible after leaving the aircraft. For example, the engines can be fitted with flow mixers to blend the cold bypass air with the hot air that passes through the combustor and the turbine. The exhausts geometry can be adapted to a wide and flat shape rather than the traditional round, increasing the mixing rate (but probably reducing thrust efficiency). Furthermore, the interaction between the exhaust stream and the airflow over the aircraft can be engineered to create an additional vortex which further promotes mixing. There are several other methods to reduce the IR signature:

¾ Have the ability to super cruise (cruise at supersonic speeds without afterburning) to restrict the temperature of the nozzle. Moreover, super cruising allows the pilot to engage on his terms, increases weapons' envelopes, minimises exposure to SAM threats and not only stretches combat radius but forces an adversary to expend his own fuel in order to get his aircraft to an Energy states where he can engage it. ¾ Use a high bypass ratio (BPR) engine to mix in cold air to reduce exhaust temperature. That said, a bypass ratio greater than about 0.4 conflicts with the requirement of the high dry thrust to achieve super cruise. ¾ Use a curved jet pipe to mask the hot turbine stages. ¾ Use two-dimensional nozzles (which increase the surface area of the exhaust plume) or ejector nozzles (which mix in ambient air) to increase the rate of cooling. ¾ Increase cooling of the outer skin of the engine bay or insulation to reduce temperature of the airframe skin. ¾ Use a curved air intake to mask, to some extent, forward emissions from the engine. ¾ Limit maximum supersonic speed to reduce IR signature due to kinetic heating.

The benefits of stealth apply not only to platforms but to a lot of weapons as well. Anti-surface munitions like the JSOW, JASSM, Apache/SCALP/Storm Shadow, Taurus/KEPD and many others are specifically shaped and treated to minimize their radar and IR signatures. This has two useful payoffs: On the one hand, the weapon itself becomes less vulnerable to enemy defensive systems, which means that fewer of the weapons launched will be shot down before reaching their target(s). This in turn means that fewer weapons and their parent platforms need to be allocated to any given mission, and finally the end result is that a greater number of targets can be confidently engaged with a given force. The other benefit is the advantage of surprise and its effect in cases where shrinking the enemyâ„¢s available reaction time is of the essence. A good example of such a situation is a typical OCA strike against an airfield. If non-stealthy strike aircraft or stand-off weapons are used, it is quite likely that they will be detected far enough out that the enemy will have some time available (even just 4-5 mins will do) to gets many of his ready-to-fly aircraft in the air and fly them somewhere else to preserve them. If the aircraft being flushed include armed hot-pad alert fighters (a common protective measure) these can immediately and actively contribute to the baseâ„¢s defence against the incoming attack. Contrast this with a situation where, as a result of using stealthy

weapons and/or platforms, the base is caught virtually napping and the attack is detected so perilously close that the enemy Has no time to get anything in the air but instead can only rely on his ground-based terminal defences. This can mean the difference between the base suffering little or no damage and being virtually obliterated.
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Presented By:
Plot no:47, Phase-2
Vijayapuri colony
Vignan Institute of Technology and Aeronautical Engineering
Abstract :
Stealth aircraft are aircraft that use stealth technology to interfere with radar detection as well as means other than conventional aircraft by employing a combination of features to reduce visibility in the infrared, visual, audio, and radio frequency (RF) spectrum..
While no aircraft is totally invisible to radar, stealth aircraft prevent conventional radar from detecting or tracking the aircraft effectively, reducing the odds of an attack. Stealth is accomplished by using a complex design philosophy to reduce the ability of an opponent's sensors to detect, track, or attack the stealth aircraft. This philosophy also takes into account the heat, sound, and other emissions of the aircraft as these can also be used to locate it.
Stealth is the combination of passive low observable (LO) features and active emitters such as Low Probability of Intercept Radars, radios and laser designators. These are usually combined with active defenses such as Chaff, Flares, and ECM.
Introduction :
Stealth or low observability (as it is scientifically known) is one of the most misunderstood and misinterpreted concepts in military aviation by the common man. Stealth aircraft are considered as invisible aircraft, which dominate the skies. With anadditional boost from Hollywood action movies, stealth is today termed as the concept invincibility rather than invisibility. Though, the debate still continues on whether stealth technology can make an aircraft invincible it was found that stealth aircraft are detectable by radar.
The motive behind incorporating stealth technology in an aircraft is not just to avoid missiles being fired at is but also to give total deniability to covert operations. This is very much useful to strike targets where it is impossible to reach. Thus we can clearly say that the job of a stealth aircraft pilot is not to let others know that he was ever there.
What is stealth ? :
In simple terms, stealth technology allows an aircraft to be partially invisible to Radar or any other means of detection. This doesn't allow the aircraft to be fully invisible on radar. Stealth technology cannot make the aircraft invisible to enemy or friendly radar. All it can do is to reduce the detection range or an aircraft. This is similar to the camouflage tactics used by soldiers in jungle warfare. Unless the soldier comes near you, you can't see him. Though this gives a clear and safe striking distance for the aircraft, there is still a threat from radar systems, which can detect stealth aircraft.
The Russian 1R13 radar system is very much capable of detecting the F-117 "Night Hawk" stealth fighter. There are also some other radar systems made in other countries, which are capable of detecting the F-117.the Gulf war the Iraqis were able to detect the F-117 but failed to eliminate its threat because of lack of coordination. The most unforgettable incident involving the detection and elimination of a stealth aircraft was during the NATO air-war over Yugoslavia. This was done by a Russian built "not so advanced" SAM (possibly the SA-3 or SA-6). The SAM system presumably used optical detection for target acquisition in the case.
Development of stealth technology likely began in Germany during WWII. Well-known modern examples of stealth aircraft include the United States' F-117 Nighthawk (1981“2008), the B-2 Spirit "Stealth Bomber", the F-22 Raptor, and the F-35 Lightning II. and the Indian/Russian Sukhoi PAK FA.
Working of stealth technology :
The concept behind the stealth technology is very simple. As a matter of fact it is totally the principle of reflection and absorption that makes aircraft "stealthy". Deflecting the incoming radar waves into another direction and thus reducing the number of waves does this, which returns to the radar. Another concept that is followed is to absorb the incoming radar waves totally and to redirect the absorbed electromagnetic energy in another direction. Whatever may be the method used, the level of stealth an aircraft can achieve depends totally on the design and the substance with which it is made of.
RAS or Radar absorbent surfaces are the surfaces on the aircraft, which can deflect the incoming radar waves and reduce the detection range. RAS works due to the angles at which the structures on the aircraft's fuselage or the fuselage itself are placed. These structures can be anything from wings to a refueling boom on the aircraft. The extensive use of RAS is clearly visible in the F-117 "Night Hawk". Due to the facets (as they are called) on the fuselage, most of the incoming radar waves are reflected to another direction. Due to these facets on the fuselage, the F-117 is a very unstable aircraft.
The concept behind the RAS is that of reflecting a light beam from a torch with a mirror. The angle at which the reflection takes place is also more important. When we consider a mirror being rotated from 0o to 90o, the amount of light that is reflected in the direction of the light beam is more. At 90o, maximum amount of light that is reflected back to same direction as the light beam's source. On the other hand when the mirror is tilted above 90o and as it proceeds to 180o, the amount of light reflected in the same direction decreases drastically. This makes the aircraft like F-117 stealthy.
Radar absorbent surfaces absorb the incoming radar waves rather than deflecting it in another direction. RAS totally depends on the material with which the surface of the aircraft is made. Though the composition of this material is a top secret. The F-117 extensively uses RAM to reduce its radar signature or its radar cross section.
The RAS is believed to be silicon based inorganic compound. This is assumed by the information that the RAM coating on the B-2 is not water. This is just a supposition and may not be true. What we know is that the RAM coating over the B-2 is placed like wrapping a cloth over the plane. When radar sends a beam in the direction of the B-2, the radar waves are absorbed by the planeâ„¢s surface and are redirected to another direction after it is absorbed. This reduces the radar signature of the aircraft.
IR :
Another important factor that influences the stealth capability of an aircraft is the IR (infrared) signature given out by the plane. Usually planes are visible in thermal imaging systems because of the high temperature exhaust they give out. This is a great disadvantage to stealth aircraft as missiles also have IR guidance system. The IR signatures of stealth aircraft are minute when compared to the signature of a conventional fighter or any other military aircraft.
If reducing the radar signature of an aircraft is tough, then reducing the IR signature of the aircraft is tougher. It will be like flying a plane with no engines. The reduced IR signature totally depends on the engine and where the engine is placed in an aircraft.
Engines for stealth aircraft are specifically built to have a very low IR signature. The technology behind this is top secret like others in stealth aircraft. Another main aspect that reduces the IR signature of a stealth aircraft is to place the engines deep into the fuselage. This is done in stealth aircraft like the B-2, F-22 and the JSF. The IR reduction scheme used in F-117 is very much different from the others. The engines are placed deep within the aircraft like any stealth aircraft and at the outlet, a section of the fuselage deflects the exhaust to another direction. This is useful for deflecting the hot exhaust gases in another direction.
Methods of avoiding detection (Reflected waves) :
There are some more methods by which planes can avoid detection. These methods do not need any hi-tech equipment to avoid detection. Some of them have been used for years together by pilots to avoid detection.
One of the main efforts taken by designers of the stealth aircraft of today is to carry the weapons payload of the aircraft internally. This has shown that carrying weapons internally can considerably decrease the radar cross-section of the aircraft. Bombs and Missiles have a tendency to reflect the incoming radar waves to a higher extent. Providing missiles with RAM and RAS is an impossible by the cost of these things. Thus the missiles are carried in internal Bombayâ„¢s which are opened only when the weapons are released.
Aircraft has used another method of avoiding detection for a very long time. Radars can use the radar waves or electro-magnetic energy of planes radar and locate it. An aircraft can remain undetected just by turning the radar off.
In case of some of the modern stealth aircraft, it uses its wingman in tandem to track its target and destroy it. It is done in the following way. The fighter, which is going to attack moves forward, the wingman (the second aircraft) on the other hand remains at a safe distance from the target which the other fighter is approaching. The wingman provides the other fighter with the radar location of the enemy aircraft by a secured IFDL (In Flight Data Link). Thus the enemy radar is only able to detect the wingman while the attacking fighter approaches the enemy without making any sharp turns. This is done not to make any sudden variations in a stealth aircraft's radar signature. Thus the fighter, who moves forward, is able to attack the enemy without being detected.
Plasma Stealth :
Plasma stealth technology is what can be called as "Active stealth technology" in scientific terms. This technology was first developed by the Russians. It is a milestone in the field of stealth technology. The technology behind this not at all new. The plasma thrust technology was used in the Soviet / Russian space program. Later the same engine was used to power the American Deep Space 1 probe.
In plasma stealth, the aircraft injects a stream of plasma in front of the aircraft. The plasma will cover the entire body of the fighter and will absorb most of the electromagnetic energy of the radar waves, thus making the aircraft difficult to detect. The same method is used in Magneto Hydro Dynamics. Using Magneto Hydro Dynamics, an aircraft can propel itself to great speeds.
Plasma stealth will be incorporated in the MiG-35 "Super Fulcrum / Raptor Killer". This is a fighter which is an advanced derivative of theMiG-29 . Initial trials have been conducted on this technology, but most of the results have proved to be fruitful.
Detection methods for stealth aircraft :
Whenever a technology is developed for military purposes, another technology is also
developed to counter that technology. There are strong efforts to develop a system that can counter the low obervability of the fifth generation stealth aircraft. There are ways of detection and elimination of a low observable aircraft but this doesn't give a 100% success rage at present.
On a radar screen, aircraft will have their radar cross sections with respect to their size. This helps the radar to identify that the radar contact it has made is an aircraft. Conventional aircraft are visible on the radar screen because of its relative size. On the other hand, the relative size of a stealth aircraft on the radar screen will be that of a large bird. This is how stealth aircraft are ignored by radar and thus detection is avoided.
A proven method to detect and destroy stealth aircraft is to triangulate its location with a network of radar systems. This was done while the F-117 was shot down during the NATO offensive over Yugoslavia.
A new method of detecting low observable aircraft is just over the horizon. Scientists have found a method to detect stealth aircraft with the help of microwaves similar to the ones emitted by the cell phone towers. Nothing much is known about this technology, but the US military seems to be very keen about doing more research on this.
Stealth aircraft of yesteryears, Today and Tomorrow :
Stealth technology is a concept that is not at all new. During the Second World War, allied aircraft used tin and aluminum foils in huge numbers to confuse German radar installations. This acted as a cover for allied bombers to conduct air raids. This method was later used as chaffs by aircrafts to dodge radar guided missiles.
The first stealth aircraft was the F-117 developed by Lockheed Martin. It was a top-secret project and implimentation developed by its Skunk Works unit. The F-117 was only revealed during the late 80s and then saw action in the Persian Gulf.
In due course of time the B-2 was developed as a successor to the B-2. Though both of them serve different purposes, the B-2 went a step ahead of the F-117. The B-2 was developed to deliver nuclear weapons and other guided and unguided bombs. On the other hand the F-117 was developed to deliver its precision laser guided bombs.
Another stealth aircraft, which made a lot of promises and in the end ended up in a trashcan, was the A-12. It was a fighter that was designed to replace the F-14 and F-18 in the future. The capabilities of this aircraft were boasted to such an extent that the project and implimentation ended up in a big mess. Billions of dollars were wasted for nothing.
Stealth technology became famous with the ATF contest. The Boeing-Lockheed YF-22 and the McDonell Douglas-Grumman YF-23 fought for the milti-billion contract to build the fighter that would take the USAF into the fifth generation fighter era. The Boeing-Lockheed won the contract and the F-22 was approved to be the replacement for the F-15 "Eagle" interceptor.
America now has a competitors, Russia decided to respond to the development of the F-22 by making the Su-47 (S-37) "Berkut" and the MiG-35 "Super Fulcrum / Raptor Killer". These fighters were developed by the two leading aviation firms in Russia Sukhoi and Mikhoyan Gurevich (MiG). The future of these project and implimentations totally depends on the funding which will be provided to the Russian defense sector. There are some hopes of increase in the funding to these project and implimentations as countries like India have started providing funds and technical assistance for these project and implimentations.
Another competition that soon came into the spotlight after the ATF competition was the JSF. This time Boeing developed the X-32 and the Lockheed.its X-35. With the experience gained from developing the F-22, they were tasked with making a replacement for the F-16. This saw great technological advances, as they had to make the first operational supersonic VSOL aircraft. Lockheed martin took the technical assistance of Russian scientists who developed the Yak-141. The Yak-141 is the first supersonic VSTOL aircraft. In the end the Lockheed team with its X-35 won the contract and the fighter was re-designated as the F-35.
Many project and implimentations remain over the horizon that will use stealth technology as its primary capability. They come from some of the most unlikely contenders. These project and implimentations include the Euro JSF, which will be designed by the team that developed the EF-2000. Russia is stepping forward with its LFS project and implimentation with the S-54 and other designs. Two new entries into this field will be India and China. India will be introducing its MCA, which is a twin engine fighter without vertical stabilizers. This fighter will use thrust vectoring instead of rudders. China will be introducing the J-12 (F-12/XXJ). This fighter that is similar to the F-22.
Avantages of Stealth :
To date, stealth aircraft have been used in several low- and moderate-intensity conflicts, including Operation Desert Storm, Operation Allied Force and the 2003 invasion of Iraq. In each case they were employed to strike high-value targets that were either out of range of conventional aircraft in the theater or were too heavily defended for conventional aircraft to strike without a high risk of loss. In addition, because the stealth aircraft do not have to evade surface-to-air missiles and anti-aircraft artillery over the target they can aim more carefully and thus are more likely to hit the target and cause less collateral damage. In many cases they were used to hit the high value targets early in the campaign, before other aircraft had the opportunity to degrade the opposing air defense to the point where other aircraft had a good chance of reaching those critical targets.
Disadvantage of stealth technology :
Stealth technology has its own disadvantages like other technologies. Stealth aircraft cannot fly as fast or is not maneuverable like conventional aircraft. The F-22 and the aircraft of its category proved this wrong up to an extent. Though the F-22 may be fast or maneuverable or fast, it can't go beyond Mach 2 and cannot make turns like the Su-37.
Another serious disadvantage with the stealth aircraft is the reduced amount of payload it can carry. As most of the payload is carried internally in a stealth aircraft to reduce the radar signature, weapons can only occupy a less amount of space internally. On the other hand a conventional aircraft can carry much more payload than any stealth aircraft of its class.
Whatever may be the disadvantage a stealth aircraft can have, the biggest of all disadvantages that it faces is its sheer cost. Stealth aircraft literally costs its weight in gold. Fighters in service and in development for the USAF like the B-2 ($2 billion), F-117 ($70 million) and the F-22 ($100 million) are the costliest planes in the world. After the cold war, the number of B-2 bombers was reduced sharply because of its staggering price tag and maintenance charges. There is a possible solution for this problem. In the recent past the Russian design firms Sukhoi and Mikhoyan Gurevich (MiG) have developed fighters which will have a price tag similar to that of the Su-30MKI. This can be a positive step to make stealth technology affordable for third world countries.
Conclusion :
Stealth technology is clearly the future of air combat. In the future, as air defense systems grow more accurate and deadly, stealth technology can be a factor for a decisive by a country over the other. In the future, stealth technology will not only be incorporated in fighters and bombers but also in ships, helicopters, tanks and transport planes. Ever since the Wright brothers flew the first powered flight, the advancements in this particular field of technology have seen staggering heights. Stealth technology is just one of the advancements that we have seen. In due course of time we can see many improvements in the field of military aviation which would one-day even make stealth technology obsolete
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Stealth aircraft


Stealth aircraft are aircraft that use stealth technology to make it
harder to be detected by radar and other means than conventional
aircraft by employing a combination of features to reduce
visibility in the visual, audio, infrared and radio frequency (RF)
spectrum. Well known examples include the United States' F-117
Nighthawk (1980s-2008), the B-2 Spirit "Stealth Bomber," and
the F-22 Raptor.
While no aircraft is totally invisible to radar, stealth aircraft limit
current conventional radar's abilities to detect or track them
effectively enough to prevent an attack. Stealth is accomplished
by using a complex design philosophy to reduce the ability of an
opponent's sensors to detect, track and attack an aircraft.
Modern stealth aircraft first became possible when a
mathematician working for Lockheed Aircraft during the 1970s
adopted a mathematical model developed by Petr Ufimtsev, a
Russian scientist, to develop a computer program called Echo 1.
Echo made it possible to predict the radar signature an aircraft
made with flat panels, called facets. In 1975, engineers at
Lockheed Skunk Works found that an airplane made with faceted
surfaces could have a very low radar signature because the
surfaces would radiate almost all of the radar energy away from
the receiver.
Reduced radar cross section is only one of five factors that
designers addressed to create a truly stealthy design. Designers
also addressed making the aircraft less visible to the naked eye,
controlling radio transmissions, and noise abatement.
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Rabindra Kumar Barik
Regd.No – 0701294331
Electronics Communication Branch


Stealth technology is a sub-discipline of electronic countermeasures which covers a range of techniques used with aircraft, ships and missiles, in order to make them less visible (ideally invisible) to radar, infrared and other detection methods.
The concept of stealth is not new: being able to operate without the knowledge of the enemy has always been a goal of military technology and techniques.
A mission system employing stealth may well become detected at some point within a given mission, such as when the target is destroyed, however correct use of stealth systems should seek to minimize the possibility of detection.

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What is it?
RADAR to detect position of objects.

Counter measures 1. ECM ( e.g. chaff cartridges) 2. Non ECM

Avoiding detection and innovating deception .

RCS reductions, acoustics, thermal and other EM emissions

F 117

B-2 Bomber

F-22 Raptor( From YF 23 Platform)

SR-71 Black bird

HMS Helsingborg

How is it achieved.
Absorbs radar waves or deflects to other directions.

Minimizes heat and other emissions from engine and other spots.

Makes difficult to detect except closely.

Smooth edges maximum radio wave reflectors.

The size of a target's image on radar is measured by RCS (σ )

For a square flat plate of 1m2 area, σ=13982 m2 at 10 Ghz

Mainly plan form alignment.

The leading edges of wing and tail surfaces set at same angles.

Use of re-entrant triangles behind skin.

Distinctive serrations used in external airframes.

Propulsion subsystem shaping.

Now in research is fluidic nozzles for thrust vectoring.

Radar Absorbing Materials (RAM)
RAMs often as paints used to absorb RADAR signals.

Iron ball paint, ferrite in polymer matrix used.

The cockpit canopy coated with thin layer of indium tin oxide .

Small cell foams painted or loaded with absorbing ink.


R- Card

Absorbing honeycomb

Transparent RAM

Other fields
Reducing RCS alone not enough.

More difficulty is reducing the IR signature.

Interest is near IR region. (shorter than 10 µm)

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Stealth aircraft are aircraft that use stealth technology to interfere with radar detection as well as means other than communicational aircraft by employing a combination of features to reduce visibility in the infrared, visual, audio, and radio frequency(RF) spectrum.
Radar is an object-detection system which uses electromagnetic waves-specially radio waves to determine the range, altitude or speed of both moving and fixed objects such as aircraft, ships, spacecrafts, guided missiles, motor vehicles etc.
Stealth means low observable. For airplanes stealth first meant hiding from radar. That is stealth refers to the act of trying to hide detection. Development of stealth technology likely began in Germany during world war II. The various aircraft designers recognized the need to design planes that did not have large radar signatures(radar cross section).
 The US scientist Jack Northrop built a flying wing in the 1940’s
 In 1964, the SR-71(blackbird), stealth airplane was launched.
 In 1982, F-117 Nighthawk was delivered.
 In 1988, the most advanced stealth fighter B-2 Spirit Multi-Role bomb was launched.
 In 1997, F-22 Raptor was delivered.
 IN 2011,China launched J-20
F-117 Night Hawk
B-2 Spirit Stealth Bomber
F-22 Raptor

1. Unusual design.
2. Outer paint.
3. Reduce heat exhaust signatures
4. Eliminate high altitude contrails.
5. Eliminate brown exhaust.

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Almost since the invention of radar, various techniques have been tried to minimize detection. Rapid development of radar during World War II led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff.
In the late 1950's the American, Central Intelligence Academy (CIA) began sending Lockheed U-2 'spy-planes' over the Soviet Union to take intelligence photographs. The U-2's were little more than jet-powered gliders built of plastic and plywood flew at 80,000ft (24,000m) to be out of range of anti-aircraft fire, but it then became clear that radar was not detecting them.
It was not until May 1960, after more than four years of over-flights, that the Russians shot one down using new radar equipment belonging to SA-2, surface-to-air missiles and even then the U-2 did not receive a direct hit.
The success of the U-2s led to highly classified research work in the US, known as 'Stealth', to create a military aircraft that was invisible to radar. The U-2 had gone undetected for so long because it was made of non-metallic materials which absorbed radar waves rather than reflecting them back to the radar ground station, as normally happens.
The Stealth program aimed at designing high-performance military aircraft incorporating, among other features, a minimum of metal and with the exterior clad in highly absorbent tiles. The aircraft would be almost invisible to radar and could make most radar-controlled anti-aircraft systems obsolete.
After being developed under a blanket of secrecy, the high-tech B-2 Stealth bomber was unveiled at the Northrop Company’s manufacturing plant in Palmdale, California, in November 1988. An audience of invited journalists and guests was kept well away from the plane, which was designed to slip through enemy radar defenses without being detected and the drop up to 16 nuclear bombs on key targets.
Modern submarines are coated in a thick layer of a top-secret resin which is highly absorbent acoustically, and reflects only a minute amount of the energy transmitted by sonar detectors.
“STEALTH”, as seen in dictionaries is ‘the act of moving, proceeding, or acting in a covert way’. Stealth technology (also known as LOT, Low Observability Technology) is a sub-discipline of electronic countermeasures which covers a range of techniques used with aircraft, ships, submarines and missiles, in order to make them less visible (ideally invisible) to radar, infrared and other detection methods.
A mission system employing stealth may well become detected at some point. With in a given mission, such as when the target is destroyed, but current use of stealth systems should seek to minimize the possibility of detection. Attacking with surprise gives the attacker more time to perform is mission and exit before the defending force can counter-attack. If a surface-to-air missile battery defending the target observes a bomb falling a summarizes that there must be a stealth aircraft in the vicinity, for example, it is still unable to respond if it cannot get a look on the aircraft in order to feed guidance information to its missiles.
Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for Radio Detection and Ranging. In infrared (IR) detection method, the IR signature of exhaust plume is the prime contributor.
Stealth is not a single technology but is a combination of technologies that attempt to greatly reduce the distances at which a vehicle can be detected in particular reducing the Radar Cross Section (RCS).
Radar is an object-detection system which uses electromagnetic waves — specifically radio waves — to determine the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.
3.1 Principles of radar
It mainly works on the two principles:
1. ECHO and
Echo is used to detect time and distance of target
Doppler shift is used to detect the speed of target approaching
3.2 Echo and Doppler Shift
Echo is something you experience all the time. If you shout into a well or a canyon, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off of a surface (either the water at the bottom of the well or the canyon wall on the far side) and travel back to your ears. The length of time between the moment you shout and the moment that you hear the echo is determined by the distance between you and the surface that creates the echo.
Doppler shift is also common. You probably experience it daily (often without realizing it). Doppler shift occurs when sound is generated by, or reflected off of, a moving object. Doppler shift in the extreme creates sonic booms (see below). Here's how to understand Doppler shift (you may also want to try this experiment in an empty parking lot). Let's say there is a car coming toward you at 60 miles per hour (mph) and its horn is blaring. You will hear the horn playing one "note" as the car approaches, but when the car passes you the sound of the horn will suddenly shift to a lower note. It's the same horn making the same sound the whole time. The change you hear is caused by Doppler shift
4.1 Vehicle shape:

One of the important factors 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.
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 aircrafts use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them.
Stealth design must also 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.
a. Propulsion subsystem shaping:
Fluidic nozzles for thrust vectoring with aircraft jet engines, and ships, will have lower RCS, due to being less complex, mechanically simpler, with no moving parts or surfaces, and less massive (up to 50% less). Fluidic nozzles divert thrust via fluid effects. Tests show that air forced into a jet engine exhaust stream can deflect thrust up to 15 degrees.
b. Non-metallic airframe:
Dielectric composites are relatively transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites used may contain ferrites to optimize the dielectric and magnetic properties of the material for its application.
c. Radar absorbing material (RAM):
RAM, often as paints, is used especially on the edges of metal surfaces. One such coating, also called iron ball paint, contains tiny spheres coated with carbonyl iron ferrite. Radar waves induce alternating magnetic field in this material, which leads to conversion of their energy into heat.
Previously, neoprene-like tiles with ferrite grains embedded in the polymer matrix were used, now RAM paint is applied directly. The paint must be applied by robots because of problems of solvent, toxicity and tight tolerances on layer thickness.
Similarly, coating the cockpit canopy with a thin film transparent conductor helps to reduce the aircraft's radar profile because radar waves would normally enter the cockpit, bounce off something random 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.
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