STEALTH TECHNOLOGY F-117 stealth attack plane Stealth technology also termed LO technology (low observable technology) is a sub- discipline of military tactics and passive electronic countermeasures , [1] which cover a range of techniques used with personnel, aircraft , ships , submarines , and missiles , to make them less visible (ideallyinvisible ) to radar , infrared , [2] sonar and other detection methods. Development in the United States occurred in 1958, [3] [4] where earlier attempts in preventing radar tracking of its U-2 spy planes during the Cold War by the Soviet Union had been unsuccessful. [5] Designers turned to develop a particular shape for planes that tended to reduce detection, by redirecting electromagnetic waves from radars. [6] Radar-absorbent material was also tested and made to reduce or block radar signals that reflect off from the surface of planes. Such changes to shape and surface composition form stealth technology as currently used on the Northrop Grumman B-2 Spirit "Stealth Bomber". [4] 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 throughcamouflage 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
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STEALTH TECHNOLOGY
F-117 stealth attack plane
Stealth technology also termed LO technology (low observable technology) is a sub-discipline of military
tactics and passive electronic countermeasures,[1] which cover a range of techniquesused with
personnel, aircraft, ships, submarines, and missiles, to make them less visible (ideallyinvisible)
to radar, infrared,[2] sonar and other detection methods.
Development in the United States occurred in 1958,[3][4] where earlier attempts in preventing radar tracking of
its U-2 spy planes during the Cold War by the Soviet Union had been unsuccessful.[5]Designers turned to
develop a particular shape for planes that tended to reduce detection, by redirecting electromagnetic
waves from radars.[6] Radar-absorbent material was also tested and made to reduce or block radar signals that
reflect off from the surface of planes. Such changes to shape and surface composition form stealth technology
as currently used on the Northrop Grumman B-2 Spirit "Stealth Bomber".[4] 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 throughcamouflage 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 infraredsignature. 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
There are two significant sources of infrared radiation from air breathing propulsion systems: hot parts and jet
wakes. The fundamental variables available for reducing radiation are temperature and emissivity, and the basic
tool available is line of sight masking. Recently some interesting progress has been made in directed energy,
particularly for multiple bounce situations, but that subject will not be discussed further here. Emissivity can be
a double edged sword, particularly inside a duct. While a low emissivity surface will reduce the emitted energy,
it will also enhance reflected energy that may be coming from a hotter internal region. Thus, a careful
optimization must be made to determine the preferred emissivity pattern inside a jet engine exhaust pipe. This
pattern must be played against the frequency range available to detectors, which typically covers a band from
one to 12 microns. The short wavelengths are particularly effective at high temperatures, while the long
wavelengths are most effective at typical ambient atmospheric temperatures. The required emissivity pattern as
a function of both frequency and spatial dispersion having been determined, the next issue is how to make
materials that fit the bill. The first inclination of the infrared coating designer is to throw some metal flakes into
a transparent binder. Coming up with a transparent binder over the frequency range of interest is not easy, and
the radar coating man probably won't like the effects of the metal particles on his favorite observable. The next
move is usually to come up with a multi layer material, where the same cancellation approach that was
discussed earlier regarding radar suppressant coatings is used. The dimensions now are in angstroms rather than
millimeters.
The big push at present is in moving from metal layers in the films to metal oxides for radar cross section
compatibility. Getting the required performance as a function of frequency is not easy, and it is a significant feat
to get down to an emissivity of 0.1, particularly over a sustained frequency range. Thus, the biggest practical
ratio of emissivities is liable to be one order of magnitude. Everyone can recognize that all of this discussion is
meaningless if engines continue to deposit carbon (one of the highest emissivity materials known) on duct
walls. For the infrared coating to be effective, it is not sufficient to have a very low particulate ratio in the
engine exhaust, but to have one that is essentially zero. Carbon buildup on hot engine parts is a cumulative
situation, and there are very few bright, shiny parts inside exhaust nozzles after a number of hours of operation.
For this reason alone, it is likely that emissivity control will predominantly be employed on surfaces other than
those exposed to engine exhaust gases, i.e., inlets and aircraft external parts. The other available variable is
temperature. This, in principle, gives a great deal more opportunity for radiation reduction than emissivity,
because of the large exponential dependence. The general equation for emitted radiation is that it varies with the
product of emissivity and temperature to the fourth power. However, this is a great simplification, because it
does not account for the frequency shift of radiation with temperature. In the frequency range at which most
simple detectors work (one to five microns), and at typical hot-metal temperatures, the exponential dependency
will be typically near eight rather than four, and so at a particular frequency corresponding to a specific
detector, the radiation will be proportional to the product of the emissivity and temperature to the eighth power.
It is fairly clear that a small reduction in temperature can have a much greater effect than any reasonably
anticipated reduction in emissivity.
The third approach is masking. This is clearly much easier to do when the majority of the power is taken off by
the turbine, as in a propjet or helicopter application, than when the jet provides the basic propulsive force. The
former community has been using this approach to infrared suppression for many years, but it is only recently
that the jet-propulsion crowd has tackled this problem. The Lockheed F 117A and the Northrop B 2 both use a
similar approach of masking to prevent any hot parts being visible in the lower hemisphere. In summary,
infrared radiation should be tackled by a combination of temperature reduction and masking, although there is
no point in doing these past the point where the hot parts are no longer the dominant terms in the radiation
equation. The main body of the airplane has its own radiation, heavily dependent on speed and altitude, and the
jet plume can be a most significant factor, particularly in afterburning operation. Strong cooperation between
engine and airframe manufacturers in the early stages of design is extremely important. The choice of engine
bypass ratio, for example, should not be made solely on the basis of performance, but on a combination of that
and survivability for maximum system effectiveness. The jet-wake radiation follows the same laws as the
engine hot parts, a very strong dependency on temperature and a multiplicative factor of emissivity. Air has a
very low emissivity, carbon particles have a high broadband emissivity, and water vapour emits in very specific
bands. Infrared seekers have mixed feelings about water vapour wavelengths, because, while they help in
locating jet plumes, they hinder in terms of the general attenuation due to moisture content in the atmosphere.
There is no reason, however, why smart seekers shouldn't be able to make an instant decision about whether
conditions are favourable for using water-vapour bands for detection.
Detection
Theoretically there are a number of methods to detect stealth aircraft at long range.
[edit]Reflected waves
Passive (multistatic) radar, bistatic radar[19] and especially multistatic radar systems are believed to detect
some stealth aircraft better than conventional monostatic radars, since first-generation stealth technology
(such as the F117) reflects energy away from the transmitter's line of sight, effectively increasing
the radar cross section (RCS) in other directions, which the passive radars monitor. Such a system
typically uses either low frequency broadcast TV and FM radio signals (at which frequencies controlling
the aircraft's signature is more difficult). Later stealth approaches do not rely on controlling the specular
reflections of radar energy and so the geometrical benefits are unlikely to be significant.
Researchers at the University of Illinois at Urbana-Champaign with support of DARPA, have shown that it
is possible to build a synthetic aperture radar image of an aircraft target using passive multistatic radar,
possibly detailed enough to enable automatic target recognition(ATR).
In December 2007, SAAB researchers also revealed details for a system called Associative Aperture
Synthesis Radar (AASR) that would employ a large array of inexpensive and redundant transmitters and
a few intelligent receivers to exploit forward scatter to detect low observable targets.[20] The system was
originally designed to detect stealthy cruise missiles and should be just as effective against aircraft. The
large array of inexpensive transmitters also provides a degree of protection against anti-radar (or anti-
radiation) missiles or attacks.
[edit]Infrared (heat)
Some analysts claim Infra-red search and track systems (IRSTs) can be deployed against stealth aircraft,
because any aircraft surface heats up due to air friction and with a two channel IRST is a CO2 (4.3 µm
absorption maxima) detection possible, through difference comparing between the low and high channel.
[21][22] These analysts also point to the resurgence in such systems in several Russian designs in the
1980s, such as those fitted to the MiG-29 and Su-27. The latest version of the MiG-29, the MiG-35, is
equipped with a new Optical Locator System that includes even more advanced IRST capabilities.
In air combat, the optronic suite allows:
Detection of non-afterburning targets at 45-kilometre (28 mi) range and more;
Identification of those targets at 8-to-10-kilometre (5.0 to 6.2 mi) range; and
Estimates of aerial target range at up to 15 kilometres (9.3 mi).
For ground targets, the suite allows:
A tank-effective detection range up to 15 kilometres (9.3 mi), and aircraft carrier detection at 60 to 80
kilometres (37 to 50 mi);
Identification of the tank type on the 8-to-10-kilometre (5.0 to 6.2 mi) range, and of an aircraft carrier
at 40 to 60 kilometres (25 to 37 mi); and
Estimates of ground target range of up to 20 kilometres (12 mi).
[edit]Longer Wavelength Radar
VHF radar systems have wavelengths comparable to aircraft feature sizes and should exhibit scattering in
the resonance region rather than the optical region, allowing most stealth aircraft to be detected. This has
prompted Nizhniy Novgorod Research Institute of Radio Engineering (NNIIRT) to develop
VHF AESAs such as the NEBO SVU, which is capable of performing target acquisition for SAM batteries.
Despite the advantages offered by VHF radar, their longer wavelengths result in poor resolution
compared to comparably sized X-band radar array. As a result, these systems must be very large before
they can have the necessary resolution for an engagement radar.
The Dutch company Thales Nederland, formerly known as Holland Signaal, have developed a naval
phased-array radar called SMART-L, which also is operated at L-Band and is claimed to offer counter
stealth benefits.
[edit]OTH radar (over-the-horizon radar)
Over-the-horizon radar is a design concept that increases radar's effective range over conventional radar.
It is claimed that the Australian JORN Jindalee Operational Radar Network can overcome certain stealth
characteristics.[23] It is claimed that the HF frequency used and the method of bouncing radar from
ionosphere overcomes the stealth characteristics of the F-117A. In other words, stealth aircraft are
optimized for defeating much higher-frequency radar from front-on rather than low-frequency radars from
above.
[edit]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
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 (i.e. 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.[29] 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,[30] though
this varies widely depending on aircraft and radar.
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 by absorbing it, the
target achieves a smaller radar cross section.[31]
[edit]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 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. To be able to fly these "safe" routes,
it is necessary to understand an enemy's radar coverage (see Electronic Intelligence). Airborne or mobile
radar systems such as AWACS can complicate tactical strategy for stealth operation.
[edit]Research
Negative index metamaterials are artificial structures for which refractive index has a negative value for
some frequency range, such as in microwave, infrared, or possibly optical.[32] These offer another way to
reduce detectability, and may provide electromagnetic near-invisibility in designed wavelengths.
Plasma stealth is a phenomenon proposed to use ionized gas (plasma) to reduce RCS of vehicles.
Interactions between electromagnetic radiation and ionized gas have been studied extensively for many
purposes, including concealing vehicles from radar. Various methods might form a layer or cloud of
plasma around a vehicle to deflect or absorb radar, from simpler electrostatic to RF more complex laser
discharges, but these may be difficult in practice.[33]
Several technology research and development efforts exist to integrate the functions of aircraft flight
control systems such as ailerons,elevators, elevons, flaps, and flaperons into wings to perform the
aerodynamic purpose with the advantages of lower RCS for stealth via simpler geometries and lower
complexity (mechanically simpler, fewer or no moving parts or surfaces, less maintenance), and lower
mass, cost (up to 50% less), drag (up to 15% less during use) and, inertia (for faster, stronger control
response to change vehicle orientation to reduce detection). Two promising approaches are flexible
wings, and fluidics.
In flexible wings, much or all of a wing surface can change shape in flight to deflect air flow. Adaptive
compliant wings are a military and commercial effort.[34][35][36] The X-53 Active Aeroelastic Wing was a US
Air Force, Boeing, and NASA effort.
In fluidics, fluid injection is being researched for use in aircraft to control direction, in two ways: circulation
control and thrust vectoring. In both, larger more complex mechanical parts are replaced by smaller,
simpler fluidic systems, in which larger forces in fluids are diverted by smaller jets or flows of fluid
intermittently, to change the direction of vehicles.
In circulation control, near the trailing edges of wings, aircraft flight control systems are replaced by slots
which emit fluid flows.[37][38][39]
In thrust vectoring, in jet engine nozzles, swiveling parts are replaced by slots which inject fluid flows into
jets to divert thrust.[40] Tests show that air forced into a jet engine exhaust stream can deflect thrust up to
15 degrees. The U.S. FAA has conducted a study about civilizing 3D military thrust vectoring to help
jetliners avoid crashes. According to this study, 65% of all air crashes can be prevented by deploying
thrust vectoring means.[41][42]
Limitations
There is no one optimum stealth design, but rather each mission requirement generates an appropriate mix of techniques. Implementation of stealth is not without penalties. Some of the materials used require special and costly maintenance. The maneuverability of an aircraft can be compromised by the introduction of stealth design features. As was the case with the F-117A, each B-2 bomber will have its own covered maintenance facility, since the B-2's low observable features require frequent performance of structural and maintenance activities.Stealth requires not only design compromises, it also imposes operational compromises. Sensors to locate targets pose a particular problem for stealth aircraft. The large radars used by conventional aircraft would obviously compromise the position of a stealth aircraft. Air-to-air combat would rely on passive detection of transmissions by hostile aircraft, as well as infrared tracking. However, these techniques are of marginal effectiveness against other stealth aircraft, explaining the limited application of stealth to the Advanced Tactical Fighter.
Aircraft for attacking targets on the ground face a similar problem. FLIR can be used for precise aiming at targets whose general location is known, but they are poorly suited for searching for targets over a wide area. A radar on the aircraft to scan for potential targets would compromise its position. In order to locate targets, stealth aircraft may rely on an airborne laser radar, although such a sensor may prove of limited utility in poor weather. A more promising approach would be to use data from reconnaissance satellites,
either transmitted directly from the satellite or relayed through communications satellites from processing centers in the United States.
There are limits to the utility of stealth techniques. Since the radar cross-section of an aircraft depends on the angle from which it is viewed, an aircraft will typically have a much smaller RCS when viewed from the front or rear than when viewed from the side or from above. In general stealth aircraft are designed to minimize their frontal RCS. But it is not possible to contour the surface of an aircraft to reduce the RCS equally in all directions, and reductions in the frontal RCS may lead to a larger RCS from above. Thus while a stealth aircraft may be difficult to track when it is flying toward a ground-based radar or another aircraft at the same altitude, a high-altitude airborne radar or a space-based radar may have an easier time tracking it.
Another limitation of stealth aircraft is their vulnerability to detection by bi-static radars. The contouring of a stealth aircraft is designed to avoid reflecting a radar signal directly back in the direction of the radar transmitter. But the transmitter and receiver of a bi-static radar are in separate locations — indeed, a single transmitter may be used by radar receivers scattered over a wide area. This greatly increases the odds that at least one of these receivers will pickup a reflected signal. The prospects for detection of stealth aircraft by bi-static radar are further improved if the radar transmitter is space-based, and thus viewing the aircraft from above, the direction of its largest radar cross section.
Several analysts claim stealth aircraft such as the ATF will be vulnerable to detection by infrared search and track systems (IRST). The natural heating of an aircraft's surface makes it visible to this type of system. The faster and aircraft flies, the warmer it gets, and thus, the easier to detect through infrared means. One expert asserts "if an aircraft deviates from its surroundings by only one degree centigrade, you will be able to detect it at militarily useful ranges." In fact, both the Russian MiG-29 and Su-27 carry IRST devices, which indicates that the Russians have long targeted this as a potential stealth weakness.
Stealth aircraft are even more vulnerable to multiple sensors used in tandem. By using an IRST to track the target and a Ladar (laser radar), or a narrow beam, high-power radar to paint the target superior data is provided.
The most basic potential limitation of stealth, is its vulnerability to visual detection. Since the ATF is 25-30 percent larger than the F-15 and 40 percent larger than the F-18, for example, it will be much easier to detect visually from ranges on the order of 10 miles. When one considers that stealth characteristics will drastically reduce the effectiveness of several types of guided air-to-air missiles, fighter engagements will probably move back to the visual range arena. In this context, the cumbersome F-22 would be at a distinct disadvantage.
Another potential "limitation" of stealth technology has little to do with its capabilities. Rather, some question the effect the pursuit of such hi tech aircraft will have on the US aerospace industry as a whole. These aircraft would not be available for foreign export until well into the next century. During that time, competitors such as the Gripen, Rafale and EFA will be peddled aggressively by European exporters. One analyst estimated that US foreign sales saved the Pentagon "about $2.8 billion through surcharges to recover part of their development costs and perhaps another $4 billion through the learning curve effect of higher production runs." Thus, America's stealth success could actually backfire, on its larger aerospace industry by causing it to forfeit sales to a new generation of top-of-the-line, although less formidable, European fighter aircraft.
DESIGN
General design
The general design of a stealth aircraft is always aimed at reducing radar and thermal detection. It is the
designer's top priority to satisfy the following conditions; some of which are listed below, by using their
skills, which ultimately decides the success of the aircraft:-
Reducing thermal emission from thrust
Reducing radar detection by altering some general configuration (like introducing the split rudder)
Reducing radar detection when the aircraft opens its weapons bay
Reducing infra-red and radar detection during adverse weather conditions
[edit]Limitations
B-2 Spirit stealth bomber of the U.S Air Force
[edit]Instability of design
Early stealth aircraft were designed with a focus on minimal radar cross section (RCS) rather than
aerodynamic performance. Highly-stealth aircraft like the F-117 Nighthawk are aerodynamically unstable
in all three axes and require constant flight corrections from a fly-by-wire (FBW) flight system to maintain
controlled flight.[13] Most modern non-stealth fighter aircraft are unstable on one or two axes only.[citation
needed] However, in the pursuit of increased maneuverability, most 4th and 5th-generation fighter aircraft
have been designed with some degree of inherent instability that must be controlled by fly-by-wire
computers.[citation needed] As for the B-2 Spirit, based on the development of the flying wing aircraft[14] by Jack
Northrop since 1940, design allowed creating stable aircraft with sufficient yaw control, even without
vertical surfaces such as rudders.
[edit]Dogfighting ability
Earlier stealth aircraft (such as the F-117 and B-2) lack afterburners, because the hot exhaust would
increase their infrared footprint, and breaking the sound barrier would produce an obvious sonic boom, as
well as surface heating of the aircraft skin which also increased the infrared footprint. As a result their