Stealth technology

Stealth technology is a subdiscipline 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. However, as the potency of detection and interception technologies (radar, IRST, surface-to-air missiles etc.) has increased, so too has the extent to which the design and operation of military vehicles have been affected in response. A 'stealth' vehicle will generally have been designed from the outset to have reduced or controlled signature. It is possible to have varying degrees of stealth. The exact level and nature of stealth embodied in a particular design is determined by the prediction of likely threat capabilties and the balance of other considerations, including the raw unit cost of the system.

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 minimise the possibility of detection. Attacking with surprise gives the attacker more time to perform its mission and exit before the defending force can counter-attack. If a surface-to-air missile battery defending a target observes a bomb falling and surmises that there must be a stealth aircraft in the vicinity, for example, it is still unable to respond if it cannot get a lock on the aircraft in order to feed guidance information to its missiles.

Stealth principles

Stealth technology (often referred to as "LO", for "low observability") 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 radar cross section reductions, but also acoustic, thermal and other aspects specifically:

Radar cross-section (RCS) reductions

Almost since the invention of radar, various techniques have been tried to minimise detection. Rapid development of radar during WWII led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff. In the context of stealth vehicles an early example of a low radar signature aircraft was the fast bomber de Havilland Mosquito designed in 1938.^{[citation needed]}

The term 'Stealth' in reference to reduced radar signature aircraft became popular during the late eighties when the F-117 stealth fighter became widely known. The first large scale (and public) use of the F-117 was during the Gulf War in 1991. However, F-117A stealth fighters were used for the first time in combat during the United States invasion of Panama (aka: Operation Just Cause), in 1989. Since then it has become less effective due to developments in the algorithms used to process the data received by radars, such as Bayesian particle filter methods. Increased awareness of stealth vehicles and the technologies behind them is prompting the development of techniques for detecting stealth vehicles, such as passive radar arrays and low-frequency radars. Many countries nevertheless continue to develop low-RCS vehicles because low RCS still offers advantages in detection range reduction as well as increasing the effectiveness of decoys against radar-seeking threats.

Vehicle shape

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

Another important factor is the internal construction. Behind the skin of some aircraft are structures known as re-entrant triangles. Radar waves penetrating the skin of the aircraft get trapped in these structures, bouncing off the internal faces and losing energy. This approach was first used on SR-71.

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

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

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

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

Shaping requirements have strong negative influence on the aircraft's aerodynamic properties. The F-117 has poor aerodynamics, is inherently unstable, and cannot be flown without computer assistance. Some modern anti-stealth radars target the trail of turbulent air behind it instead, much like civilian wind shear detecting radars do.

Shaping does not offer stealth advantages against low-frequency radar. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. However, low-frequency radar is limited by lack of available frequencies which are heavily used by other systems, lack of accuracy given the long wavelength, and by the radar's size, making it difficult to transport. A long-wave radar may detect a target and roughly locate it, but not identify it, and the location information lacks sufficient weapon targeting accuracy. Noise poses another problem, but that can be efficiently addressed using modern computer technology; Chinese "Nantsin" radar and many older Soviet-made long-range radars were modified this way. It has been said that "there's nothing invisible in the radar frequency range below 2 GHz". [1]

Ships have also adopted similar techniques. The Visby corvette was the first stealth ship to enter service, though the earlier Arleigh Burke class destroyer incorporated some signature-reduction features [2]. Other examples are the French La Fayette class frigate, the USS San Antonio amphibious transport dock, and most modern warship designs.

Stealth Radar

The ability of designing a radar in such a manner as to enhance backscatter radiation detection, from a large radar cross-section, which was created by radar frontal cross section reducing airframe shape design. Additionally, the ability to make the radar hard to detect, which inhibits the ability to be attacked with high speed anti-radiation missiles (HARMed).

1. The ability of designing a radar in such a manner as to enhance backscatter radiation detection: The first key to the approach is found in a high school solid geometry course. There is the GPS technology, essentially finding a distance from each of four moving (but known position at any one instant) satellites, called pseudoranges. By the intersection of four hollow basketball shapes of pseudoranges from four widely separated satellites, a unique point can be determined. Four spheres intersect in an obvious point, if their centers are highly separated; and the mathematics was once taught in the public school system.

Passive radar will be like that, but substituting ellipsoids for spheres.

2. The ability to make the radar hard to detect: There is the F-117 technology old schematic diagram showing a beam of electromagnetic radiation coming from the old time radar as densely packed parallel pointed arrows. When the beam strikes the stealth shaped airframe from the front, it scatters with each single arrow of the former beam pointing off in a different dissipated direction from the stealth shaped airframe. Therein is found the second key to the approach, from a high school physics course, where it is discovered that electromagnetic wave propagation can occur bilaterally. Different emitters from "out there" would reflect off from the stealth airframe in a concentrated direction toward the receiver. Put the radar transmitters away from the receiver, and use many. The radar location is now that of the passive receiver.

The reflection locus of points from an emitter to a receiver by an indirect path can be easily determined from complex signal ghosting. Visible in the early days of black and white television, a clear picture would exhibit a “ghost” if an airplane flew in the area. The ghost came about from two paths from the emitter to the receiver, one being longer by a constant in time. The ghost picture was displaced “to the right”, later in time than the direct path, and diminished in intensity by the “radar cross section” of the reflecting point. From high school solid geometry, we know that the focus points of the emitter and receiver are surrounded by the locus of points of possible reflector position, having a constant time difference, an ellipsoid, a hollow foot ball shape. The amount of time difference sets the volume of the ellipsoid.

The reflector position can be uniquely determined from three hollow ellipses of revolution, one of whose foci is the common point of receiver location. The other focus, for each ellipse of revolution (hollow football surface), would be the location of the electromagnetic emitter for that ellipsoid. By the intersection of three hollow football shapes of ellipsoids from three separated transmitters, the unique point of reflector position can be determined (the second point solution would be below ground level). Three ellipsoids intersect in a point above ground, and the mathematics was once taught in the public school system.

Old radar used to use single frequency high power emitters. This makes detection of the radar transmitter location trivial by radio direction homing. This makes targeting by a high speed anti-radiation missile possible. By using a low power wideband signal emission, the reception can occur even below the noise floor (such as six orders of magnitude below the noise floor). The stealth radar can use common existing emitters that have appropriate recognizable patterns of modulation, such as the horizontal sync pulse from a television transmitter, or signal properties from cell telephone towers. The passive receiver radar does not indicate what emissions in the area are being used for the radar location of the reflection point in the sky.

The emitters may, of course, be located above ground, but that would require a four ellipsoid solution to determine unique position.

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.

Radar absorbing paint

Radar absorbing paint or radar absorbent material (RAM), is used especially on the edges of metal surfaces. The RAM coating, known also as 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. Early versions of F-117A planes were covered with neoprene-like tiles with ferrite grains embedded in the polymer matrix, current models have RAM paint applied directly. The paint must be applied by robots because of issues relating to solvent toxicity and tight tolerances on layer thickness.

In a similar vein, coating the cockpit canopy with a thin film transparent conductor (vapor-deposited gold or indium tin oxide) helps to reduce the aircraft's radar profile because radar waves would normally enter the cockpit, bounce off something random (the inside of the cockpit has a complex shape), 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.

Acoustics

Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. Some early stealth observation aircraft utilised very slow-turning propellers in order to be able to orbit above enemy troops without being heard. The B-2 Spirit is said to have acoustic stealth features, as is the Boeing Bird of Prey.

The presence of supersonic and jet-powered 'stealth' aircraft such as the SR-71 Blackbird indicates that acoustic signature is not always a major driver in aircraft design, although the Blackbird relied more on its high speed and altitude and had very poor stealth capabilities by modern standards.

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

Visibility

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

Infrared

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

Reducing radiofrequency (RF) emissions

Infrared emissions and sound aren't the only detectable emissions generated by ships or aircraft. The stealth vehicle must not radiate any energy which can be detected by the enemy, such as from onboard radars, communications systems, or RF leakage from electronics enclosures. The F-117 uses passive infra-red and "low light level TV" sensor systems to aim its weapons and the F-22 Raptor has an advanced LPI radar which can illuminate enemy aircraft without triggering a radar warning receiver response.

Measuring stealth

The size of a target's image on radar is measured by the Radar Cross Section or RCS, often represented by the symbol σ and expressed in square meters. This does not equal geometric area. A perfectly conducting sphere of projected cross sectional area 1m² (ie a diameter of 1.13m) will have an RCS of 1m². Note that for radar wavelengths much less than the diameter of the sphere, RCS is independent of frequency. Conversely, a flat plate of area 1m² will have an RCS of almost 14,000m² at 10GHz if the radar is perpendicular to the flat surface. If you rotate it, the amount of energy reflected directly back to the transmitter is reduced, as some is reflected to the side, so the RCS is reduced. Modern stealth aircraft are said to have an RCS comparable with small birds or large insects, though this varies widely depending on aircraft and radar.

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

Stealth tactics

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

References

Sequential Monte Carlo Methods in Practice, by A Doucet, N de Freitas and N Gordon. Published by Springer.
Countering stealth
How "stealth" is achieved on F-117A

External links

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