Stealth technology

Stealth technology covers a range of techniques used with aircraft, ships and missiles, in order to make them less visible (ideally invisible) to radar and other detection methods.

Radar avoidance technology was first used on a large scale 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 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 stealth vehicles.

The concept of stealth itself is not new. Being able to operate without the knowledge of the enemy has always been a goal of military technology and techniques. But "stealth technology" redesigns the vehicle itself to dramatically reduce its observability.

A mission using stealth will obviously become common knowledge eventually, such as when the target is destroyed. But if the attacking force maximizes stealth and speed, it can also gain the element of surprise. Attacking with surprise gives the attacker more time to perform its mission and exit before the defending force can counter-attack. With stealth technology the defender might not be able to respond at all. 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 is not a single technology but is a combination of technologies that attempt to greatly reduce the distances that a vehicle can be detected at; in particular radar cross section reductions, but also acoustic, thermal and other aspects specifically:

Radar cross-section reductions

Vehicle shape

The possibility of designing aircraft in such a manner as to reduce their radar cross-section was recognised in the late 1930's, 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. see Brewster's angle. 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 even jet turbine blades) give 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 aircraft skin there is a special structure known as re-entrant triangles. Radar waves penetrating the skin of the aircraft get trapped in this structure, bouncing off its 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 three metal plates at right angles to each another (corner reflector), perpendicular to the incident radar wave. This configuration occurs in the tail of a conventional aircraft, where the vertical and horizontal components of the tail are set at right angles. A stealth aircraft must use a different arrangement. Often, a stealth design has the vertical element of the tail tipped at an angle, as in the F-117. The most radical approach is to eliminate the tail completely, as in the B-2 Spirit. As well as 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. The shape of the aircraft must be devoid of complex bumps or protrusions of any kind if it is to be stealthy. This means that all weapons, fuel tanks, and other stores may not be carried on under-wing pylons but must be stored internally. Furthermore, a stealth aircraft becomes unstealthy when it opens its bomb bay doors.

Planform alignment is also often used in stealth designs. Planform alignment involves using a small number of angles 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.

The shape 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.

This is of no help against low-frequency radar, whose wavelength is roughly twice the size of the airplane or its surface structures, using the half-wave resonance effect. 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 tell its operator that something is there, but not where it is, at least not with enough accuracy to target surface-to-air missiles on an enemy plane. Another problem is posed by noise, 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". ^*

Ships are also adopting 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 ^*. Other examples are the French La Fayette class frigate, the USS San Antonio amphibious transport dock, and most modern warship designs.

Non-metallic airframe

Composites are transparent to radar, whereas metals reflect waves back to the radar transmitter if the metal happens to be perpendicular to the radar, or else the metal is involved in an unstealthy shape. If metals are to be used, some elements and alloys reflect less electromagnetic radiation than others. The composites used often contain high amount of ferrites as filling.

Radar absorbing paint

Radar absorbing paint or radar absorbent material, 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 aircraft must be painted by robots, not just because the solvent used is toxic, but because the thickness of the sprayed layers must be tightly controlled.

In a similar vein, it is known that coating the cockpit window with a thin film of gold 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 very complex shape), and possibly return to the radar - but if the gold reflects the incoming radar waves, most of the energy is likely to go straight up rather than back to the radar. The gold film is thin enough that it doesn't significantly affect the pilot's vision.

Sound

Stealth aircraft need to stay subsonic to 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.

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 ^*, and mission planning also considers altitudes where the probability of their formation is minimized.

Infrared

The primary means of reducing the infrared signature is generally to have a non-circular tail pipe (a slit shape) in order to minimise the exhaust area 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. ^*

Reducing radar 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 that of a height finding radar, terrain following radar or search radar. 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 plane'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 plat 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 aircraft's cross-sectional area, the only way to reduce it would be to make the aircraft's physical profile smaller. Rather, by reflecting much of the radiation away or absorbing it altogether, the aircraft has an effectively 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 centres or surface-to-air (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 very 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. Most low RCS radars exploit Doppler filter to increase the SNR, knowing the exact location of the radars enables to design a flight path that has zero radial speed, therefore invisible. Note that in order to be able to fly these "safe" routes, it is necessary to understand the enemy's radar coverage - see Electronic Intelligence. Also note that if the enemy has mobile radars, such as AWACS, this 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

Aircraft | Military technology | radar

Gourt Home::Gourt :: Stealth technology