- For the 2005 movie, see Stealth.
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 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 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 technology is not a single technology but a combination of technologies, that attempt to reduce the radar cross section, specifically:
a) Vehicle shape. 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 (2.4 metre) 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 two metal plates at right angles to one 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.
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 take advantage of the poor aerodynamics of the stealth planes and 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 radars, whose wavelength is roughly twice the size of the airplane or its surface structures, using the half-wave resonance effect. However, low-frequency radars are limited by their size, making them difficult to transport, and lack of available frequencies which are heavily used by other systems, lack of accuracy given the long wavelength. 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, such as the British Type 23 frigate, which has no right angles on its superstructure, and French La Fayette class frigate, and like so much of other ships nowadays.
b) Use of non-metallic materials called composites for the 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.
c) Radar absorbing paint, or RAM (Radar Absorbent Material) coating, 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, because the solvent used is highly toxic.
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.
d) Technologies to reduce other signatures such as infra-red, visible, and sonic. 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. Most stealth aircraft use matte paint and dark colors, and operate only at night. 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.
e) Technologies to reduce 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 a passive infra-red system to navigate and the F-22 has an advanced Low Probability of Interception (LPI) radar which can illuminate enemy aircraft without triggering a radar warning receiver response.
The size of a plane's image on radar is measured by the Radar Cross Section or RCS. Imagine a metal plate of area A square metres held perpendicular to the beam of radar transmitter. It reflects most of the radar energy back to the source, and thus is easily detected. It is said to have an RCS of A square metres. 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 and the value will be less than A. 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.
The RCS is not directly related to the aircraft's cross-sectional area; if it was, the only way to reduce the RCS 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.
Stealthy strike aircraft such as the F-117 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.
- 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