IR decoys

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Lockheed AC-130 during decoy ejection

IR decoys, also known as decoy flares , are decoys against guided missiles with an infrared seeker head . The heat radiation that arises when using flares is intended to deflect the seeker head sensors from the actual target, ideally to the heat radiation of the heat flare. This is done by generating IR clutter from many different heat sources, generating specific emission spectra to offer the seeker a decoy target , and generating heat walls to cover the actual target.

IR decoys contain either pyrotechnic charges, pyrophoric solids or liquids or highly flammable solids as energy stores . When igniting, a strongly exothermic reaction is triggered which, depending on the chemical composition of the energy storage device, is accompanied by a more or less strong visible flame and smoke development.

From 1981 to 2002 more than 50% of all aircraft losses were caused by IR-guided weapons. During the Gulf War in 1991 it was even said to have been 78%. Between 1985 and 2010, 90% of all US aircraft losses are believed to be attributable to IR-guided weapons.

overview

IR viewfinder

In order to understand the effect of flares on IR-guided missiles, an understanding of the structure of the seeker is necessary. In physics, infrared radiation refers to electromagnetic waves in the spectral range between visible light and longer-wave terahertz radiation . Water vapor, which absorbs IR radiation, only results in windows in the wavelength range of 1-6 µm and 8-14 µm in the atmosphere, in which the radiation can travel relatively far. At an altitude of over 10 km, however, the absorption is negligible. CO 2 , dust and water droplets also reduce visibility, with the CO 2 share practically constant up to an altitude of around 50 km.

Planck radiation spectra for different temperatures

The Wien's displacement law states that the greatest radiant power occurring at a nozzle plane at a wavelength of 3 microns, with afterburner microns at about 1.5. The aircraft's exhaust plume is brightest in the 3–5 µm range, hot parts on the fuselage are 3–5 and ≥8 µm. The trunk is best visible at ≥8 µm. Since older viewfinders only consist of one detector element, a mechanical front end is necessary to enable target tracking in space ( rosette scanning ). As far as possible, the detectors have an automatic gain control in order to be able to adapt to different brightnesses. The problem of target, clutter and flare detection had to be solved mechanically in older models, which will be discussed below.

In order to avoid “contamination” of the contact with sources of interference, a narrow field of view of the viewfinder was aimed for. This field of view is expanded by scanning movements. Older viewfinders such as the 9K32 Strela-2 connect a thin, rotating diaphragm between the detector element and the optical components of the viewfinder head. This is alternately coated with IR-permeable and impermeable material, and thus resembles a propeller. This achieves two things: The IR target flickers with the rotation frequency of the shutter, whereby the deviation from the visual axis can be inferred from the duration of the obscuration / visibility: the further out, the longer the duration. Furthermore, aircraft and torch (point source) can be distinguished from clouds or ground (clutter): the latter are voluminous and are therefore not covered by the screen in a staccato manner. An analog filter removes the more or less constant signal so that only the point source (target or flare) is tracked. The viewfinder tries to keep the target in the middle, where the IR energy gets through all the spokes equally well and the flickering is practically zero.

The disadvantage of the viewfinder was that they are relatively insensitive to the detection of target movements if the target is already held in the middle. As a result, the older IR missiles fly on a "wobbly" flight path. The following viewfinders were therefore conically circling: The “propeller” is fixed in front of the detector, instead a secondary mirror rotates. The IR radiation from the optical components reaches the primary mirror on the outer edge of the viewfinder, reflects the radiation onto the rotating secondary mirror, and this through the “propeller” onto the detector (beam path similar to a Cassegrain telescope ). Due to the rotating mirror, the IR target point circles around the center axis of the detector field over the spokes of the diaphragm. If the viewfinder looks directly at the IR source, the point of light circles in a neat circular path around the center axis of the detector field, which, thanks to the “propeller”, leads to a constant illumination frequency of the detector. If, on the other hand, the viewfinder looks at an angle, the IR point circles on an ellipse and the illumination frequency changes as the mirror is rotated, from which a computer can calculate a course correction.

While the IR viewfinder was initially not cooled, active cooling was later introduced to increase sensitivity and to be able to locate longer wavelengths. The Strela-2, for example, has a viewfinder made of lead (II) sulfide , which is most sensitive at 2 µm and can therefore only differentiate between the nozzle and the background, which only allows shots from behind. The latest IR-guided weapons use imaging viewfinders, which either stare at an object or scan it. These viewfinders are fully digital and see the target with a gimbaled IR video camera. They can discover aircraft through image recognition and track them safely, as well as control specific parts of them.

Trigger response

In order to reduce the effect of IR decoys, a flare detection ("trigger") and the counter-reaction of the viewfinder ("response") are programmed into the viewfinder logic. As mentioned above, clouds are discriminated against by older viewers due to their spatial distribution. A heat flare, like the flight target, represents a point target and is in principle treated the same by the viewfinder. The oldest seekers have no protection against flare, but only detect the hottest target at around 2 µm, which then represents the IR interfering body. The viewfinders with a conically circling target point have an inherent protection against flares: Since the target IR contact is kept on a circular path on the detector (constant illumination frequency), and a torch quickly falls off the aircraft, it appears as an IR contact on an elliptical path (frequency-modulated sinusoidal illumination frequency), which disappears (relatively) quickly from the detector field. A narrow field of view also helps as the torch quickly falls out of the field of view. The flares must reach the maximum radiation value shortly after being ejected. The most common methods of flare detection ("triggers"), which are also combined, are:

Failed trigger response: An AIM-9M Sidewinder hits the heat flare, ejected from an F / A-18C Hornet.
  • A sharp increase in IR energy from the target triggers the "trigger" and switches it off again when a threshold value is undershot. The threshold value for triggering the flare detection must be above the value of the aircraft's afterburner. This method can easily be outwitted if the IR decoys burn relatively slowly.
  • Detectors that can use two bands in the infrared work with a band comparison: Airplanes emit more radiation in the long-wave spectrum than in the short-wave spectrum; the opposite is true for heat flares. This method can be outwitted if multiple torches in different bands burn with the same intensity.
  • A kinematic “trigger” exploits the fact that IR decoys quickly fall to the ground due to air resistance. A connected torch leads to a relatively large change in the angle of the viewfinder in a short time, which triggers the "trigger". If the change in the angle difference between the target IR point and the torch IR point is too small, this method will fail. Several IR decoys are ejected in a short sequence.
  • The spatial “trigger” sets the viewfinder’s field of view between the aircraft and the torch, i. H. quasi in the middle between the possible correct goals. Both IR points are so distinguishable for the viewfinder, which triggers the flare detection. Since the real target is at the edge of the viewfinder field of view, and the viewfinder tends to average indistinguishable IR sources, this method fails when many IR decoys are ejected in very short sequences.

If the flare emission is recognized, the countermeasure of the viewfinder is triggered ("response"). The most common countermeasures, which can also be combined, are:

  • The searcher's inputs to the control logic are ignored; the missile maintains its current flight maneuver until the heat flare leaves the viewfinder field of view or the trigger timeout occurs. If there is still a flare in the field of view of the viewfinder after the timeout, it is activated.
  • With the push-ahead response, the viewfinder’s field of vision moves forward in the direction of movement of the target. The torch falls out of the field of view more quickly, reducing the amount of time the viewfinder cannot track the target. If the movement is carried out too strongly, the field of vision moves too far forward, so that the guided weapon loses the target and has to re-acquire.
  • A spatial trigger must be used for the push-pull response. If the target and flare are on opposite sides of the field of view and can thus be discriminated against, the viewfinder intentionally targets the weaker IR source, which is the aircraft.
  • A countermeasure, also for modern imaging seekers, can be to attenuate certain sectors of the field of view with filters. The viewfinder is put on sunglasses in these areas to avoid being dazzled by the heat flare. In the case of older non-imaging seekers, the target can only be pursued further if the attenuated intensity of the torches does not exceed that of the flight target.

Actual charges

Older seekers or their detectors cover the wavelength of 1–5 µm. Since the classic heat flare has its radiation peak at around 1.5 µm, the radiation intensity must be significantly higher than that of the aircraft in order, according to the law of displacement, to still achieve a greater intensity than the target at longer wavelengths. Long-wave viewfinders work in an area where the radiation intensity of classic heat flares is significantly lower. Active charges can be roughly divided into two different types: pyrotechnic active charges, which carry the oxidizer with them for combustion, and pyrophoric active charges, which use the oxygen in the air for oxidation.

Sectional drawing of an MJU-7A / B

Pyrotechnic active charges burn very hot and therefore emit most strongly at short wavelengths. These heat flares also burn brightly in the visible spectrum, creating a plume of smoke. The burn time is about 5 to 10 seconds. If such a pyrotechnic charge hits the ground while burning, fires can be triggered there. Since the introduction of IR decoys in 1959, which initially used Al / WO 3 - Thermite in graphite spheres, these have been using magnesium - fluorocarbons . Modern flares consist of a solid, pyrotechnic compound made of magnesium, polytetrafluoroethylene (PTFE) and Viton as a fluorine copolymer, or with a synthetic elastomer as a binder. These so-called MTV flares are ejected and simultaneously ignited by an ignition charge. Due to the high temperature (over 2000 K), the highest radiation intensity is in short-wave bands, which makes the MTV flares very effective against older IR seekers that could only search on these bands. Modern active charges also use a spectrally adapted active mass. The reducing agent is overbalanced so that the oxygen in the air has to contribute to the combustion. The further development are kinematic flares, which are used in the latest generation of aircraft. Instead of simply falling to the ground, these flares move along predetermined paths next to the aircraft. These torches, for. B. MJU-47, equipped with a rocket motor at the stern and vector nozzles.

A MJU-7A / B decoys, a typical example of older MTV flares, is shown opposite. It consists of an outer aluminum shell (1), an electrically ignitable pulse cartridge (2), which causes the discharge of the active charge and, if necessary, its ignition, and a sabot  (3) designed as a pipe fuse, which ignites the active substance (4) with the ignition (5) and the enveloping mostly self-adhesive aluminum foil (6) should only allow outside of the cartridge case. The cartridge is closed at the front with a cover plate (7).

In contrast to pyrotechnic active charges, pyrophoric substances take the oxygen required for the reaction from the air. Therefore, the performance of pyrophoric decoy targets is basically dependent on the altitude, i.e. the oxygen partial pressure. In the eighties, the spraying of triethylaluminum was tried out, which was very effective, but too expensive. Modern systems use coated solids. The oxidation process is almost invisible to the eye, which is why they are also suitable for preventive use. The thin strips of the material are distributed in the room like chaff and release infrared radiation during the rapid oxidation. These heat walls made of thin nickel, steel or iron strips or their alloys with a length of about 1 cm are coated with propylene oxide and can reach up to 1255 K. Ignition occurs as the material is distributed when it comes into contact with oxygen. Porous metal wafers (around 500 pieces per cartridge) are also used for this. The heat walls created in this way are also effective against modern viewfinders. Point-shaped, pyrophoric, smoke-free and "dumb" flares such as MJU-50/51 are also purchased in order to be used preventively in place of the pyrotechnical, smoke-generating flares (e.g. MJU-47).

tactics

The tactic of the ejection depends on the available charges and the IR seeker of the threat. For example, German Transall transport aircraft emit flares on their landings in Kabul as soon as the missile warning device indicates a threat in order to make possible attacks with flying fists such as the FIM-92 Stinger or Strela-2 more difficult. In most cases, several flares are ejected like a volley , creating a large heat curtain next to and behind the machine. They are ejected by a small propellant that brings them to a speed of around 150 km / h. The exact ejection rates and patterns are controlled by an associated computer system, which varies the deployment according to the threat, the target to be protected and the parameters of the flare.

F / A-18C throwing a torch

Older viewfinder models work e.g. B. only on short wavelengths, so that pyrophoric active charges are less effective. Here pyrotechnic active charges are the method of choice. The ejection triggers the "trigger", the missile selects course as the "response". If the target does not change course, a collision still occurs. A few torches and moderate maneuvering are sufficient for defense. Rotating mirror seekers will use kinematic and spatial “triggers” and respond with push-ahead response. Here masses of heat flares have to be ejected in a short time, or a few kinematic flares, and maneuvered hard so that the missile might lose its target during the push-ahead maneuver. Cooled two-band detectors with push-pull response require the ejection of pyrotechnic (short-wave) and pyrophoric (long-wave) IR decoys and maneuvers to present the viewfinder with various decoys on all bands. With the most modern, imaging viewfinders with image recognition, punctiform, pyrotechnic active charges are "seen" and recognized as such by the viewfinder. Then a (software-based) attenuation of flares is probably switched on in order to avoid overexposure to the scene. Due to the spatial expansion of pyrophoric heat walls, the aircraft can try to hide behind them, or at least obstruct the image recognition software.

Throwing a torch, usually also preventively over enemy territory to make it more difficult to lock on, is often supplemented by other measures: Maneuvering in or near clouds or towards the sun is helpful. Infrared flashing lights such as AN / ALQ-144 are only useful against viewfinders with rotating mirrors, as the lighting frequency is disturbed. Compared to older models with rotating diaphragms or modern imaging viewfinders, these systems are counterproductive because they draw attention to the target. Directed optronic countermeasures (DIRCM) are effective against all types of viewfinder, provided the glare laser fully covers the spectrum of the viewfinder.

The trend in IR-guided weapons is therefore towards multispectral seekers: If the detector element of the Stinger was already able to cover IR and UV, this principle is also implemented in the most modern IR-guided air-to-air guided weapons. While the detector element of the Israeli Python 4 was already able to discriminate on two bands (long and short wave IR), the imaging viewfinder with 128 × 128 pixels of the Python 5 is based on the Hughes AMOS. This covers three bands, with electro-optical as a third band. The viewfinder “sees” the target in the visible spectrum of light, which undermines IR countermeasures.

Since air-to-ground guided missiles have been using imaging IR seekers for a long time, sometimes also in the optical spectrum, IR decoys from ground vehicles (tanks, ships, etc.) always generate a warm smoke screen to hide the target in the viewfinder image. The transition to smoke throw systems is fluid, e.g. B. with red phosphorus .

Web links

Commons : Decoy flares  - collection of images, videos and audio files

Individual evidence

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