Understanding the infrared threat.
The AIM-9 became a standard of Western air forces, and IR-missile technology spun off into many forms and variants for air- and ground-launched operations. The Soviets acquire the technology through espionage and quickly reverse-engineered it into the AIM-9 clone that we called the ATOLL.
A Technical History
The enabling technology for practical IR missiles came from WWII research on detector materials. Materials that could be used to fashion detectors sensitive in the IR bands allowed engineers to devise the scanning seekers. These initial materials, such as lead sulfide (PbS), were sensitive to radiation in the near-IR (1- to 2-micron wavelength) band. The primary source of targeting IR radiation came from the hottest ([greater than]1000 [degrees] C) metal parts of jet engines those seen when looking directly up the tail pipe. Much of the research on these detectors took place in the 1940s, and there is evidence that the Nazis were well on their way to developing an IR missile at the close of the war. It certainly would not have boded well for the Allies had ME-262s been equipped with "heaters" to use against the bomber streams. That was not to be; however, the IR missile, once developed, has gone on to account for more aircraft shootdowns, at a very high kill rate, than any other anti-aircraft weapon system regardless of cost.
One can argue the relative capabilities of IR versus radio-frequency (RF) guided missiles or point out the weather limitations of any IR device, but the metrics and net results are undeniable. From Vietnam through the Six-Day War, Bekka Valley Afghanistan and the Falklands, the IR missile has dominated. Air forces have been brought to their knees or had to radically change their tactics to avoid or overfly these relatively simple, low-cost threats. Allied kill statistics from Desert Storm, presented in Table 1, show the trend continuing.
Another IR-missile forte are those contingency scenarios labeled "operations other than war," where a single shootdown can cause a radically changed national strategy. Every day, special-operations and mobility aircrews are faced with the prospect that someone may decide to take a shoulder-fired IR missile out of a suitcase and let loose. They fully recognize that the missile will guide on their engine heat until slamming into their aircraft with the kinetic energy of a passenger car traveling 95 mph. The shooter could be a terrorist with just ten minutes of training, consisting of how to take off the end cap, fire up the battery, cool the head, point the missile, lock up the target, fire and disappear.
This simplicity is much of the reason why this threat has been so effective; has proliferated around the world; and is in the hands of virtually every military or paramilitary force, legitimate or not.
Infrared-Missile Advances Continue
Over the last 20 years, the most proliferated and hence most likely to be encountered shoulder-fired threat has been the Soviet's SA-7 Strella. This relatively simple Vietnam-era missile is gradually being supplanted by much more capable third-generation missiles that can reach farther, turn harder and are resistant to our best IR countermeasures (IRCM).
Advanced designs take advantage of better detector materials, moving the bandpass from the near-IR (1-to 2-micron) into the mid-IR (3-to 5-micron band), where engine plume provide a superior signal for all-aspect engagement. The 3- to 5-micron band, recognized as band IV, also has less atmospheric attenuation and clutter factors. Missile designers moved to cool the seekers - normally using liquid nitrogen to remove thermal "noise" and improve overall detector performance. Seeker slew and track rates, field of regard, guidance laws, maneuverability and kinematic performance have all been improved as newer generations have gradually been fielded.
These advances have expanded engagement parameters for fire teams or pilots from the restrictive rear-aspect shots (which target hot engine parts) to all-aspect (front, side and back) shots with limited views of hot parts or plume. Engagement envelopes have expanded two- and threefold, allowing detection and tracking of large-signature targets beyond 10 kilometers for shoulder-fired missiles and much further for the larger air-to-air weapons. Air-to-air missiles generally have better lock-on performance because they have larger apertures with which to collect IR energy. With the addition of better rocket motors, shoulder-fired missiles now can reach 6-8 km; larger weapons can reach 10-20 km. Performance against maneuvering and high-speed targets has also improved dramatically with latest-generation missiles, the most advanced of which now use vectored thrust to make the hard turns in the early stages of flight. The Russians, known for fielding telephone-pole-sized missiles, have variants that rival astronomical telescopes for aperture size. Pilots should be concerned, since these missiles may have lock-on and kinematic ranges rivaling their RF counterparts. Combined with IR search-and-track and EW techniques to counter the opposing aircraft's RF-missile capability, the first shot may be taken by the guy with the better IR missile. It could be disconcerting to hear "Fox 2 [inches] called before you can fire your AAMRAM. Hopefully, the IRCM will work, and the good guys will still prevail.
Fundamental Infrared Physics
Anyone attempting to develop countermeasures against IR missiles immediately runs into the immutable laws of physics. Intuitively, one knows that the higher the temperature of an object, the more heat it radiates. One also intuitively senses that the larger the object (the greater surface area), the higher the signature. Stefan and Boltzman quantified this with their famous namesake equation, shown below:
R = [Epsilon] x [Sigma] x [T.sup.4]
R is total radiant flux emitted per unit area;
E is emissivity based on the material type (a blackbody has [Epsilon] of 1);
[Sigma] is Stefan-Boltzmann constant (5.6686 x [10.sup.12] watts/[cm.sup.2] [deg.sup.2]); and
T is temperature in degrees Kelvin.
In other words, the rate that radiant energy is transmitted from an object rises dramatically as its temperature increases and, conversely, slows as the temperature drops. That is why, for example, a cup of coffee cools rapidly from hot to warm - but then lingers at tepid temperatures for much longer. In the IR business, the hotter the target, the more energy there is to exploit. Unfortunately that's not the whole story. The temperature also effects the peak wavelength and the total bandwidth of the radiant energy emitted. The hotter the temperature, the shorter the dominant wavelength, which is why the filament in your dining-room rheostat glows red or orange when you turn it down and "white hot" as you turn it up. If you turn the rheostat down to the lowest setting, it may cease to glow in your visual range, but actually it is still radiating brightly in the near infrared because of the wavelength shift but with much less total radiated power. You will need a near-IR camera to see this (several camcorders now have that capability).
Max Planck explained this phenomenon in his brillant postulation that objects radiate photons at probabilistic wavelength peaks determined exclusively by the temperature or energy level of the object or surface. Planck reasoned that as temperature increases, higher-energy photons (shorter wavelengths) would be emitted at higher rates, thus shifting the peak wavelength. He was also able to calculate the radiant energy emitted by an object at any wavelength, given its temperature. Any surface will radiate light energy based on its temperature, surface area and another factor called emissivity. All real objects absorb, reflect, transmit (allow energy to pass through them) and radiate light at different percentages. Some are good absorbers (black surfaces), some good reflectors (white surfaces or mirrors), and some good transmitters (windows). A perfect theoretical radiation absorber is called a black body in infrared physics and is given an emissivity of one - meaning it absorbs all radiation. A black body also radiates at a predictable peak radiant wavelength based on its temperature. Objects that do not absorb perfectly, either by transmitting through and/or reflecting parts of any incident energy, are called gray bodies. Gray bodies have an emissivity of less than one and obviously are what make up the majority of the real objects. This information is useful because engineers can determine, at a given wavelength where a detector operates, how much radiant energy will fall on his sensor aperture by knowing the operating temperature of the object.
A consideration of IR engineers is that the radiant energy from objects operating at IR-band temperatures will be considerably lower than that coming from the same object at visible-band emission temperatures (e.g., glowing white hot). Objects at high temperatures (such as missile plumes) will radiate in the IR but with less power because of the wavelength shift up into the visible band. Objects operating at temperatures where IR emissions predominate will also have lower relative brightness levels than the same object at higher temperature. The impact of this is seen in the design of IR sensors. For example, long wave (8-to 12-micron band) IR sensors, such as those found in forward looking infrared sensors (FLIRS) and targeting pods, will generally have relatively large apertures with which to collect the maximum amount of IR radiation. This is because the majority of their targets operate at low temperatures (in and around ambient temperature) and radiate very little energy per unit of surface area. Tribute should be given to IR engineers who can produce long-wave IR imagery showing glowing tanks and recently laid tread tracks, knowing what little radiant energy they have to work with in the IR band.
One final fundamental that must be grasped is atmospheric transmission windows. Most of what humans see - the visual range - is reflected energy from the sun or lights. Only when we look at high-temperature, high-energy surfaces do we see radiated light such as fires, lamps or glowing gases - as in missile plumes or afterburners. The visible band (0.7- to 0.4-microns) generally has high transmissivity across the entire red-to-violet spectrum. This allows visible light to propagate long distances in clear air. Atmospheric gases, aerosols and particulates in the atmosphere between sensor and target cause attenuation of radiant energy - light. In the IR bands, we see the same effects, but some bands are completely blocked by absorption and/or scattering of the signal. These zero-transmission bands are avoided for IR sensors, whose signals must travel through the atmosphere. IR engineers must take advantage of the windows where transmission is high and target signature, based on temperature, is usable.
IR Countermeasures: The Cat-and-Mouse Game Continues
These fundamental IR-radiation concepts explain some of the challenges in developing infrared countermeasures. For example, a magnesium flare needs to have a jamming-to-signal (J/S) ratio of greater than one to one versus the protected aircraft's signature to seduce a missile away from the aircraft. Since the observed surface area generally given by the plume of the flare is quite small, it has to operate at very high temperatures to match or exceed the inband aircraft signature. But high temperatures shift the wavelength, and we are forced by the laws of physics to increase the size of the flare to provide enough energy at the correct wavelength.
Much of the energy of the flare is wasted in bands where IR-missile seekers don't look. The same effect also applies to jammer sources. Trying to get enough inband J/S from small sources operating at peak wavelengths well outside of the missiles' tracking bands has long been a key concern for countermeasures designers.
Despite the challenges, EW designers were quick to develop countermeasures that handily defeated the relatively simple scanning seekers of first-generation threats. These missiles used null-tracking scanning sensors that only updated the guidance loop when the target moved off boresight. Called spin-scan seekers, they operated in the near-IR range, using an ingenious combination gyro and cassegrain-mirror mechanism to collect IR energy. The energy was focused through a simple chopping reticle and onto the detector to provide angle and angle-rate information to the guidance loop. These missiles were easily spoofed by simple, low-power jammers modulated at the spin rate of the seeker. In fact, lock-on to a target could be denied by using the jammer in the preferred pre-emptive mode. Since the missile detector operated in the near-IR band, hot arc lamps, carbon rods or even fuel-fired sources were used as jammer sources, modulated by mechanical choppers. Rudimentary decoys or expendables, including revamped illumination flares, were effective but had to be used pre-emptively due to a lack of reliable missile warning. This presented a problem that persists to this day - how to store enough expendables on the aircraft.
The cat-and-mouse game so familiar in RF technology plays out in the IR world as well, as missile designers move rapidly to counter the countermeasures. IR missiles have continued to bring down aircraft at astonishingly high rates. In many cases - in most cases, in fact - the aircrews never knew they had been fired on until missiles hit their aircraft. Even effective countermeasures were often of little value unless used pre-emptively, as unreliable missile launch detectors (MLDs) and/or missile-approach warning sensors (MWS) remained unreliable.
IRCCM: Upping the Ante
IR missile designers made the next move, designing seekers that were resistant to countermeasures and had improved overall capability. Missile designers took advantage of the laws of IR physics and the fact that jammers and flares operate at much higher temperatures than target aircraft. They used new detector materials, such as indium antimonide (InSb), with high sensitivity in the 3-to 5-micron band, moving away from the peak transition bands of flares and jammers. Designers also reduced the field of view (FOV) of seekers, forcing flare designers to develop flares with faster rise times to ignite nearer the target aircraft. Reduced FOV also limited the effectiveness of jammers situated away from the aircraft engines by reducing seeker response to off-boresight sources. Seekers were designed to be capable of sensing the presence of a flare or jammer, triggering the initiation of an IR counter-countermeasures (CCM) response and rejecting the countermeasure signal.
A discriminant frequently used to detect the presence of a flare is rapid intensity rise. The flare's intensity is presumed to rapidly rise to several orders of magnitude higher than that of the target signature. This intensity rise is used to trigger an IRCCM response. The response to a flare could simply be to turn the seeker off momentarily, allowing the flare to fall down and aft, out of the seeker's FOV [ILLUSTRATION FOR FIGURE 1 OMITTED.] A similar approach is to apply a push-ahead or limiting bias into the seeker-track-loop essential, resulting in the same countermeasures effect. More sophisticated seekers use a separate detector to provide the discriminating factor. A two-color seeker might use a second detector operating in the near-IR and compare it to the primary mid-IR detector. The presumption is that aircraft will have a lower signal in the near-IR than flares, which can be rejected as targets. A similar approach can be implemented with a single-detector seeker by comparing separated target intensity. The increasing variety of novel counter-countermeasures techniques has made it difficult to come up with a single flare countermeasure that defeats all threats.
The IR seeker transition to mid-IR or band-IV seeker was a fundamental counter to the first-generation jammer. The lack of any significant power in that band made first-generation jammers ineffective on all but the lowest-signature aircraft. Seeker designers also implemented the frequency-modulated conscan (FM conscan) or other continuous tracking schemes to further lessen the effect of jamming. FM-conscan seekers forced the jammer to compete directly with the aircraft signature - increasingly difficult with large-signature aircraft and high-temperature jammer sources. Jamming difficulty increased as more missiles came online. With a more diverse IR order of battle, picking the right jam code or devising an effective generic code became more difficult. Staring imaging seekers are also likely to be difficult for low-power jammers to effectively defeat. One way to deal with the imagers is to increase the J/S to a point where the target blooms from scattering and reflection effects within the optical path of the seeker itself.
Advanced Countermeasures Approaches
IRCM designers, clearly behind in the game, have pulled out all the stops in the search for effective techniques against second- and third-generation missiles. Facing a variety of missiles and IRCCM approaches, countermeasures designers have attempted to put up generic multi-fold techniques that defeat at least a few priority threats.
Flare designers have adopted fly-along or kinematic flares to defeat push-ahead or forward-bias CCM responses in the seekers [ILLUSTRATION FOR FIGURE 2 OMITTED]. To combat a wider range of IRCCM-equipped missiles, kinematic flares have been combined with multiple-component flares, literally presenting IR fireworks shows to the seekers.
Yet even the combinations of decoys counter only a limited number of specific threats. Spectral CCM, or two-color seekers, remain a concern for operational commands. Finding effective decoy techniques has proven difficult because of the temperature dilemma. The use of large spatial sources operating at lower temperatures, such as the mysterious Special Material Decoys (SMDs), has been pursued. The SMD radiates more energy in the same band as the aircraft, where the more advanced mid-IR seekers operate. The SMD has shown better effectiveness against missiles using spectral CCMs, but its disadvantage is its high separation rate from the aircraft, as the SMD quickly falls out of the missile's FOV. Service laboratories have been working on fly-along variants of the SMD to improve trajectory performance. Future flight testing of prototypes will determine if this is an effective countermeasures concept.
IR-jammer designers seeking to put more inband energy on the missile (higher J/S) have moved towards directional sources. Initially, flashlamps with mirrors were employed to increase the jamming power along expected axis of attack (normally to the rear). Directable mirrored systems using flashlamp sources mounted in turrets are now in development and should be fielded in the near term. Developers, finding directed flashlamps inefficient and bulky, have turned to laser sources to provide highly collimated inband jamming energy across a range of missile bands. Laser sources in the mid-IR have been considered low powered and technically immature, but recent years have seen major breakthroughs on several fronts. Frequency-doubled C[O.sup.2] lasers have achieved multiwatt performance in fieldable packages. Both solid-state and semiconductor laser sources have shown tremendous growth, reaching the point where operational designs are practical.
The use of laser sources can greatly increase the J/S but adds its own challenges. The relatively narrow (1- to 3-milliradian) beam width of the lasers requires precise and highly stable tracking for accurate pointing. IRCM lasers generally have small aperture requirements, allowing a drastic reduction in the size of the pointing system. Laboratories and industry are pursuing designs for "minihead" pointers as small as a softball. These designs reduce the integration and performance impacts seen with the pointers for directed flashlamp jammers.
A robust IRCM jammer would cover all missile bands; provide very high jamming energy; and use real-time, on-the-fly information to select optimal jamming modulations. A concept that provides this capability is called "Closed-Loop Laser IRCM" and is being pursued under the Air Force Research Laboratory (AFRL) Electro-Optical/Infrared Countermeasures Integrated Technology Thrust Program (EO/IRCM ITTP). Closed Loop uses innovative processing and tracking techniques to optimize jam codes to the specific threats. This allows a much tighter laser beam to be directed into the seeker dome, providing a much higher J/S. This concept is scheduled for testing in FY00 and, if successful, should provide technology for upgrades to the first-generation directable jammers now moving into operational use.
Directed Energy Techniques
The service laboratories continue to pursue directed-energy approaches to defeating the IR threat. High-energy laser techniques for defeating imaging-seeker detectors are also being investigated under the AFRL EO/IRCM ITTP. Longer-range plans are to use even-higher-energy lasers to effect catastrophic damage to the missile seeker or structure. This generic kill capability may provide countermeasures effective against EO, IR and RF missiles. In another thrust, high-power microwaves seem to be gaining new life as an IRCM technique in the AFRL's Directed Energy Advanced Tactical study. This concept, which envisions putting a defensive bubble around the aircraft, is being touted as the most robust solution to the problem.
Possibly the most critical element of any advanced IRCM concept is the missile-warning function. Operational missile-warning systems are functional and provide reasonable warning capability but are not without problems. Limited detection range under certain conditions and the bugaboo of false alarms are the operational community's recurring complaints. Developments in IR missile-warning sensors and processing are providing some hope of improving the situation, though at a cost. Single-color and two-color IR architectures are being pursued, but both are challenged by the age-old background and clutter problem. New processing techniques, higher-resolution arrays and multispectral discriminants are being refined to provide high probability of detection at long ranges with few false alarms. Service laboratories have strong programs underway collecting flight-test data on IR sensor and processing performance. Two-color sensors, initially tested using mechanical filter wheels to detect the separate bands, are now moving to multispectral staring focal-plane arrays based on stack diodes or quantum-well imaging photodiode devices. Another novel industry design, using a multiple-aperture split-imaging approach has been flight-tested and is showing promise as well. Motion detection and optical flow clutter reduction techniques are also under investigation. Many issues remain to be solved in the missile-warning arena, but confidence is running high that the technology is finally here to address this critical need.
While there is much to cheer about in IRCM developments, it is clear that the threats are still in the lead. Most military aircraft do not have onboard IRCM. Many that are equipped with IRCM do not have missile warning. Every contingency is taken on with the full knowledge that someone out there may be willing to fire on a US aircraft. We know that as time goes on, the old Strellas and ATOLLs are being replaced with much more capable third-generation missiles. Even more capable IR-imaging missiles, not specifically addressed in this article, are likely to show up on threat lists in the next ten years. Looking for sales, the Russian, Chinese, South Africans, French, Israeli and, yes, the UK and US will be selling IR missiles. Even if major nations are diligent in selecting to whom they sell them, the "previously owned" market will continue to flourish. How many Stingers from Afghanistan are still floating around the international arms bazaar? Bosnia-like scenarios will get more difficult as the threat changes. IR-missile dominance in the anti-aircraft kill record is likely to go unchallenged. So be careful out there.
Bill Taylor works for the Air Force Research Laboratory's Electro-Optical Technology Division, (Wright-Patterson AFB, OH), in the field of infrared countermeasures, supporting the Large Aircraft Infrared Countermeasures Advanced Technology Demonstration. He is a retired Air Force Lt Col with operational experience in F-4 Phantom II fighter operations and a tour in Wild Weasels. He subsequently was a requirements officer at HQ TAC and Program Manager for Infrared Countermeasures in the Aeronautical System Center, running the Advanced Strategic and Tactical Infrared Expendables (ASTE) program. Mr. Taylor finished his USAF career as the Deputy Chief of Electronic Warfare Division of (the former) Wright Laboratory and later was the Deputy Chief of Electro-Optical Technology Division in the Air Force Research Laboratory.
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|Title Annotation:||infrared guided missile|
|Comment:||Understanding the infrared threat.(infrared guided missile)|
|Author:||Taylor, William (American government official)|
|Publication:||Journal of Electronic Defense|
|Article Type:||Cover Story|
|Date:||Feb 1, 1999|
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