Infrared countermeasures for helicopter applications.
The helicopter's operational characteristics and the signature observability or radiometric makeup of the platform have a profound influence on the successful implementation of IRCM. IRCM development is a montage of interrelated technical disciplines.
Supporting technologies necessary to the successful development and production of an IRCM system as shown in Figure I must as a minimum include weapon system exploitation, optical radiometry, weapon system simulation facilities, material technology, IRCM applications analysis and development and production facilities. FOOLING SMART WEAPONS
The topic of IRCM is linked with that of precision-guided munitions or "smart" weapons. IRCM provides a defensive response to smart weapons - specifically those designed for the optical region of the electromagnetic spectrum. The optical spectrum of IRCM encompasses emissions in the ultraviolet, visible, near infrared (IR) and the far IR. The target sensing guidance unit of the smart weapon is the optical receiver of these IRCM emissions. The smart weapon can change its direction or react in flight to hit its target. Helicopter engagements with smart weapons primarily are focused upon the surface-to-air or air-to-air passive homing missile. The passive missile guides itself with energy emitted by the target or natural sources, such as the sun, reflected by the target (Figure 2). The homing missile has a target tracking device contained within it. This device generates the missile's steering signals. Since passive missile systems do not require any further intervention from the launcher, they are true fire and forget" weapons.
Smart weapon defenses have had important implications for the use of tactical aviation everywhere. A recent example was the war in Afghanistan. The hidden mountainous retreats of the Mujahideen were impossible for the Soviet Army to reach with its armored units. This forced the Russians to strike from helicopters and bombers.
The first smart-weapon defense the Mujahideen received was a small, portable, shoulderfired IR homing missile, the Russian SAM-7. The Russians knew their own missiles and the best way to defeat them. Their countermeasures gave them air power superiority until the spring of 1986 when the Mujahideen received US Stinger IR missiles. The Stinger was the best missile of its type in the world. It had a longer range, was more maneuverable and it better differentiated between aircraft and decoy flares than the SAM-7. It could pick out aircraft hugging the ground and its heat-seeker was sensitive enough to home in on any part of an enemy aircraft.
Stingers made the use of helicopters costly. The Mujahideen claim the missiles destroyed more than 400 Soviet planes, or more than a third of the Soviet Union's annual production. Although this figure may be exaggerated, there is no doubt they forced the Soviets to shift tactics. The use of aircraft was curtailed and air support became less certain.
Unlike US Redeyes and Soviet SAM-7s (which are tail-pursuit missiles), the Stinger can be fired at incoming aircraft from more than three miles away. It is also much harder to decoy with IRCM. As the Stinger approaches, its guidance system steers it away from the target's exhaust plume and into vulnerable areas of the fuselage.
State Department sources say, with the Stinger, the Mujahideen brought down one Soviet aircraft a day. A Soviet-made Hind-D helicopter costs about $8 million; a Stinger, $50,000. The arsenal of small, cheap, smart weapons is growing as illustrated in Table 1. IRCM TECHNIQUES
All SAM systems have weaknesses, however. They are susceptible to countermeasures.
Since the distinguishing feature of the IR missile system is it homes on its target, the countermeasures to be used against it are therefore governed by the detailed nature of missile sensor. IRCM approaches are based on two concepts - the generation of false targets and the suppression of radiation that will reach the missiles detection/homing system.
The longest established IRCM consists of camouflage paints and direct helicopter signature suppression. Paints have been applied to aircraft to minimize its visibility by reducing optical contrast between it and its surroundings. This reduces its detection by weapons operators. Evasive maneuver or mission tactics may be included in the category of camouflage, i.e., reducing the radiation by presenting the least favorable aspect to the weapons systems.
The use of paints to minimize temperature contrasts between the skin of the helicopter and its surroundings is one way of suppressing the radiation that will reach the weapons detection system. The temperature differences necessary to minimize the distinction between the vehicle and its surroundings are typically said to be no more than 2' to 4'. This is not easy to achieve but the situation is helped by the fact the temperature of the surroundings can vary by more than twice this amount.
The desire to produce an IR, low-observable vehicle is a complexity of elements (Figure3), anyone of which can dominate in a given engagement. The general approach to suppressing the IR radiation from a platform is to "hide what cannot be cooled and cool what cannot be hidden." Further reduction of the helicopter's, IR signature is largely a matter of shielding (hiding) hot spots. The most difficult spots are the engine exhausts, even though the exhaust can be reduced (cooled) by diluting it with engine cooling air. The most successful implementation of IR signature suppression has been achieved for the helicopter.
To see without being seen is an old recipe for victory. One of the oldest means of IRCM is deploying smoke when the helicopter is fired upon. Of the different screening smokes, the conventional white type is effective only against visual observation. This led to the development of multiband screening agents that are effective against IR sensors. They can be based upon a combination of absorption, scattering and reflection produced by clouds containing relatively large particles or on clouds of IR radiating material, the emission characteristic of which provide effective screening in the IR as well as the visual regions of the spectrum.
The effective deployment of the screening agent to achieve area coverage on a moving platform remains a difficult problem. As with all such reactive countermeasures such as smoke and decoys, the number of agents that a platform can carry is limited. In general, these countermeasures are used only in a reactive mode, which to date has meant deployment in response to observed threats. For all the success of passive countermeasures in making the helicopter less conspicuous they alone are not enough to make the difference in reducing threat lethality to an acceptable level.
The false IR target or expendable decoy has been shown to be an effective countermeasure for the protection of aircraft against the IR homing missile. When SAM-7s were first used by the Vietcong against US helicopters in 1972 they reportedly scored a hit for every three missiles fired. The system's effectiveness diminished after learning the missile could be neutralized by dropping decoy flares, a solution that was quickly followed by bolt-on shields that cloaked the IR radiation of the engine exhausts.
The SAM-7 also had an unimpressive success rate in the 1973 Yom Kippur war. As a heat sinking missile it was known that it homes in on other heat sources such as the sun or decoys. The Israeli Air Force used preemptory deployment of flares during an attack on a target that might be defended by IR missiles. As the aircraft entered the probable IR missile threat zone the flares were dispensed at timed intervals until the zone was exited.
The decoy has proved to be an effective IRCM. By means of simple deception the decoy gives the incoming missile a more attractive target than the intended helicopter. The path of the helicopter and the decoy diverge sufficiently so the missile misses the helicopter.
The decoys employed so far have been simple devices aimed at deceiving the missile's guidance during its Right. The decoy is considerably less weapon dependent than on-board active countermeasures. However, the factors affecting decoy effectiveness are surprisingly similar. Thus, weapon characteristics, platform emissions, how the countermeasures are to be employed and the weight and volume restrictions of the platform are like dependencies. The decoy must also consider the means for detecting the weapon launch, the range of the detection and the decoy trajectory with respect to the dispensing helicopter.
Historically, the decoy was the first IRCM to be used operationally. The advanced development models of the air-to-air IR missiles in the 1950s prompted development of decoys, beginning from photoflash cartridges formerly used on aerial reconnaissance aircraft. The IR pryotechnic flares began with the RITA I on B-47 and B-52 aircraft in 1958. The IR decoy flare) continues to be the most widely deployed countermeasure to the IR missile. The M-130 aircraft general-purpose dispenser is one of the widely deployed systems. The M-130 was designed for use on helicopter platforms and is presently installed on UH-60A, OH-58A/C, AH-LS, UH-LH/V and CH-47C/D aircraft. This chaff and flare dispenser weighs 48 pounds with a double dispenser and carries the M-206 countermeasures flare and the M-1 countermeasure chaff.
Current research and direction in the flare decoy area may be driven by its own success. The flare decoy has been so successful that most modern IR missiles include the capability to identify and reject the IR flare decoy. Flyalong decoys and the coordinated use of decoys and on-board active countermeasures are presently under study.
Whatever the type of expendable countermeasures, the effectiveness is critically dependent on timely deployment. For this to be achieved the IR missile threat must be detected at the earliest possible stage and responded to quickly. This is particularly acute in the case of the helicopter environment. This response, in most cases, cannot be done by the crew relying strictly upon visual and manual controls. As a consequence automatic missile warning systems integrated with the countermeasures dispenser provides automatic deployment.
A JED article in June 1989 surveyed the active and passive missile warning systems. The functions of an integrated warning system drawn from that article are shown in Figure 4.
The integrated concept is now moving from concept to advanced development stage. A contractor team of Northrop Electronic System Division and Westinghouse Electro-Optics Division is developing integrated demonstration hardware for helicopter IRCM application. This hardware will illustrate the feasibility of warning and on-board countermeasures response as its first objective. Integrated IRCM suite development will follow closely as an advanced development proof-of-principal system beginning in late 1991. ACTIVE SYSTEMS
The active IRCM system or IR jammer is a platform-based transmitter that generates optical signals superimposed on the energy emitted by the helicopter. These signals, when received by the smart IR missile, cause the missile to deviate from its intercept trajectory and miss its intended target.
There is a strong case to be made for active IR self-protection systems; they provide the only nondepletable and feasible defense against the omnidirectional modern IR-guided missile smart weapon now facing helicopters.
A helicopter active on-board airborne IR jammer is unique in its application. The most critical factor in the determination of the IR jammer's effective radiated power is the helicopter IR signature. This is one area where the platform and the IRCM work to match observability with the jammer's effective radiated power, leading to the simultaneous trade-off of low-observability features with active on-board jamming technologies. The comparative light weight of the helicopter airframe dictates that the IR selfprotection equipment be as lightweight and power-efficient as possible. The ideal situation is to maximize the use of multifunction systems within the countermeasure, thus eliminating duplicate hardware.
From 1962-1972, active IR jammer development was underway for countering the air-to-air Atoll AA-2B IR missile. These efforts intensified as the proliferation of the shoulder-held, surface-to-air missile increased. These earlier smart weapons could be countered by causing optical break lock, wherein the active countermeasure system forces the missile to lose guidance, "go ballistic" and miss its target.
Also in the early 1970s it was realized future missiles would use the longer-wavelength IR optical spectrum and a higher degree of sophistication for target-sensing guidance. Active countermeasure applications were directed to the Block III Redeye missile. It was illustrated that the conicalscan IR missile could also be countered. These investigations showed on-board active countermeasure jammers would produce large deviations or miss distances for a wide range of both surface-to-air and air-to-air IR missiles exclusive of the type of target sensing guidance that smart weapons used.
The Sanders ALQ-144 IRCM system is representative of the current generation of helicopter on-board self-protection jammers. This lightweight system is an omnidirectional, spectrally broad emitter. The success of this broad-area jammer was in the optimization of the source temperature/size trade-off in favor of large source size and lower temperature. The reason for favoring this is intimately tied to the broad area and intensity of the helicopter's signature and the effective temperature which produces the IR emission. The current ALQ-144 systems, as reported in September 1989, are "being upgraded t,; include several additional bands of the IR spectrum not covered by the original systems." The report went on to say that the original or its upgrade "uses a piece of graphite in a glass tube that is electrically heated. The system then uses lenses with special collecting geometry to gather the energy efficiently so it can be radiated. A reflector rotating inside the housing projects a beam out. The beam moves in a 360' are to provide omnidirectional coverage to counter a missile approaching from any angle." The ALQ-144 weighs only 28 lbs., but operates on a continuous 1,200 W, which is a substantial load for a helicopter. ACTIVE IRCM DESIGN
Because the IR missile smart weapons are developing at such a rapid pace, it is continually necessary to re-examine the protection envelopment provided by current active IRCM. The development of the optical modulation techniques is a concurrent effort as well. The key to countermeasure development is to provide a general operational capability rather than one dedicated to a specific smart weapon.
One common element that all the active jammers must possess is the ability to produce the required optical energy consistent with the jamming technique employed. It follows that this energy must also be projected into a field of view whose angular extent covers the direction from which the smart weapon poses a threat.
The rule of thumb is the more optical interference the countermeasure creates the larger will be the distance that the weapon misses the aircraft. There is currently renewed interest in directed energy and power managed active countermeasures to create more optical interference more efficiently.
The evolutionary process of the active on-board IR jammer is typically to increase the output power when dealing with more complex IR missiles. This approach has its limitations, both in the jammer volume vis a vis allowable output aperture size and the prime power consumption of the jammer. The quandary of installation trade-offs, using large-size and low-temperature sources for aperture-limited systems, is alleviated by optimization of incoherent directed energy jammers that incorporate small-source areas and high radiance. The directed energy IRCM produce a high-radiance beam of IR energy that is directed to the incoming missile by an on-board missile warning system.
Northrop Electronics Systems Division is pursuing just such a development. The goal of matching the jammers field of view to that of an existing missile warning devices has been achieved by a Northrop and Westinghouse development team. The integrated jammer and missile warning receiver insure that the countermeasure is continuously on the missile and providing a continuous tracking capability.
The electrical demands that active on-board countermeasures place on the helicopter power units are being addressed by electrical power management of the countermeasure resources. The directed IRCM system is one example of a class of jammers that are activated only when alerted by the warning receiver - and then only to a power level equivalent to that required by the particular spatial location of the missile. That is, spatially match the required countermeasures power to the power emitted by the platform.
Advanced IRCM for helicopter aircraft has entered one of the most encouraging initiatives that has been seen in a long time. Systems now in the development stage feature integration and automation. While the small, high-speed, passively guided IR missiles are changing the rules of air combat, current IRCM initiatives are employing the optimum exploitation of combined active, passive and tactical countermeasures to ensure helicopter survivability for the future threat environment.
The key to new IRCM development is to provide a countermeasures' response only when necessary and then only against specific threat modes of operation. This approach maintains the IRCM's robust nature while leveling the system complexity with a corresponding lower cost, weight and size.
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|Publication:||Journal of Electronic Defense|
|Date:||May 1, 1991|
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