Infrared Countermeasures Systems
The US military has recognized the increasing threat to its tactical aircraft from anti-aircraft infrared (IR) guided missiles. By one estimate more than 500,000 shoulder-fired surface-to-air missiles exist and are available on the worldwide market. The lethality and proliferation of IR surface-to-air missiles (SAMS) was demonstrated during the Desert Storm conflict. Approximately 80% of U.S. fixed-wing aircraft losses in Desert Storm were from ground based Iraqi defensive systems using IR SAMS. Both IR SAMS and IR air-to-air missiles have seekers with improved Counter-Countermeasures (CCM) capabilities that seriously degrade the effectiveness of current expendable decoys.
Man Portable Air Defense Systems (MANPADS) are the most serious threat to our large, predictable, and slow flying air mobility aircraft. These systems are lethal, affordable, easy to use, and difficult to track and counter. According to a 1997 CIA Report, MANPADS have proliferated worldwide, accounting for over 400 casualties in 27 incidents involving civil aircraft over the previous 19 years. This proliferation has forced air mobility planners to frequently select less than optimal mission routes due to lack of defensive systems on airlift aircraft.
All Infrared [IR] direct threat weapons require line of sight [LOS] to be established prior to launch and the in-flight missile must maintain LOS with the target heat source until impact (or detonation of the proximity fuse). IR missiles require the operator to visually detect the target and energize the seeker before the sensor acquires the target. The operator must track the target with the seeker caged to the LOS until it is determined that the IR sensor is tracking the target and not any background objects (natural or man made objects to include vehicles, the sun, or reflected energy from the sun off clouds, etc.). The IR sensor is also susceptible to atmospheric conditions (haze, humidity), the signature of the aircraft and its background, flares, decoys, and jamming.
When an aircraft has been detected, targeted, locked-on, and the missile fired, the emphasis has to shift to defeating the in-flight missile. Of course, except in the case of autonomously guided missiles, countermeasures against the ground (or hostile aircraft) tracking and command guidance system could still be effective (as in the case of conventional RF countermeasures). There are already a number of countermeasures against RF seekers.Man portable air defense systems (MANPADS) which are shoulder launched missile systems typically include heat seeking or infrared (IR) missiles and are a threat to aircraft and other types of transportation. IR missiles include an IR detector, which allows the IR missile to detect and track a target. More particularly, IR missiles detect the heat signature (i.e., infrared light) which is emitted by hot structures, for example, engines of the aircraft, to track the aircraft in an attack. Heat-seeking MANPAD (Man Portable Air Defense) systems such as the FIM-92 Stinger missile present a critical and pressing terrorist threat to commercial air transport aircraft. The most vulnerable phases of flight are during landing approach and immediately after takeoff. Also many landing approach profiles require prolonged flight at low altitudes over populated areas. Nevertheless, although the stakes are high, the probability that any particular transport would ever be attacked is very low. Pyrotechnic flares are used traditionally for this purpose, but have short effective time durations. Routinely dispensing flares to draw possible MANPAD missiles away from a transport is clearly unacceptable. Dispensing flares or recoverable decoys when an attack is detected requires a sophisticated and costly missile attack sensing system. Recurring false alarms would likely cause unacceptable hazards from flares to people and property. Tethered decoys have also been proposed. Non-predeployed recoverable decoys must be deployed quickly after receiving a warning, which places stringent requirements on the tether line and requires a complex release and recovery system. Decoys must also radiate considerable IR power, which limits operating duration or requires significant power-carrying capacity by the tether. Fueled decoys must be refueled and battery-operated decoys need to be recharged or replaced, requiring costly and time-consuming ground operations. Another issue is handling potentially hazardous materials at passenger terminals. To be effective, a passive decoy must radiate IR energy at levels comparable to or exceeding that of an airliner. The hot parts (mainly the turbine plates) of transport aircraft engines present areas of about 0.5 m.sup.2 at temperatures ranging around 750 K radiating at about 1500 W/sr, primarily in the rearward direction. Conventional MANPAD-launched missiles include an infrared sensor that is sensitive to heat, for example the heat emitted from an aircraft engine. The missile is programmed to home in on the infrared heat signal using a steering system. Using a rotating reticle as a shutter for the sensor, the incoming heat signal is modulated, and, using the modulated signal, an on-board processor performs the calculations necessary to steer the missile to its target. Owing to its portable size, MANPAD missiles have a limited range, and a burn time of a few seconds from launch to extinguishing. In recent years, missile guidance systems have become increasingly sophisticated, and, as a result, there are a number of different types of missiles in existence. In some embodiments, the missile is outfitted with multiple sensors that detect infrared radiation at multiple wavelengths, using reticles that are encoded at different patterns. More recently, missiles that employ a focal plane array (FPA) have been developed. Such FPA-based systems attack aircraft based on image processing, rather than heat or radar signatures, and are trained to attack vulnerable locations of the aircraft, i.e. cockpit and rudder, which are more susceptible to fatal attack than the engines. In view of the threat, various countermeasure techniques have become popular. A missile warning system scans the region for rocket launch signals, such as the infrared or ultraviolet signature of a rocket tail. Upon the detection of a missile launch, various countermeasure systems are activated. In one example, hot flares or chaff are released from the aircraft to confuse the infrared or radar system of the launched missile. Other approaches broadcast light energy in order to confuse the missile infrared sensors. In one example, light energy emitted by non-coherent flashlamps is directed toward the missile sensors, in order to confuse them and render them ineffective ("jamming"). IR missiles are vulnerable to high powered IR carrier signals which blind the IR detector of the incoming IR missile. In addition, IR missiles are vulnerable to lower powered IR carrier signals that are modulated using certain modulating signals that confuse its tracking system and cause the tracking system to track a false target. Conventional countermeasures to an IR missile threat include jamming systems which confuse or blind the IR missile using either IR lamps and/or IR lasers. These jamming systems transmit either a high powered IR carrier signal to blind the IR detector of the incoming IR missile or, otherwise, transmit a lower powered IR carrier signal modulated with a modulating signal to confuse the IR detector of the incoming missile. In Closed-Loop InfraRed CounterMeasure (CLIRCM) systems, the optical subsystem of the missile sensor is remotely interrogated to determine its optical modulation frequency. Coherent laser energy that is specifically encoded in a suitable format by the countermeasure system is then directed toward the missile sensors, thereby confusing, or jamming, the missile sensors, causing the missile to be steered off course. High-power laser energy can also be used to counterattack FPA-based systems, by disabling the focal plane array. The IR lamp and/or IR laser jamming systems are heavy, complex, consume a great deal of power, and require significant space. Real estate in airborne platforms, as well as in most other transportation is typically at a premium or may not be available. Further, systems using IR lasers include precise pointing and tracking devices, which are hard to implement and produce drag on an aircraft platform. Owing to their extremely high cost, such countermeasure systems have enjoyed only limited use, primarily on military aircraft. The countermeasure systems are commonly integrated into the aircraft, for example, in the fuselage, wing, or nose of the aircraft, or fixed onto an outer portion of the aircraft. Depending on where the countermeasure systems are mounted to the aircraft, they can lead to an increase in drag, reducing flight performance and increasing operating costs. Also, servicing, maintenance, upgrading and testing of the systems are expensive and time consuming procedures. In addition, such procedures require grounding of each aircraft for a period of time. What is needed is a system that may jam IR missiles and that may have reduced size, weight and power (SWAP) requirements. Also needed is a system with a reduced time for pointing and having increased reliability and reduced drag on the aircraft platform.
The real challenge is posed by the shoulder-launched "fire and forget" type of IR guided missiles. In most cases, such missiles require lock-on prior to launch; they do not have autonomous reacquisition capability. Given an adequate hemispheric missile warning system (such as that in development), it is quite conceivable that the missile can be defeated in flight. One approach is to use an RF weapon (directed from the aircraft under attack, or counter-launched) to defeat the guidance electronics. For optical or IR seekers that are obviously not "in-band" to the RF weapons, a "back-door" means of coupling the RF energy into the attacking missile must be used. Such back-door mechanisms exist; however, they are notoriously unpredictable and statistically diverse, differing by orders of magnitude from missile to missile, even those of the same class, depending on the missile's maintenance history.
Rather than simply providing a second bright IR source in an attempt to draw an approaching missile away from a targeted aircraft, Directed infrared countermeasures systems [DIRCM] systems use beams of light, produced by a variety of means such as flashlamps, to exploit knowledge about the design of reticle-scan MANPADS seekers to defeat their homing mechanisms. In many MANPADS, a reticle within the seeker causes pulses of light from the target aircraft to "shine " on the missile 's infrared detector. The IR detector senses the IR radiation and sends an electric signal to the guidance package, which determines the target location and allows the missile track the target aircraft's location and movement through the sky. By shining a modulated light towards the seeker, an IRCM system provides the infrared detector with extra "false "data, which deceives or "jams "the missile, causing it to miss its intended victim. Northrop Grumman 's Nemesis system is a widely-utilized flashlamp-based DIRCM system. There are more than 3,000 IRCM systems deployed world-wide that protect against infrared guided missile threats.
Despite the advantages that DIRCM systems have over flares, these systems have limitations that have prompted a move towards laser-based systems, such as the Navy 's TADIRCM system and the Air Force 's new LAIRCM system. LAIRCM builds upon the NEMESIS platform but replaces the flashlamp source of IR radiation with a laser source.
Another approach is to use a laser to attack the threat in its seeker band. For highly dynamic aircraft that can maneuver to avoid the threat, it may suffice to simply blind the missile and assume it can be avoided. Current open-loop systems confuse missiles with random false targets or IR energy, making the missile wobble in flight, but not necessarily break lock. With a "smart" jammer, the oncoming missile is first identified, and then a tailored in-band jamming signal is sent to cause break lock or actual deflection. Both the Naval Research Lab (for ship defense applications) and the Air Force Wright Laboratory are developing multiwavelength laser systems to accomplish this countermeasure. This technology is one step beyond the near-term ATIRCM system in that the laser waveform is tailored to the specific threat and hence will cover a wider class of missile seekers.
In 1994 the Joint Directors of Laboratories/Technology Panel for Electronic Warfare (JDL/TPEW) published the Tri-Service Infrared Countermeasures (IRCM) Techbase Master Plan. Under the sponsorship of OSD DDR&E, the services compiled a comprehensive plan with all the services' 6.2 through 6.3 IRCM programs, including the Advanced Research Projects Agency (ARPA) IRCM laser program. The plan took more than 2 years to complete and was the work of the EO/IR Countermeasures Committee members and the TPEW principals. What was needed was an integrated, single program that addressed the needs of all three services to protect helicopters, high performance tactical aircraft, and large transport vehicles. The main technology areas addressed were missile warning, expendables, multi-line laser sources, pointer trackers, band four fiber optic cable, and jamming waveforms. The plan was separated into near-, mid-, and long-term programs because of an urgent need to get countermeasures against some of the IR missile threats fielded and the need to focus the service technology teams. Each of the services was given specific areas of research, many of which were critical to the other services' technology demonstrators and advanced technology demonstrations (ATD). In addition to the threat to aircraft, the plan also addresses the IR anti-shipping missile threat and the IR top attack munition/antitank guided missile threat to ground vehicles.
The infrared imaging missile seeker represents a leap in technology that may require more robust infrared countermeasures to defeat it, such as expendables that mimic an aircraft plume in several key ways. Resembling the aircraft to be protected would potentially defeat an IR tracker that may use several aircraft plume characteristics for tracking.
On 28 November 2002 an Israeli passenger plane taking off from Mombasa narrowly escaped being hit by two missiles. Few passengers were even aware that two rockets were fired at the Arkia Boeing 727 jet shortly after take-off. The pilot thought that a bird had hit the bottom of the plane. The crew saw "two white stripes, coming up from behind the airplane on the right side and a bit above us, and passing us from behind to the front of the airplane and disappearing after a few seconds." While there was early speculation that the Israeli Arkia Charter Co. airliner may have used protective technology to evade the missiles, this was apparently not the case. Instead, the aging missiles proved inaccurate or that the people who fired them weren't properly trained. But civilian aircraft owned by Israel's flagship carrier, El Al, are equipped with countermeasures against the heat-seeking weapons. Saudi authorities found an empty SA-7 tube in May 2002 near Prince Sultan Air Base in Saudi Arabia. According to some reports, the SA-7s fired in Kenya were from the same production lot. The serial numbers were not sequential but they are close to each other, leading one to believe that they might have come from a similar source.
Vulnerability reduction techniques are needed to insure the survivability of military and civil transport aircraft engaged by MANPADS missile threats. The MANPADS missile is a highly effective weapon proliferated worldwide. Typically containing an IR seeker, the missile offers little opportunity for a warning before impact. Impacts are often lethal. Examples of lethality include 1) the Afghan mujahedeen killing of 269 Soviet aircraft with 340 such missiles, 2) Desert Storm evidence that IR missiles produced 56% of the kills and 79% of the Allied aircraft damaged, and 3) civil aircraft experiencing a 70% probability of kill given a MANPADS hit. Such high kill ratios are unacceptable and require immediate solutions. Recent military engagements, such as Desert Fox, demonstrate curtailment of daytime operations as a result of the MANPADS threat. Civil aircraft remain virtual "sitting ducks" to terrorists. Delaying solutions may prove catastrophic.
Whereas susceptibility reduction (hit avoidance) should be regarded as the primary means of aircraft defense, optimal survivability can be achieved through an integration of susceptibility and vulnerability reduction (hit survival) techniques. Vulnerability reduction techniques are particularly necessary during take-off and landing when restrictions to tactics and countermeasures are in-place. Vulnerability reduction techniques are also particularly important for commercial aircraft in that the use of flares and rapid G-maneuvers is not appropriate. However, some solutions may prove applicable to all aircraft and threats encountered. Low risk example solutions for military-commercial aircraft application include relocating critical components away from hot-spots, locally hardening fixed critical components, moving hot-spots to less vulnerable locations, using sacrificial structure, and improved fire suppression techniques. While each example is expected to enhance transport aircraft survivability, proposed vulnerability reduction techniques need prioritized based on various orders of merit (i.e., cost, weight, effectiveness, aircraft type limitations, retrofitability, implementation time, etc.). Highly ranked concepts will be evaluated using modeling and simulation to identify probabilities-of-effectiveness as compared to unprotected aircraft systems. The most promising vulnerability reduction concepts will be transitioned into an advanced development stage of the program. Modeling and ground-based vulnerability testing will be performed to determine the success of competing systems.
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