Shake it off your tail! The sky is a hostile environment for military aircraft, especially with threats from fighters and ground defences to be reckoned with. In the past few years these threats have extended from the traditional combat aircraft to support aircraft and now to civilian airliners, driving a universal requirement for self-protection.

The threat comes less from the weapon than the sensor, and self-protection is therefore based on thwarting the sensor. For the past 60 years the prime threat has come from radar-guided/directed weapon systems and the first line of defence has been the radar warning receiver (RWR). This provides warning of radars (on land-based or airborne weapon control sensors) or by the sensors of surface-to-air or air-to-air missiles.

A wide variety of systems are on the market, of which typical examples include the AMS Sky Guardian 200, BAE Systems North America AN/ALR-56, the Elta EL/M-2160, the Elettronica ELT/156-158 and the Thales TDS family. These are all installed on a variety of platforms, often including fixed and rotary-winged, and while usually produced by one manufacturer this is not always the case. The AN/ALR-67, for example, is produced by Northrop Grumman (V2) and by BAE Systems/Lockheed Martin/Raytheon (V3).

All tend to be direct detection sensors using either crystal video or instantaneous frequency measurement receivers. These detectors actually provide more than mere warning, for they are digitally controlled processing systems and form the foundation of a comprehensive defensive aids suite. These systems can sort and identify all threats to the aircraft by comparing the incoming signal with an extensive threat library containing parameters of all known hostile and friendly radars. This library is usually prepared and updated by the customer.

Having identified the threat a radar warning receiver can then initiate the appropriate countermeasures where necessary, normally on a priority basis. These can be designed to shield the aircraft using electronic jammers and/or to decoy threats away by the launching of chaff or infrared expendables.

Infrared Biggest Plague, but ...

The growing electro-optical threat has also had to be addressed with ground-based weapon systems using laser targeting while the infrared seeker, which homes on the infrared signature of the aircraft's engines, has become an especially deadly threat.

Northrop Grumman has noted that only 20 percent of aircraft shot down between 1958 and 1992 fell to radar-guided threats and the remainder were lost to missiles with infrared seekers. During the first Gulf War (1990 to 1991) 75 per cent of American aircraft losses were due to the same threat that is ignored by radar warning receivers. For this reason missile warning systems integrating radar and optical seeker detectors, such as the Thales MWS-20, are frequently deployed in defensive aid suites to detect both radar and infrared guided missiles, particularly those approaching from the rear of the aircraft.

Two typical electro-optical detection systems are the Eads Missile Launch Detection System (Milds)--designated AN/AAR-60 by the United States--for rotary-wing aircraft, and the Northrop Grumman AN/AAR-54, which is compatible with high performance, fixed-wing combat aircraft and rotary-wing and support aircraft. Both use up to six sensor heads designed to detect the ultraviolet radiation of missiles, which allows the system to track them and to interface with a defensive aids suite computer to initiate the appropriate countermeasures.

To co-ordinate and integrate the operation of all aircraft protection suites a controlling system is required. Terma produces the Electronic Warfare Management System for F-16s, although it is used in a variety of other aircraft including transports. It integrates an RWR, a jammer and a dispenser system. The system was originally designed to reduce pilot workload in the F-16.

The range of threats facing modern high-performance combat aircraft means that an increasingly sophisticated defensive aids suite is required. One example is being produced by BAE Systems Avionics, Elettronica and Indra for the Eurofighter Typhoon as the Eurofighter Defensive Aids Sub-System (EuroDass). It consists of a defensive aids computer, an integrated radar-band electronic support and countermeasures package and a Missile Approach Warner (Maw). A Laser Warning Receiver (LWR) will be installed in British aircraft and EuroDass has a Counter-Measures Dispensing System (CMDS) associated with, but not part of it

The defensive aids computer provides system control and allocates system resources, while the electronic support and countermeasures package gives the pilot immediate warning of potential threats, analyses received signals and generates information co-ordinated with aircraft sensors. The countermeasure element provides the most effective jamming techniques including the use of a towed decoy. The Maw, developed by BAE Systems and Elettronia, is an active pulse-Doppler sensor. It appears that the Typhoon Batch 1 aircraft will receive a basic system with defensive aids computer and CMDS with installation of the complete system in Batch 2 aircraft.

In service on the Rafale is the Thales Spectra (Systeme de Protection, d'Evitement et de Contre-mesure du Rafale) integrated electronic warfare suite, which provides long-range detection, identification and accurate localisation of electromagnetic, infrared and laser threats. It consists of a phased array radar jammer, the MBDA Samir (Systeme d'Alerte Missile Infra Rouge) and a countermeasures dispenser.

Given the greater threat from missiles with infrared seekers (some 4000 are reported to have recently vanished from a depot in Iraq), and the complexities of the subject of radar countermeasures, this article will focus on the problems they pose to aircraft and the self-protection products required. The threat from manportable air defence systems is especially serious for, with some 500,000 delivered worldwide, they pose a threat not only to combat and support aircraft but also to civilian aircraft. In conventional combat a single soldier firing a missile worth some $ 500,000 can bring down an aircraft worth $100 million but the terrorist threat has steadily grown with some 30 aircraft brought down at the cost of some 1000 lives.

Infrared or ... Infrared?

The threat to aircraft has meant that air force transports have been routinely equipped with countermeasure systems and there are moves to extend this protection to civilian aircraft. Aircraft usually have four potential heat sources: exposed engine parts, the exhaust plume, structure warmed by aerodynamic friction and cockpit glazing, which reflects sunlight. In the two to three-[micro]m band exploited by first-generation heat seeking missiles, which remain in widespread service, hot metal is the predominant source. Energy emanating from the engines (including the exhaust plume) accounts for more than 99 per cent of the total heat signature. Skin heating becomes more important in the three to five-[micro]m band employed by more modern infrared seekers, but generally represents only about 20 per cent

The solution has been to use flares to confuse and to distract the missiles to prevent them homing in on the energy radiated by the engine, and this has led to the development of dispenser systems which ensure the flares are be dispersed at a set rate and in a sequence which would create a confusing pattern behind the aircraft. In turn the missile manufacturers developed dual-band seekers operating in the ultraviolet end of the spectrum, or the 0.2 to 0.45-[micro]m band, and in infrared, the 1.1 to 10.6-[micro]m frequency band, to distinguish between decoys and real targets and to seek the target through other sources of radiated energy. Other methods involve using processors to detect and to reject sudden rises in energy levels caused by a flare or noting and rejecting the flare to re-acquire the target.

Dual and single-band sensors continue to be available to meet differing roles. More sensitive seekers are preferred for air-to-air missiles because infrared radiation is more focused to provide an improved angle-of-arrival in most atmospheric conditions. The uncluttered background makes them easier to detect but the detectors, like the seekers, are vulnerable to atmospheric conditions and to improve sensitivity. The sensors, therefore, require cooling systems, which effect both reliability and availability.

Ultraviolet seekers are more effective at lower and slower targets and are less vulnerable to false targets such as decoys. The sensors are smaller, lighter and require less cooling but are more vulnerable to atmospheric conditions while sensitivity and resolution are poorer.

Dispenser systems have traditionally consisted of a control unit and a programmer that controls the rate and type of cartridges expended from the dispenser. Dispensers, which use both chaff (against radar seekers) and flares, are either conformal or podded. The conformal systems, such as the BAE Systems AN/ALE-40 and AN/ALE-47, consist of cartridge magazines attached to the aircraft fuselage, pylons or even internally while podded systems, such as the CelsiusTech Bop and Boz family, consist of dedicated underwing units which have a larger number of dispensers and can provide longer protection.

The prime producers of chaff and flares are BAE Systems North America, Chemring, and Pyrotronics, as well as Israel Military Industries. Dedicated flare producers for aircraft include Alloy Surfaces and Kilgore Flares (part of Chemring) in the United States as well as Britain's Wallop Defence Systems. They tend to be ejected at a speed of 25 to 45 metres/second and to burn for between two and four seconds with compositions that are designed to 'dazzle' or to confuse heat-seekers.

Flare deployment is often made on a contingency basis; an aircraft entering an area where shoulder launched missiles might be encountered will automatically begin deploying infrared munitions as a safeguard. The result is that aircraft appear to deploy a trail of 'comets' with bright heads and smoky trails--the problem is that this also provides a visual warning to missile operators.

If an aircraft is flying at some 4500 metres (15,000 feet) and if a shoulder-fired missile system is some eight kilometres away the operator will typically require about three seconds to identify it, two to initiate the missile, three to allow the gyros to spin up and four seconds to acquire, aim and launch. With the missile likely to take between ten and twelve seconds to reach the target the total engagement sequence is about 24 seconds but this can be reduced if the target advertises its presence.

Refining the Art

Pyrotechnic manufacturers have recognised this problem and are reportedly reacting to it. The solution is to have cartridges that no longer radiate in the visual 0.4 to 0.9-pm-band but only in the ultraviolet and infrared bands.

Alternative means of aircraft self-protection do exist, and the United States has taken the lead in the production of infrared jammers. One of the most common is the BAE Systems North America AN/ALQ-144A, which is used by some 20 customers for their helicopters. It consists of a cylindrical infrared source surrounded by a modulation screen that directs the energy at the missile.

For transport and even business jet aircraft, with customers including Air Force One, the company produces the AN/ALQ-204 Matador: a modular system consisting of multiple transmitters and controls. Each transmitter contains a four to twelve-kW source that emits pulsed infrared radiation to combat multi-IR guided missiles. The Matador configuration is certified by the US Federal Aviation Administration.

An alternative system developed by Northrop Grumman, BAE Systems North America and Boeing is the AN/AAQ-24 Nemesis Directional Infra-Red CounterMeasures. Now generically known as the Dircm, the Nemesis incorporates a 256 x 256 staring array fine tracking subsystem and turreted defence system. The Nemesis detects the missile's exhaust plume using the AAR-54(V), which provides bearing data to the turret. A tracking subsystem establishes a target box in which the threat is calculated to appear and maintains turret alignment in this box for the duration of the engagement. The jamming source initially is a high-power, modulated arc lamp but this will be upgraded to a laser with demonstrations using the Fibertek Viper solid-state, diode-pumped, multiband laser source. The system is being produced for the C-130 Hercules and C-17 Globemaster III.

A European Dircm programme was announced at this year's ILA exhibition in Berlin. Diehl, Eads and Thales are cooperating on electro-optical countermeasures developments to meet new threat scenarios. They have developed the Flash laser-based Dircm demonstrator, which has been trialled since last year and is intended to be part of the A400M self-protection system with later applications on other aircraft. MBDA in France is developing the Iris-400 missile warning system to complete the airlifter's self-protection system.

The demand for aircraft self-protection systems continues. In July Eads revealed it had won a contract from Denmark for 30 Milds systems, which will be installed in F-16s. Germany has also ordered the system for its helicopters; the CH-53s scheduled for Afghanistan are to be the first recipients. Thales and Eads have won a contract worth the equivalent of $ 120 million for threat warning equipment which will be installed in Eurocopter Tiger and NH90 helicopters for European and Australian customers. The equipment consists of a Thales radar warner and an Eads laser warning receiver and will be complemented by an MBDA Saphir-M decoy system, of which more than 500 were ordered at the beginning of September in what the company claimed was largest single contract placed for such a system in Europe.

Of growing importance is the requirement to protect aircraft that would not normally be exposed to man-portable missile threats. New Delhi, for example, has requested aircraft protection systems worth $ 40 million for three Boeing 737s used to transport government officials. The Indians want the AAQ-24, ALE-47 and the AN/ALQ-211 electronic warfare systems. The threat to government leaders and officials is credible; indeed it was the shooting down in 1994 of the Falcon 50 business jet carrying the presidents of both Burundi and Rwanda that contributed to the genocide that took place in the latter country.

Airliners in the Crosshairs

Recent incidents have shown that airliners are now becoming terrorist targets, one of the most recent being in 2002 when an Israeli aircraft leaving Mombasa in Kenya had two missiles launched against it, fortunately without success.

Since early last year the US Department of Homeland Security has had a requirement for a counter-man portable missile protection system that can be installed on airliners. In August it was announced that BAE Systems Information and Electronic Warfare Systems and Northrop Grumman have each won $ 45 million 18-month Phase II system development and demonstration contracts to that end.

The BAE Systems' proposal is based on the AN/ALQ-212 advanced threat infrared countermeasures system along with the AN/AAR-57 common missile warning system, which is in production as a podded system for the US Army, the British Army and the Royal Air Force. This includes a laser jammer in a turret. The counter-man-portable missile version would be housed in a conformal pod to reduce drag.

The Northrop Grumman system, Guardian, is based on the AAQ-24 Nemesis repackaged into a conformal pod fitted onto the rear fuselage. The company will complete the design and fabrication of the prototype system for flight testing and certification in two commercial Boeing 747 and MD-11 aircraft.

When trials are completed successfully one system will be recommended for production and will undoubtedly be well placed to win orders outside the United States. Within the United States the initial application for the system is expected to be the approximately 300 wide-body airliners making up the US Civil Reserve Air Fleet, upon which the country's armed forces rely heavily for transport during times of conflict. Both companies estimate the unit cost of their system, assuming production of at least 1000, to be less than one million dollars.

The Israeli government has a similar programme and, during the 2004 Farnborough Air Show, it was revealed that Elbit's Music multi-spectral system had recently been selected to meet this requirement. The Music is essentially a Dircm-like system integrated with a missile warning suite and uses a thermal imager to acquire and to track missiles and a laser jammer supplemented by a dispenser system.

The Airliner Requirement

The requirement is to protect Boeing 737-size or larger airliners during the ten-minute periods covering take-off and landing--the envelope in which shoulder-fired missiles operate. The typical vulnerability zone around an airport is 80 kilometres long and eight kilometres wide, an area of 640 k[m.sup.2], extending up to 4500 metres. The system must have a 90 per cent probability of success against multiple missile launches or more than 80 per cent against two missiles that will hit the aircraft simultaneously. False alarms should occur no more than once every 100 takeoffs or landings, or 17 hours of operation, whichever is the lower, while the maximum specified weight is 450 kilograms. The system should raise total drag by no more than one per cent at cruise speed and altitude.