Fighting an invisible threat: air warfare involves both visible and invisible battlefields and the latter becomes more demanding every year--having extended from combat aircraft to support aircraft and now down to the helicopters and even civil aviation.

In the early days of air defence missile warfare, the weapon was remotely guided to its target by a ground-based or an airborne radar. Later, the missiles had the ability to guide themselves autonomously or semi autonomously with onboard sensors. This threat continues to exist, but it is essentially one for conventional or semi-conventional operations rather than asymmetrical ones, for these are extremely sophisticated weapons that require considerable maintenance support.

But while the threat from radar-guided weapons may have grown fainter it is not one that can be ignored. An aircraft's first line of defence remains the radar-warning receiver (RWR), which is now a common fit on both fixed- and rotary-wing combat aircraft as well as on the support aircraft that may have to operate within a combat zone.

The warning receiver uses either crystal video or instantaneous frequency measurement receivers to detect radio-frequency waves in the ranges employed for weapon guidance such as the I/J-band (1 to 18 GHz). The very first generation merely alerted the crew of the presence of the enemy within their vicinity and were then followed by systems that not only provided an audio warning (the famous 'Sam song' of the Indochina conflict) but also an indication of the direction of the threat.

Most receivers now feature digitally-controlled processing systems and a library of signal signatures that provides the pilot not only with an alert and an indication of the threat but also the latter's nature. It does this by comparing the incoming signal with an extensive threat library, usually one prepared by the user, that contains the parameters such as wavelength, pulse repetition frequency and pulse width of both hostile and friendly radars.

One major problem is that in areas such as Western Europe, but now also many areas of the Third World, the wide variety of emitters--including radios and wireless communications systems as well as air traffic control radars--create a dense electronic environment. This may confuse or even overwhelm the radar detector, even worse, create numerous false alarms that could encourage the crew to disable it. The systems must consequently be sensitive enough to detect the threat even in such an environment and sufficiently robust to operate in that very same environment, a performance that is akin to detecting a whisper in a football crowd.

A typical modern radar warning receiver is the widely used BAE Systems North America/Northrop Grumman Electronic Systems' AN/APR-39A family, a digital version of the analogue AN/APR-39 that weighs between 7 and 16 kg. The sensor is used mostly in helicopters such as the AH-1 Cobra and the AH-64 Apache, the UH-60 Blackhawk and the UH-1 Iroquois as well as in transport aircraft such as the C-130 Hercules. This system consists of a digital signal processor, two H/M-band (6 to 100 MHz) receivers providing continuous coverage, pairs of spiral antenna/detectors, a single C/D-band (0.5 to 2 GHz) blade antenna providing correlated data on the status of detected emitters, a radar signal indicator and a detecting set control unit. The signal processor incorporates a main system control microprocessor, a video processor, display and articulated audio units, a multiplexer, a C/D-band receiver, a power supply and a user data module that programmes the sensor.

The versatility of modern receivers may be gauged not only from the APR-39 but also from the 30-kg BAE Systems Australia ALR-2002. This consists of four quadrant receivers, a low band receiver, a data processor, track and interface processor and azimuth display indicator. Its dual receiver architecture is claimed to provide nearly 100 per cent probability of intercept for all emitter types and densities. It is part of the F/A-18 Hornet upgrade programme and equips both the S-70A-9 Blackhawk and CH-47D Chinook helicopters.

Asymetric Warfare Threat

A growing threat in the era of asymmetric warfare is the missile with electro-optical sensors, also known as a heat seeker. Indeed Northrop Grumman has noted that 80 per cent of aircraft shot down between 1958 and 1992 fell to these types of missile and only 20 per cent to radar-guided threats. During Operation 'Desert Storm' in 1991 alone heat-seeking missiles accounted for 75 per cent of American aircraft losses.

Particularly affected are helicopters and transports, because many of these fall victim to man-portable missiles. These are easy to conceal and fire and the weapons themselves require relatively little servicing which make them a useful tool for insurgents, indeed the recent downing of a Royal Air Force Hercules in Iraq is likely to have been to one of these weapons. What makes them even more significant is that terrorists have shown themselves willing to use them against civil aircraft with some 30 shot down in the past 40 years at the cost of 1000 lives.

These missiles home in on the infrared emissions from the aircraft. The passive nature of the missile seeker meant it was initially extremely difficult to detect but sensors equivalent to the RWR are now available. A typical one is the CMC Electronics AN/AAR-44 infrared missile warning receiver used by the US Air Forces Special Operations Command as well as the Belgian Air Force. CMC, by the way, officially became L-3 Cincinnati Electronics in October 2005.

The system weighs some 28 kg and consists of a track-while-search processor, a control/display unit and scanning infrared sensor heads to provide hemispheric or spherical fields-of-view. The latter detect emissions from the missile's exhaust plume and provide an indication of the direction of the threat together with visual and audio alerts. The system includes multi-spectral detection to provide multiple, simultaneous threat detection, positive missile warning with a minimum false alarm rate and includes resistance to decoy flares and angular detection of better than 1[degrees].

Most of these detection systems form the basis of a more sophisticated system, which usually integrates them with countermeasures systems to compose a defensive aids suite. The most common element is the dispenser, which usually consists of a control unit, and a programmer that controls the rate and type of cartridges expended. Originally chaff was attached in packets which would be released by the pilot at the appropriate time but later decoy dispensers were mounted in pods under the wing. They have the advantage of carrying more munitions to provide longer protection. However, they have the disadvantage of creating drag and reducing the number of underwing stations for fuel or ordnance. Therefore conformal systems were developed with the magazines attached to the aircraft fuselage. In some of the latest aircraft they are mounted within the fuselage so as not to compromise the aircraft's radar signature.

The ordnance itself consists of chaff or flares against radar- and infrared-guided weapons. The chaff is comprised of myriads of reflecting surfaces, some of silver paper others of aluminium-coated glass filaments, but all designed to create within the threat-range electronic envelope a false return that will seduce the hostile seeker and allow the would-be target to escape. The chaff must deploy rapidly but, simultaneously, it must occupy a relatively small volume of sky to give the hostile emitter sufficient time to be decoyed. With both chaff and flares the dispenser system ensures the decoys are dispersed at a set rate and in such a sequence that it creates a confusing pattern behind the aircraft.

Multiple-band Seekers

The flares pose very different and, in some ways, more demanding problems. Initially the infrared seekers aimed for the hottest part of the sky, which would be the rear of the aircraft engine or engines. Simple flares, and even sharp manoeuvres against the sun were sufficient to fool them, but seeker designs have become more sophisticated and now exploit other heat sources--including the structure warmed by aerodynamic friction and cockpit glazing, which reflects sunlight and is especially vulnerable to 3 to 5[micro]m seekers.

Dual-band seekers have been developed to operate both in the ultraviolet (0.2 to 0.45-[micro]m) end of the spectrum and in the infrared (1.1 to 10.6-]micro]m) band, in an effort to distinguish between the decoys and the targets and also to seek out the target through other sources of radiated energy. In some weapons, processors have the ability to reject sudden rises in energy levels caused by a flare or, alternatively, they note it and reject it 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 the uncluttered background makes them easier to detect their target, but the warning detectors (like the seekers by the way) are vulnerable to atmospheric conditions. To improve their sensitivity they thus require cooling systems, which affect both reliability and availability.

Ultraviolet seekers are more effective at lower and slower targets and are less vulnerable to false acquisitions 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.

A variety of products are available. Armtec Defense Products (formerly BAE Systems North America, Integrated Defense Solutions) produce both chaff and infrared decoys. Square format infrared decoys include the M-206 (often used in helicopters) and MJU-7 (often used in combat aircraft such as the F-16 and the F-15), while circular cartridges include the MJU-32, which is used in older combat aircraft and helicopters. The RR-129 is a circular format chaff cartridge cleared for a variety of fixed-wing combat aircraft including the F/A-18.

Alloy Surfaces (a subsidiary of the British Chemring Group) for instance, focuses on infrared decoys and has developed Special Material Decoys (SMD) which use patented pyrophoric materials to radiate in-band infrared energy as a decoy. Most decoys are extremely visible, but the SMDs are invisible by both day and night and can be combined with other systems to provide a cocktail of self-defence measures which can fit any current or projected flare band and can be tailored to match most specific aircraft signatures.

The ASD-118L has been designed specifically for RAF helicopters while the M211 is part of the US Army's Suite of Integrated IR CounterMeasures programme and is designed to protect its helicopters against manpads threats. With increasing use of low-signature aircraft the company has produced flares specifically for this threat, including the MJU-27 for the US Navy and the MJU-50 for the US Air Force. They are compatible with all existing dispenser systems.

An alternative means to chaff and one increasingly being introduced is an infrared jamming system, such as the BAE Systems North America AN/ALQ-157, which was designed for transport aircraft and large troop-carrying helicopters. Two transmitters are installed for unobstructed protection in azimuth together with a power supply, an electromagnetic interference filter assembly and a pilot's control indicator. In-flight the crew can select one of five pre-programmed jamming codes, although more can be added, and the system then generates confusing IR signals.

A more sophisticated approach is the Northrop Grumman Electronic Systems, Defensive Systems Division and Selex Sensors and Airborne Systems (formerly BAE Systems Avionics) Directable IR Counter-Measures (Dircm) system known to the US forces as AN/AAQ-24 Nemesis and to the Royal Air Force as ARI 18246. This uses the Northrop Grumman AN/AAR-54 passive missile warning sensor, a high-resolution (256 x 256) staring array fine tracking subsystem, a control indicator unit, a system processor and up to four, four-axis, steerable tracking/ jamming turrets.

Within the Dircm the AAR-54 detects the threat missile's exhaust plume and provides bearing data to the steering subsystem. With the turret (or turrets) suitably aligned, the 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 initial jamming source is a high-power, modulated arc lamp, which can be upgraded to a laser source, and is aimed directly at the missile seeker which can be distracted or destroyed. The system is being installed in C-17 Globemaster III and Special Operations' C-130s.

Radar jammer systems, once confined to Suppression of Enemy Air Defences (Sead) aircraft, are increasingly deployed in combat aircraft for self-protection. A typical system is the Saab Avitronics BOQ-A110 (formerly Erijammer A110); a 235-kg underwing pod with internal control and display units. The pod features three 120[degrees] antenna assemblies for hemispherical coverage with continuouswave/high-pulse doppler, direction-finding and set-on receivers covering the H-J-band (six to twelve GHz). The continuous-wave/high-pulse doppler receiver alerts the system to the presence of threat emitters, while the direction-finding unit identifies the sector in which the threat is located. The narrow-band set-on receiver is used to support some of the jamming modes and also analyses threat signals. Four additional reception antennas are located in the pod's side-mounted radomes and are used to support the equipment's Doppler jamming modes.

Selex and Northrop Grumman have also produced the Zeus radar warner and jammer, which is integrated within the airframe. It uses instantaneous frequency measuring and fast superheterodyne techniques to measure all potential radar threat characteristics including direction of arrival, time of arrival, frequency, pulse repetition interval, pulse width, amplitude and scan interval and rate. The system identifies the types of radar threats, measures the threat parameters and then uses the transmitters to jam both pulse and continuous-wave radars, while control features ensure that home-on-jam weapons are not allowed enough time to acquire the aircraft. New hardware, including a techniques generator and digital frequency memory, are now being added.

Another internally mounted system is the Northrop Grumman Electronic Systems' and ITT Avionics AN/ALQ-165 Airborne Self-Protection Jammer. This automatically selects the best jamming techniques against the threat in a two-band frequency range, based upon the system's library but, like most systems, the computer software can be modified to accommodate new threats as they arise.

Active and passive systems are also increasingly integrated into a defensive aid suite. Terma, for example, has developed the Electronic Warfare Management System, or AN/ALQ-213, initially to improve the defensive capabilities of the F-16 Fighting Falcon which includes the AN/ALR-69 RWR, the AN/ALQ-162 jammer and AN/ALE-40 dispenser. Most of the controls and indicators for these systems are replaced by a computer-based electronic warfare management unit and an electronic warfare prime indicator to create self-protection programmes, with the pilot having manual, semi-automatic or automatic options. The system can select the most effective combination of countermeasures by automatically analysing the incoming threat. The system has growth potential including increased computer capacity, control of new countermeasures including towed decoys, direct infrared countermeasures and laser warners while a tactical threat display can be introduced. Several countries have used the system in transport aircraft and helicopters.

One system for the latest combat aircraft is the Thales Airborne Systems/ MBDA Systeme de Protection, d'Evitement et de Contre-mesure du Rafale (Spectra) which covers radar, infrared and laser threats. This incorporates a range of technologies that includes interferometry, digital frequency memory, electronic scanning, multi-spectral IR detection, image processing, artificial intelligence, monolithic microwave integrated circuitry and very high-speed integrated circuitry.

The Spectra includes a phased-array radar jammer and is internally installed within the Rafale airframe with the elements integrated through a dedicated databus and central system processor. MBDA provides the infrared missile launch detector with internal infrared and chaff dispensers.

Similar systems are now being found in attack helicopters. A typical one from Selex is the Hidas which is incorporated in the WAH-64 Apache attack helicopter as well as Greek and Kuwait Apaches. It consists of the company's Sky Guardian 2000 RWR and Series 1223 laser warning receiver, the BAE Systems North America AN/AAR-57 missile warning system and the Thales Optronics Victon 78 Series 455 dispenser system, as well as a defensive aid suite controller. The system can be expanded with a radar jammer and a directable IR countermeasures system such as the Nemesis.