Defensive aids systems: electronic armour

Defensive aids systems: electronic armour

The dozens of aircraft shot down in Afghanistan and Iraq over the last decade continue to underline the importance of self protection systems for aircraft, especially helicopters operating within easy range of man portable air defence systems. At the same time, advanced aircraft like the Chinese J-20 and Russian / Indian PAK FA are adding to the number of threats modern military aircraft face. The rapidly growing multi-billion dollar aircraft self protection market is testimony to the fact that such systems are an essential component of modern warfare and are something that no military aircraft can afford to fly without.

An aircraft operating in a hostile military environment may be exposed to a variety of threats, including small arms fire; anti-aircraft artillery (AAA); surface to air missiles (SAMs) and enemy aircraft. Taking appropriate countermeasures involves first detecting the threat and then neutralising it. The combination of self-protection sensors and countermeasures, with a controlling system to integrate them, is referred to as a defensive aids subsystem/system/suite (DASS/DAS/Def-Aids). Such a system detects, prioritises and counters threats automatically, and typically employs chaff and flares or directed countermeasures, such infrared (IR) lamps or lasers. However, many defensive aids systems include electronic warfare (EW) equipment that blurs the boundary between defence and offence.

The first step to protecting an aircraft is identifying potential threats through radar, laser and missile warning receivers.  The staple of just about every defensive aids system is the humble radar warning receiver (RWR). A typical RWR is able to detect a wide range of radar signatures and match them to an internal library of thousands of emitter signatures. By carrying at least four RWR antennas, an aircraft is able to determine the direction of the radar. A typical RWR system is able to cover the most common radar frequencies from 2 to 18 GHz. Once identified, the signals are then prioritised and sorted. The data is then sent to a cockpit display or onboard jammers, which are another key part of an aircraft’s electronic armoury. A wide variety of RWRs are on the market, typical examples of which include the Selex Sky Guardian 2000, BAE Systems North America AN/ALR-56, Raytheon AN/ALR-67 and Elettronica ELT-156-158.

Once a threat has been identified by a radar warning receiver, either countermeasures can be dispensed or the threat jammed. Electronic countermeasures (ECM) equipment is usually carried internally, but many podded systems are also available.

A typical jammer will feature a receiver, a processor and a transmitter that is tuned to the frequency of the hostile transmitter. There are two main types of jammers: noise and deception.  A noise jammer transmits a signal reminiscent of electrical/white noise, which causes the radar return from the aircraft to be obscured. At long ranges noise jamming may cause the aircraft to disappear from the operator’s display entirely. However, at close range a large amount of power is needed to overcome hostile signals. Noise jammers are most effective if they can target a specific frequency range so they can focus power into a narrow spectrum. It is difficult generating enough power to jam many frequencies at once.

It should be noted that many modern radars are frequency agile and can often change wave properties on a pulse-by-pulse basis, making noise jamming an unpredictable process.  In addition, many modern radar systems have a home-on-jam (HOJ) mode. Furthermore, by employing sidelobe reduction and artificial intelligence techniques, radars can become resistant to many forms of jamming. Low Probability of Intercept (LPI) radars are almost invisible to some medium-capability radar warning receivers, especially advanced active electronically scanned array (AESA) units like the F-22’s AN/APG-77.

Deception jamming is a more sophisticated technique as it involves transmitting signals that mimic the real radar return. By transmitting a radar signal slightly before or slightly after the real return, the radar receiver gets the impression the target is closer or farther away than it really is. By transmitting a signal matching that of the radar, a false target jammer can create multiple targets on the victim’s radar. Although a radar operator may know he is being deceived, he may be unsure which target is real.

Track breaking techniques are used mainly against tracking/guidance radars. The technique works by sending back a stronger signal than the genuine return, causing the radar to lock onto the fake target, which will often become distorted and jump around. Conical scan radars can be jammed by varying the amplitude of the jamming signal at a rate similar to the rotation rate of the antenna. Such Amplitude Modulation (AM) ECM drives the antenna off target.

Monopulse radars are harder to jam and require techniques like cross eye jamming. In this case, two repeaters transmit a radar signal with time delays, distorting the shape (and thus the perceived direction) of the returned echo. The monopulse radar then tracks the area of the false signal.

There are some fifty plus radio frequency jamming systems on the market. For example, the AN/ALQ-162 from Northrop Grumman is an important and popular radar jammer, used by three US military services and allied nations including Canada, Denmark, Italy, Kuwait and Spain. The 40 pound (20 kg) unit is installed internally on many helicopters, inside the BAE Systems ALQ-164 pod on the AV-8B, and inside a wing pylon on F-16 and F/A-18 aircraft. It is able to counter both continuous wave and pulse-Doppler radars. Other examples of (mostly pod-mounted) radar jammers include the Northrop Grumman (NG) AN/ALQ-131(V) and AN/ALQ-135(V); ITT/NG ALQ-165; ITT AN/ALQ-136; Selex Galileo Sky Shadow; Elta EL/L-8212/22 and Elettronica ELT/553 and ELT/555 family.

Complex electronic countermeasures techniques and jammers do not always defeat hostile radars, and so Towed Radar Decoys (TRDs) were developed, to either jam hostile radars or to lure missiles away from the aircraft. The first recorded operational use of a towed decoy occurred during the 1991 Gulf War when a number of Royal Air Force Nimrod MR.2 maritime patrol aircraft deployed the BAE Systems (Selex) Ariel TRD on Operation Granby.

TRDs are deployed on some type of cable, usually Kevlar with a fibre optic link, and can be either recovered or jettisoned after use. Decoys have moved from being threat specific to wideband devices capable of countering a wide range of threats. The Raytheon ALE-50 is a typical example of a TRD. First deployed on the F-16 in 1996, it is also used on the Hornet, Super Hornet and B-1B Lancer. Other examples of towed radar decoys include the Rafael X-Guard, Cassidian (EADS) Sky Buzzer and BAE Systems AN/ALE-55.

Not all active decoys are towed, however. The Raytheon GENeric eXpendable (GEN-X) cartridge is a small, active expendable device that emits signals to lure radar-guided missiles from their intended target. It follows on from the Primed Oscillator Expendable Transponder (POET) system. It has a wider frequency range than the system it replaced and can be released from standard dispensers such as the AN/ALE-39, AN/ALE-40 or AN/ALE-47. GEN-X entered service in 1994, with the US Navy and Marine Corps.

Northrop Grumman has noted that only 20% of aircraft shot down between 1958 and 1992 fell to radar guided weapons, as missiles with infrared seekers posed a far greater threat.  Radar warning receivers and towed decoys are less than half the defensive aids battle – during the first Gulf War, 75% of American aircraft losses were to threats not detected by radar warning receivers. As a result, missile approach warning (MAW) systems form another essential part of defensive aids suites.

Missile approach warning systems that use active sensors (radar) can detect all aspects of a projectile launch, with almost zero false alarms, while passive UV/IR sensors only detect the launch and booster plume of the projectile. Passive sensors are most common as they are discreet and less complex than active systems. Like a RWR, a central computer analyses threats and displays them in the cockpit or automatically launches countermeasures. Similarly, laser warning systems operate in the laser band of the spectrum to counter laser-guided missiles and those with laser proximity fuses.

Some examples of missile launch detectors and approach warners include the Cassidian (EADS) AN/AAR-60 MILDS (Missile Launch Detection System), BAE Systems AN/ALQ-156, MBDA DDM, and Elta EL/M-2160. The Joint and Allied Threat Awareness System (JATAS) is a project by the US Navy and Marine Corps to protect their tilt-rotors and helicopters from missiles and RPGs. Initiated in 2008, JATAS will use infrared and laser warning sensors to detect and track small arms, rocket and missile fire.

Once a missile has been detected, the next step is to deploy countermeasures against it. The most common forms of aircraft self protection in such instances are chaff and flares, which are cheaper than complex self protection ECM systems. The 42 lb (19 kg) Saab BOL countermeasures dispenser is a typical example of a podded system, and is operational on Harriers, Tornados, Typhoons, F-15s, Gripens and F/A-18s. It holds 160 chaff/flare packages, which is up to five times more than conventional dispensers. It is placed on the wings so vortices will assist in dispersing the expendables. The market for countermeasures dispensers is quite large and includes such systems as the MBDA Saphir, Saab BOP-L, Terma MCP, Thales Vicon 78, BAE Systems AN/ALE-40, ALE-45 and ALE-47, and MBDA Spirit.

The most commonly used expendable is chaff, which consists of finely sliced metal foil (usually aluminium) or plastic strands coated in metal, cut to a length appropriate for air interception radar frequencies. Chaff can be used by dispensing small bundles (to give the impression of small targets) or by creating large chaff clouds to completely obscure an aircraft’s position. Although dumping chaff is still quite effective, modern radars are less easily fooled by chaff than they once were because of its low Doppler (speed of movement) content.

Flares are launched to confuse heat seeking missiles or make them think the flare is the real target. Typically, flares are ejected at a speed of 80-150 feet/25-45 metres per second and burn for between two and four seconds. Early versions of the SA-7 Strela and similar SAMs that homed only on hot metal had a roughly one in four chance of hitting their target. Flares were generally effective against such basic missiles, which sometimes homed on the sun. In response, missiles with dual bands (such as UV and IR) were introduced, allowing them to distinguish secondary heat sources as well as flares. Later generations of SA-16 and SA-18 missiles, which also homed in on heat from the rest of the aircraft, improved their hit rate to around 60%.

Missile manufacturers began using other methods to counter flares, such as processors to detect and reject sudden rises in heat caused by a pyrotechnic. Consequently, dual-spectral flares are commonly loaded into dispensers along with conventional flares. There are even cooler pyrophoric flares that give off radiation that is invisible to the naked eye and therefore do not give away an aircraft’s position, especially at night. Most aircraft are fitted with a dual chaff/flare launching system but the greatest weakness of chaff and flares is they offer no protection against missiles coming from the front of the aircraft.

Dedicated infrared missile jammers are useful complements to flares. BAE Systems offers a number of IR jammers, including the AN/ALQ-157 (800 produced) and AN/ALQ-144. The latter has long been used on many helicopters – over 6 000 were produced for many countries and around 3 000 are still in service with US forces. The device uses a wide-angle heat lamp surrounded by a modulation screen that directs the energy at the missile seeker. However, such systems do not radiate enough energy in all directions to allow all-aspect jamming, and radiation is often blocked by the helicopter’s fuselage.

According to the US Air Force, 90% of all US air combat losses over the last 25 years can be attributed to infrared missiles. As a way of countering these threats, directional infrared countermeasures (DIRCM) systems are being used together with flares and infrared jammers – in many cases they are replacing these older methods.

After experience in Iraq and Afghanistan, DIRCM systems are becoming increasingly popular – the US Army, for instance, said the AN/ALQ-212 Advanced Threat Infrared Countermeasures (ATIRCM) fitted to a Chinook successfully countered a multi-missile ambush soon after the system was fielded in 2009. Meanwhile, Marine Corps CH-53E Sea Stallions are able to once again fly in areas of Afghanistan previously denied to them by the threat of SAMs. The BAE Systems AN/ALQ-212 consists of the AN/AAR-57 Common Missile Warning System (CMWS), Electronic Control Unit, two steerable infrared jamming heads, laser jammer and countermeasures dispenser.

The AAQ-24 Nemesis Directional Infrared Countermeasures (DIRCM) and Large Aircraft Infrared Countermeasures (LAIRCM) systems are two of the leading offerings aimed at defeating infrared guided missiles. The Nemesis has become one of the foremost DIRCM systems, being fitted to Chinooks, Merlins and Dutch Apaches as well as US Navy and Marine Corps CH-53D/Es and CH-46Es.

The AAQ-24(V) Nemesis was developed by Northrop Grumman in conjunction with BAE Systems (Selex Galileo) and Boeing. It consists of the UV AN/AAR-54(V) or two-colour IR passive missile warning sensor, tracking system and steerable tracking/jamming turret that uses laser power to disable missile seeker heads. The project was initiated by the UK in 1989 and joined by the US in 1993. Production for the UK began in 2001. Using new technology, it has been made more compact, lighter and less expensive: new lightweight transmitter units include the Small Laser Transmitter Assembly (SLTA) and Mini-Pointer/Tracker Assembly (MPTA).

Northrop Grumman developed the AAQ-24 Nemesis into the LAIRCM system to protect large military aircraft. A variety of American aircraft have been fitted with the product, including some of the US Air Force’s C-17, C-130, KC-135, KC-10 and C-5 transports and tankers, and Marine Corps CH-46Es and CH-53D/Es. Eleven different platform types use the LAIRCM system in the UK, including C-17s, Lockheed TriStar tanker/transports and VIP BAe 146s, while the A330 tankers will also get the system. The LAIRCM system equips NATO’s fleet of E-3As. LAIRCM systems have been installed on or are scheduled for installation on more than 500 military aircraft, including 50 different fixed wing and rotary wing aircraft.

Australia is another big user of LAIRCM, which has been selected for integration onto the A330 MRTTs, C-130Js and 737 Wedgetail airborne early warning and control (AEW&C) aircraft.  When it ordered the LAIRCM system for its C-130Js in 2008, Australia became the first country outside Britain and the US to procure the system for non head-of-state aircraft. The Royal Australian Air Force’s (RAAF’s) fifth C-17 Globemaster III will also be fitted with LAIRCM, as will its other four. The C-17s countermeasures suite is completed by the ATK/BAE Systems AN/AAR-47 missile warning system and AN/ALE-47 countermeasures dispenser.

LAIRCM’s closest counterpart is the lightweight (60 lb/25 kg) and compact Multi-Spectral Infrared Countermeasures (MUSIC) system by Elbit. It is designed to counter IR-guided SAMs, and can be fitted to military and commercial aircraft. The system comprises a system processor, missile warning system (IR, UV or radar), a thermal imaging camera for acquiring and tracking missiles, a High Speed Turret for missile seeker tracking and a laser that blinds the missile’s sensors, either confusing the seeker away from the aircraft or overloading its sensors. The Commercial-Music (C-MUSIC) system was ordered in June 2009 by the Israeli Ministry of Transportation to protect Israeli passenger aircraft.

Northrop Grumman’s LAIRCM has the lion’s share of the DIRCM market, but new entrants are arriving, such as the ELT/572. The latter is a collaborative effort between Elbit’s Elop and Italy’s Elettronica. The system gives all-round protection, even when missiles are launched from above. In January this year Elettronica was contracted to install the two-turret DIRCM system on Italian C-27J, C-130J and AW101 aircraft. Meanwhile Raytheon used its AIM-9X Sidewinder seeker head to develop the Scorpion and Quiet Eyes low cost, compact DIRCM systems for helicopters and other small platforms.

Most DIRCM systems are heavy and expensive and so the US Army is planning a more capable and reliable Common Infrared Counter Measures (CIRCM) system. Plans call for test flights next year and service entry in 2017, with 1 076 systems being acquired for Apache, Black Hawk, Chinook and Kiowa Warrior helicopters. The CIRCM will weight just 85 lb (40 kg), or 35 lb (15 kg) for small helicopters and less than 120 lb (55 kg) fully installed. In comparison, LAIRCM for helicopters weights 196 lb (89 kg) for a two-turret system. CIRCM will be compatible with the JATAS and CMWS. ITT, Northrop Grumman/Selex and Raytheon/BAE Systems are competing for the contract.

There are many integrated self-protection systems on the market that primarily use flares/decoys to disable IR threats, rather than lasers and other directed energy devices. The Selex Galileo Praetorian (formerly EuroDASS) created especially for the Eurofighter Typhoon, is a good example. The system incorporates a Defensive Aids Computer (DAC), an integrated radar band Electronic Support and Countermeasures (ES/ECM) package that includes two towed radar decoys, an active pulse-Doppler missile approach warner, laser warning receiver and countermeasures dispensing system. Other defensive aids systems include the Elta EL/M-2160, BAE Systems AN/ALQ-178, BAE Systems/ITT AN/ALQ-214, Saab Integrated Defensive Aids Suite (IDAS) and Compact-IDAS (CIDAS), ITT AN/ALQ-211, Thales Spectra and NII Ekran L370 Vitebsk (President-S), to name but a few.

In Australia, Project Echidna was initiated in 1998 to provide an electronic warfare self protection suite for Australia’s helicopters and transport aircraft. It was meant to comprise the BAE Systems Australia ALR-2002 radar warning receiver, Thales Vicon 78 countermeasures dispensing system and EADS AN/AAR-60 MILDS (Missile Launch Detection System; fitted to Australia’s AP-3Cs, NH90s and Tigers), all tied together through an Electronic Warfare Controller computer.

However, serious delays with the ambitious ALR-2002 caused the defence ministry in 2006 to cancel the integration of the RWR onto its 75 F/A-18A/B Hornets. They were supposed to receive the ALR-2002 as part of the Hornet Upgrade (HUG) project Phase 2.3. Instead the Hornets received the ALR-67(V)3 from 2008. Upgraded Hornets are also receiving Elta EL/L-8212/8222 advanced radio frequency jamming pods. As part of HUG Phase 2.1, RAAF Hornets had the ALE-47 dispenser, and this is augmented by the addition of Saab’s BOL countermeasures dispenser as part of HUG Phase 2.3.

Meanwhile, RAAF C-130Hs received the Elisra SPS-1000(V)5 RWR, AAR-47 missile warning system and ALE-47 countermeasures dispenser. The Army’s CH-47Ds received the AAR-60 and Vicon 78 while some S-70 Black Hawks followed suit with a basic version of the Echidna suite.