Stealth has revolutionized the Air power by vastly enhancing the penetration capabilities of fifth generation aircrafts like the F-22A Raptor developed by Lockheed Martin and Boeing, L.M. F-35 Lightning II. Stealth in military is often described to mean, all the methods, techniques and measures intended to deny or delay the detection and tracking of friendly forces by the enemy including electronic signature masking, Infra-red and acoustic signature suppression.
For the last four decades or so, the US has had total dominance in the area of stealth technology, but this is now coming to an end with China and Russia developing stealthy fighters and unmanned combat aircraft. China is developing the Chengdu J-20 and J-31. The smaller Shenyang J-31 has been flying for more than two years. Chengdu Aircraft Corporation has rolled out “2017,” the eight in the line of J-20 jets that China has been developing in the past few years giving China the largest number of stealth fighters in the world after the United States. The Sukhoi PAK FA is being developed by Sukhoi for the Russian Air Force. Its prototype, T-50, flew for the first time in 2010.
The rise in stealth technology has complicated the air defense of countries as it creates lots of gaps in Air defense coverage therefore militaries have been developing many counter stealth solutions and technologies. Many technologies have potential to detect stealthy targets including Over-the-horizon radars, active / passive multistatic radars, very low frequency radars, and sensitive IR sensor systems, laser radar imaging, hyperspectral imaging, e.t.c. . The beauty of infrared search and track technology (IRST) technology? It is completely passive and does not highlight the location of the aircraft, unlike when a pilot decides to use the on-board radar, which can give away its position as radiofrequency energy bursts out.
IRST systems are becoming essential elements of advanced fifth generation aircrafts to detect ultra stealthy targets like F-22. IRST sensors also critical to detect heat sinking missiles and to locate and destroy enemy forces in radar-denied environment . US F-35 and Russia’s Su-35 Flanker have IRST systems, as well as Eurofighter Typhoon that has PIRATE (Passive InfraRed Airborne Track Equipment) suite.
Selex has claimed hat its IRSTs have been able to detect and track low-RCS targets at subsonic speeds, due to skin friction, heat radiating through the skin from the engine, and the exhaust plume. The U.S. Navy’s Greenert underscored this point in Washington in early February, saying that “if something moves fast through the air, disrupts molecules and puts out heat . . . it’s going to be detectable.”
The U.S. Navy recently placed an order with Boeing for the procurement of 12 Infrared Search and Track (IRST) systems intended for the F/A-18 Super Hornet to provide “see first, strike first” capability. The Super Hornet’s IRST is a long-wave infrared detection system that can detect and lock weapons onto enemy aircraft without using radar. In addition to detecting airborne threats, IRST significantly enhances multiple target resolution compared to radar, providing greater discrimination of threat formations at longer ranges. Data from the IRST sensor is fused with other on-board F/A-18 sensor data to provide maximum situational awareness to the warfighter.
Wang Yanyong, technical director for Beijing A-Star Science and Technology, has confirmed that its two systems – the EOTS-89 electro-optical targeting system (EOTS) and the EORD-31 infrared search and track (IRST) – are in development for China’s J-20 and J-31 fighters. Marketing brochures on A-Star’s booth suggest that the J-20 could use the passive sensors to detect and aim missiles against the Northrop Grumman B-2 bomber and Lockheed Martin F-22 fighter, even while its radar is being jammed by a Boeing EA-18G Growler. “It lists detection ranges for the B-2 at 150km and for the F-22 at up to 110km,” as reported by Stephen Trimble.
To detect ultra-stealthy aircraft like the F-22 and Chengdu J-20, India needed to strengthen the capabilities of the Su-30MKI and now the country has decided to design and develop a long-range dual-band infrared imaging search and track system i.e., IRST. The system will enable Sukhoi with additional capabilities. The need was felt more when India find it difficult to detect China operationalized the J-20 fighter jets which are made up of radar-absorbing materials last year
Even amid electronic attack or heavy RF and infrared countermeasures, IRST provides autonomous, tracking data that increases pilot reaction time, and enhances survivability by enabling first-look, first-shoot capability, Lockheed Martin officials say. Giorgio Balzarotti, vice president of IRST programmes at Leonardo’s Nerviano site in Italy, said having an IRST sensor is like being in a dark room with your enemy. You are able to see your enemy because of the temperature emitted by them, but they would have to shine a light (analogous to turning on a radar) to be able to see your position. “You are at an advantage…you can be completely silent,” he added.
Counter stealth technologies
For more than half a century, the radar has been indisputably the most important sensor in the battlefield, especially in the air domain. Radars have always been competing with electronic warfare systems, which are trying to hinder detection and tracking with the use of various jamming techniques. However, the apparition of stealth or low observable technology since the late ’80s has been the game changer
which has really contested the radar dominance.
Low frequency band (VHF/UHF) radars exhibit serious anti-stealth capabilities. It is noted that the only case of a stealth aircraft lost due to enemy action (the F-117A shot during the Kossovo war) was associated with the use of a modified soviet-made VHF radar (P-18 “Spoon Rest”). Russian Nebo-M multiband radar designed by the Nizhny-Novgorod Research Institute of Radio Engineering (NNIIRT) uses fusion of data from the three radars to create a robust kill chain. The VHF system performs initial detection and cues the UHF radar, which in turn can cue the X-band RLM-S. The three radars of system are Active Electronic Scanned Array(AESAs) type: the VHF RLM-M, the RLM-D in L-band (UHF) and the S/X-band RLM-S.
However, Radars are active systems and are easily detectable by adversary too; All 4th and 5th generation fighters have installed modern digital Radar Warning Receivers (RWR) and Electronic Support Measures (ESM) which can accurately estimate not just the direction but also the distance, and thus the geolocation, of enemy radar emissions.
IRST as preferred counter stealth technology
Infrared radiation (IR) is invisible electromagnetic radiation, with longer wavelengths than red light, typically covering the spectrum from 300 GHz (wavelength 1 mm or 1000 μm) up to 430 THz (0.7 μm). Every object with a temperature above absolute zero emits electromagnetic radiation, mainly in the IR band. Objects at higher temperatures emit also visible light, as in the case of the incandescent light bulb. The spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium is given by Planck’s law.
As IR travels through the atmosphere, it is absorbed by water vapour, carbon dioxide, carbon monoxide, nitrous oxide, etc., leaving only certain “windows” (the sub-bands of 3-5 and 8-12 μm) allowing for decent propagation.
An infrared signature of any object depends on many factors, including the shape and size of the object, temperature and emissivity, reflection of external sources (earthshine, sunshine, skyshine) from the object’s surface, the background against which it is viewed and the waveband of the detecting sensor.
Concerning the case of an aircraft, it exhibits a complex thermal signature, emanating from the following components:
Engine “hot parts” (aft turbine face, engine centre, body and interior nozzle surface) because of heating due to fuel burning.
Engine exhaust plumes (emissions from the fuel combustion, mainly carbon dioxide and water vapour).
Airframe, which includes all of the external surfaces of the wings, fuselage, canopy etc., as well as solar and terrestrial reflections and Mach shock wave (aerodynamic heating) due to compression of air especially nose especially leading edges of wings, tails and air intakes.
the hot engine parts including aft turbine face, engine centre, body and interior nozzle, Engine
surface) including exhaust nozzle the exhaust plume and airframe
The IRST systems are getting more and more importance due to increased reluctance to use radar sensors because of threat of’ anti-radiation missiles and dramatic improvements in IR sensor performance, resulting in long-range detection capability. InfraRed Search & Track or IRST systems offer significant advantages with respect to traditional radar systems, such as passive operation, resistance to jamming, and long detection ranges (under certain conditions). Furthermore, the last decade or so has brought phenomenal improvements in computing power in algorithms, meaning that IRST systems can now filter out false positives and do an excellent job of passively scanning the skies for large, hot, and fast moving targets.
Because the atmosphere attenuates infra-red to some extent and because adverse weather can attenuate it also, the range compared to radar is limited. However, due to its shorter wavelength, IRST provides better resolution than radar thus giving a fighter aircraft ability to Identify other aircraft at longer ranges and its better angular resolution provides better possibly up to 40 times more accurate capability for differentiating aircraft in formation than radar.
In contrast to radars, IRST systems provide passive means of detecting and tracking, along with required target information often without the target being warned that it was being tracked. The IRST or Infrared Search and Track system uses the heat or infrared radiation emitted by the target to generate data for the weapon system of an aircraft. The modern infra-red search and track systems, pose serious threat against aircraft optimized only for minimal radar observability like F-22, B-2, RQ-170 etc. Furthermore, they cannot be jammed as easily as the radar. They also offer much better angular resolution with respect to the radar but they cannot measure distance directly.
“As a result, IRST is most useful for air superiority fighters, which typically operate at 30.000 ft and above – well above normal cloud cover and in relatively thin atmosphere. Only clouds typically present at altitudes above 8 km (~26.000 ft) are those of cirrus variety, which are IR transparent,” he further writes.
Even though stealth aircraft make use of various techniques for reducing their thermal signature, IR radiation cannot be totally eliminated. Such techniques are the omission of an afterburner (as in the case of the F-117 and B-2), the use of high bypass ratio turbofan engines (where the bypass stream is used to cool the exhaust gases), and the placement of the exhaust duct on the top, in an effort to hide the hot gases from below (as in the case of B-2). Some aircraft use their fuel as coolant, transferring waste heat to it, e.g., the F-35. The fundamental variables available for reducing radiation are temperature and emissivity, and the basic tool available is line of sight masking. Even if significant efforts have been exerted in order to minimise the IR signature of fighter aircraft, it is impossible to make a fast flying jet, propelled by hot exhaust gases, completely disappear, in the IR spectrum. Therefore, any vehicle flying into the air unavoidably emits thermal radiation, which can be detected, if it exhibits sufficient contrast against the cold background. Therefore, IRST systems appear to be a viable anti-stealth approach.
“In theory, state of the art IRST could find and track F22 at quite long range,” said Justin Bronk, a Research Fellow specializing in combat airpower at the Royal United Services Institute. Bronk, went on to explain that stealth planes are generally bigger requiring larger wings to create more lift. Even worse, the radar absorbing materials these planes are coated in “heat up quickly” leading to a “good infrared return even if the jet exhaust is shrouded”
Most modern stealth fighters (excepting the F-35 and J-31 tactical bombers) are intended to operate at high altitudes – above 50.000 ft – where ambient temperatures range from -30 to -60 degrees Celsius. At high altitudes, the atmosphere is less dense and it absorbs less infrared radiation – especially at longer wavelengths. The effect of reduction in friction between air and aircraft does not compensate the better transmission of infrared radiation. Therefore, infrared detection ranges are longer at high altitudes. At high altitudes, temperatures range from −30 to −50 °C – which also provide better contrast between aircraft temperature and background temperature. Jamming IRST with an infrared laser is also very difficult against a maneuvering aircraft. As a result, actual tracking and engagement range of IRST can be expected to be greater than that of radar, even if latter has a major advantage in initial detection range.
Wide Adoption of IRST in Air forces
On the other hand, IR systems are more sensitive than radar to adverse weather conditions. Therefore, taking into account the advances in radar technology, IRST systems were abandoned in the U.S. 15-20 years ago, with the venerable F-14D Tomcat being the last IRST-equipped fighter. Contrary to the U.S., the Russians have never given up on IRST, equipping most of their fighters since the ’60s with such systems. The Chinese, who are advancing fast on stealth design, equip their Chengdu J-20 and Shenyang FC-31 with advanced electro-optical systems, which appear to be similar in concept to the ones of the F-35. Furthermore, during the last decades, all modern European fighters (Rafale,
Eurofighter Typhoon, Gripen NG, which are unofficially referred to as “eurocanards”) are featuring latest generation IRST systems, with advanced capabilities.
Americans have recently re-discovered IRST systems, trying to catch up with Europe and Russia. U.S. Air Force (AW&ST Sept. 22, 2014, p. 42) is the latest convert to the capabilities of IRST. The U.S. Navy’s IRST for the Super Hornet, installed in a modified centerline fuel tank, was approved for low-rate initial production in February, following 2014 tests of an engineering development model system, and the Block I version is due to reach initial operational capability in fiscal 2018. Block I uses the same Lockheed Martin infrared receiver—optics and front end—as is used on F-15Ks in Korea and F-15SGs in Singapore.
Commanders will use IRST in a radar-denied environment to locate and destroy enemy forces. The IRST system is intended to allow the F/A-18E/F to operate and survive against existing and emerging air threats by enhancing situational awareness and providing the ability to acquire and engage targets beyond visual range.
Northrop Grumman has launched a partnership with Italian firm Selex ES under which Selex’s infrared know-how will enter the US and possibly be turned around for export products for Foreign Military Sales customers. The deal pushes into the US market the technology Selex has worked on for the Eurofighter’s PIRATE (passive infrared airborne tracking equipment) sensor, for the European Neuron UCAV technology demonstrator, and, most recently, for Sweden’s Gripens. By installing an IRST system in a low observable aircraft makes the aircraft even stealthier and deadly, that can see clearly even when its radars are jammed.
The IRST consists of a passive longwave infrared receiver, a processor, inertial measurement unit, and environmental control unit. The infrared receiver, processor, and inertial measurement unit fit inside the sensor, which attaches to the front of the fuel tank mounted on the Super Hornet’s BRU-32 bomb rack.
Modern variations of IRSTs can search out to intermediate ranges, track multiple targets and even engage other aircraft using its telemetry data alone. Operating modes are similar to radar: multiple target track (permitting engagement of multiple targets; similar in nature to radar’s track while scan), single target track and slaved acquisition (where IRST is slaved to another sensor, such as radar or RWR).
Infrared Search and Track system is usually fitted in a spherical glass enclosure on the front of a fighter aircraft. IRST can use scanning or staring array. Scanning system can use a single element, which then sequentially scans the instantaneous field of view (determined by the aperture) ahead of the jet for heat or Infrared signatures; the Scanning system is typically a rotating mirror.
This system is cheaper than a staring array. Its output is serial as only IFOV is directed on the detector at any one time. Dwell time is determined by both frame rate and number of pixels in the image; a system with 30 Hz refresh rate and standard VGA monitor of 640×400 pixels has a dwell time of 1/7.680.000 of a second, which leads to increased noise in the system and reduced sensitivity.
Modern systems use Staring sensor uses one detecting element for each part of the image within field of view. This means that all detecting elements are simultaneously exposed to the image of the object, or a frame. Standard frame rate is 30 Hz, and dwell time is equal to the frame rate (1/30 of a second). Longer dwell time results in a more sensitive detector and less noise.
Some examples of IRST systems are the following:
a) IRST21 is the next generation of Lockheed Martin’s legacy IRST sensor system, which accumulated more than 300,000 flight hours on the U.S. Navy’s F-14 and international F-15 platforms. The long-range IRST21 sensor uses infrared search and track technology to detect, track and enable the Super Hornet to engage threats with air-to-air weapons.
b) AN/AAQ-37 Electro-Optical Distributed Aperture System (EODAS) of the F-35 (Northrop Grumman): It comprises of 6 Imaging IR sensors, providing 360° spherical situational awareness. It provides day/night vision, IRST capabilities with fire control, missile detection and tracking, etc
c) AN/AAQ-40 Electro-optical Targeting System (EOTS) of the F-35 (L.M.): This is the second electro-optical system of the F-35, combining FLIR, IRST and laser. It provides detection, tracking and designation of ground targets, and also identification of aerial targets. This is a single channel midwave IR system, limiting its performance against nonafterburning targets and in air-to air role. It is based on the L.M. AN/AAQ-33 Sniper Advanced Targeting Pod.
d) PIRATE (Passive Infra-Red Airborne Tracking Equipment) of the Eurofighter Typhoon (EUROFIRST consortium): A 2nd gen. system (Imaging IR), combines FLIR and IRST, allowing the tracking of up to 200 targets, 90 km from the front and 145 km from the rear. It has an ID range of 40 km, has 140-degree field of regard in azimuth, with -15-degree depression angle. PIRATE is a dual-band system (3-5 and 8-10 microns), combining long range detection capability of the longwave IRST with high resolution and all-weather performance of midwave one.
e) OSF (OptroniqueSecteur Frontal) of the Rafale (Thales Optronique – SAGEM): Comprises an IR subsystem (with IRST and FLIR) with a range of 100 km and a second subsystem with a high resolution TV camera (range up to 40 km) and laser for range-finding. OSF is fully integrated to the aircraft weapon system. Like PIRATE, its IR sensor is dual-band, using 3-5 and 8-12 micron bands.
f) OLS-35 of the Sukhoi Su-35 BM (NIIPP): Includes an IRST, with a maximum range of 50 km on the frontal and 90 km on the rear aspect of the (non afterburning) target, a TV camera and a laser for range-finding and target designation.
g) SpectIR (L.M.): An IRST system based on the AN/AAS-42 of the F-14D Tomcat. It is in advanced testing stage. Is is designed to be installed in the front part of an external fuel tank (for the F-18 E/F) or as an external pod (for the F-15, F-16 etc).
U.S. Navy air combat experts are asking electro-optics engineers at Lockheed Martin Corp. to build five infrared receivers and four control processors for the infrared search and track (IRST) system aboard F/A-18E/F Super Hornet jet fighter-bomber. The Super Hornet combat aircraft IRST is a long-wave infrared detection sensors system that targets enemy aircraft in conditions where the Super Hornet cannot use its radar. The Navy and Boeing first flew the IRST Block II pod on an F/A-18E/F Super Hornet in late 2019. The IRST Block II is part of the Super Hornet Block III upgrades to keep the F/A-18 in active service for decades to come. Block III upgrades also include enhanced network capability, longer range with conformal fuel tanks, an advanced cockpit system, signature improvements, and an enhanced communications system.
Even amid electronic attack or heavy RF and infrared countermeasures, IRST provides autonomous, tracking data that increases pilot reaction time, and enhances survivability by enabling first-look, first-shoot capability, Lockheed Martin officials say.
IRST system Limitations
Due to relatively shorter wavelength, IRST is more sensitive than radar to adverse weather conditions. Much of the infrared radiation is absorbed by water vapor, carbon dioxide, methane and ozone. IR sensors are effective in determination of azimuth and elevation fairly accurately. However, being a passive sensor, IRST alone has issues with range finding and are required to be coupled with radar or other means such as lasers for range determination.
Other techniques for rangefinding are triangulation through multiple aircrafts, Target motion analysis can also be combined with atmospheric propagation model and/or apparent size of the target in order to provide a more accurate rangefinding, or these modes can be used as standalones. IRST could also use sensitivity model (Atmospheric Propagation Model) to roughly estimate range and velocity of target without using any active sensors. “Radiance difference between target and the background is also a possibility,” writes picard578.
Hence, despite the limitation of the variable atmospheric transparency affecting their performance, IRST system enhances performance against low radar cross-section targets while providing immunity to electronic detection and RF countermeasures.Another Limitation of IRST is that it has narrow field of view. But even on a good day, looking for fifth generation aircraft in the open skies with IRST is like “looking through a drinking straw,” said Justin Bronk, a Research Fellow specializing in combat airpower at the Royal United Services Institute. “The [IRST] field of regard is quite small… and it’s much much harder to perform a wide sector scan in a way that a radar can,” said Bronk.
Selex-ES, which is the lead contractor on the Typhoon’s Pirate IRST and the supplier of the Skyward-G for Gripen, has claimed openly that its IRSTs have been able to detect and track low-RCS targets at subsonic speeds, due to skin friction, heat radiating through the skin from the engine, and the exhaust plume. The U.S. Navy’s Greenert underscored this point in Washington in early February, saying that “if something moves fast through the air, disrupts molecules and puts out heat . . . it’s going to be detectable.”
IR detector technology
Large part of infrared radiation is blocked by atmosphere, However, there are two wavelength “windows” in which very little infrared radiation is absorbed by the atmosphere. These windows are at midwave infrared (3-5 ) and long wave infrared ( 8-12) microns. The 3–5 microns window corresponds to higher peak emission temperature (~450 degC), and is better suited for detecting hot spots such as afterburner exhaust plume. The 8–12 microns band has lower peak emission temperature (~17 degC), and is generally used for emissions from larger surfaces at lower temperatures like detection of subsonic or supercruising airframe because of aerodynamic heating of skin.
Comparing both bands, 3-5 mM band is less affected by aerosoil while 8-12 mM band has longer detection range and is less affected by clouds. “Consequently, up until appearance of dual-band systems, midwave band was preferred for ground attack while longwave band was preferred for air-to-air usage,” writes picard578.
Over the years, significant developments have taken place in IR-detector technology, towards increasing their sensitivity i.e. reducing their Noise Equivalent Irradiance (NEI). Currently cooled GaAs/AlGaAs (AluminiumGallium Arsenide) and HgCdTe (Mercury CadmiumTelluride) detectors are being used that operate in the mid-wave (3–5 mm) and long wave (8–12 mm) bands. They are capable of detecting IR radiation in a wider spectrum, and are also capable of locking-on to aircraft from all aspects, including from the front. Such systems are inherently immune to commonly used countermeasures like IR flares that appear as a point source.
New generation IR detectors are based on Quantum Well IR Photodetectors (QWIP) technology. QWIP IRST such as PIRATE or OSF has some very useful advantages over “legacy” IRST. Aside from longer range, they can be tuned for sensitivity in certain IR band. While normal IRST operates in microwave to longwave IR bands, QWIP IRST can operate in very longwave bands, allowing for easy detection of objects that are only slightly hotter than the background, with difference being in single digit degrees of Cenzius.
They use multi-colour thermal-imaging systems that employ an array of detectors to build a spatial map of the scene. While modern QWIP IRSTs offer the best performance, they have to be cooled to extremely low temperatures: 65 K is not uncommon. The longer the wavelength of light, the less energy the light has to give the electrons and the colder the detector must be to avoid excessive thermal excitations.
Leonardo has revealed that it has been contracted to deliver its infrared search and track (IRST) technology to an undisclosed Far Eastern partner for integration on a new unmanned aerial vehicle development. The so-called Skyward-AB will be provided for integration on a tactical UAV that is nearing the end of its development phase, and two units will be delivered to the aircraft’s manufacturer in 2019, Giorgio Balzarotti, vice president of IRST programs at Leonardo, told AIN at its Nerviano, Italy facility in December. This will be integrated onto a tactical UAV, which “is not the first time we have tested on an unmanned aircraft, but it is the most important one,” he noted.
It is believed that the tactical UAV is still in its development phase and will be ready next year, with the IRST sensors also being delivered next year for use in an air-to-air detection and tracking role. This the first time an IRST sensor has been used on a UAV in an air-to-air role – the past example of nEUROn was exclusively air-to-ground – although no further details are known at this point owing to its confidential nature.
Leonardo also unveiled details of a new distributed IRST system in development, known as the Multi-Aperture Infrared (MAIR) system. This differs from past systems in that it uses multiple IR cameras around a platform to aid detection and tracking of targets and threats, rather than just a single aperture as on traditional systems. “If we are able to have a single head sensor, we can have more than one head to perform full coverage of the scene,” explained Balzarotti.
MAIR has been company funded and will be tested on a helicopter next year. Leonardo is also exploring additional functions including interfacing with countermeasures – to detect incoming rockets or missiles, and dispense flares or cue laser jamming – as well as a pilot aid to increase situational awareness by projecting imagery of their surroundings onto a helmet-mounted holographic display
The huge investment made by the government in the intelligent tracking systems leads to rapid development of this market. However, the high designing and manufacturing cost of this technology restrains the market growth.
The infrared search and track (IRST) market is segmented based on component, platform, industry vertical, and geography. On the basis of component, the market is divided scanning head, processing & control electronics, and display. On the basis of platform, it is categorized into airborne, naval, land, and others. According to industry vertical, it is bifurcated into aerospace and defense. By geography, the market is analyzed across North America, Europe, Asia-Pacific, and LAMEA.
Top manufacturers/key players are Leonardo S.p.A,Safran S.A.,Lockheed Martin Corporation,Thales Group,HGH Systmes Infrarouges SAS,Rheinmetall AG,Hughes Network Systems LLC, Aselsan A.S.,Northrop Grumman Corporation and Tonbo Imaging Private Limited.