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. 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 smaller Shenyang J-31 has been flying for more than two years. 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.
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. have been found useful for detection of stealth targets.
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.”
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.
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. 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 IRST21 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.
Counter stealth technologies
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.
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.
The IRST system consists of a passive long-wave infrared receiver (IRR), a processor, inertial measurement unit (IMU), and environmental control unit (ECU).
“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”
IRST as preferred counter stealth technology
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.
The major IR signature contributors in a stealth aircraft are the hot engine parts including exhaust nozzle because of heating due to fuel burning, the exhaust plume and airframe due to compression of air especially nose especially leading edges of wings, tails and air intakes.
Even though stealth aircraft make use of various techniques for reducing their thermal signature, IR radiation cannot be totally eliminated. The fundamental variables available for reducing radiation are temperature and emissivity, and the basic tool available is line of sight masking.
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. 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.
Wide Adoption of IRST in Air forces
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.
IRST preferred solution for Stealth detection
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.
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.
“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.
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.
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.”
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)
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.
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