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In recent years, precision guided weapons play more and more important role in modern war. One of the greatest strengths of a precision strike missile is a reduction in the number of weapons or aircraft sorties required to destroy a target. Strategy Analytics forecasts the global Smart Weapons (SW) market will grow to over $41.8 billion in 2025, representing a CAGR of 3.7%. A renewed emphasis on advancing Smart Weapon capabilities to counter evolving threats such as A2/AD (anti-access Area Denial) envelopes, combined with on-going demand from asymmetric wars and continued force modernizations in emerging countries is driving spending across the full range of Smart Weapons.
Precision guided weapons may include a variety of imaging or non-imaging sensors to detect and track potential targets. Sensors used to guide projectiles to an intended target are commonly referred to as seekers. One of the key contributors to the missile accuracy, lethality, and adverse weather capability of precision strike missiles is the advancements in missile seekers.
Seekers may operate in various portions of the electromagnetic spectrum, including the visible, infrared (IR), microwave, and millimeter wave (MMW) portions of the spectrum. Some projectiles may incorporate multiple sensors that operate in more than one portion of the spectrum. A seeker that incorporates multiple sensors that share a common aperture and/or common optical system is commonly called a multimode seeker.
Infrared homing is a passive weapon guidance system which uses the infrared (IR) light emission from a target to track and follow it. Infrared seekers are passive devices, which, unlike radar, provide no indication that they are tracking a target. This makes them suitable for sneak attacks during visual encounters, or over longer ranges when used with a forward looking infrared system or similar cuing system. This makes heat-seekers extremely deadly; 90% of all United States air combat losses over the past 25 years have been due to infrared-homing missiles.
India’s Defence Research and Development Organisation (DRDO) successful test-firing on 27 March 2019 of its first anti-satellite (ASAT) missile that destroyed one of the country’s own satellites in space was also based on IIR seeker. Secretary DRDO, Satheesh Reddy stated that the 13 m-high, three-stage interceptor missile, which was fitted with two solid-propellant rocket motor stages and a hit-to-kill capable ‘Kill Vehicle’ (KV), was employed to target the satellite under ‘Mission Shakti’ (Strength).
The KV’s onboard advanced terminal guidance system, which featured a strap-down (non-gimballed) imaging infrared (IIR) seeker and an inertial navigation system that used ring-laser gyroscopes (RLGs), detected and tracked the 740 kg Microsat-R Earth observation satellite at an altitude of 283 km in low-Earth orbit (LEO). Reddy explained that after the two rocket motor stages had taken the ASAT missile to the required height and velocity, the nose tip heat shield was ejected and the IIR seeker, located within the very front of the nose, locked onto the target satellite, guiding the KV towards it at a “closing speed” – the velocity of the target and KV combined – of 10 km per second.
Infrared Homing and IIR Seeker
Missiles which use infrared seeking are often referred to as “heat-seekers”, since infrared is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and emit heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background. Some statistics suggest that heatseeking missiles have been responsible for more than 80% of all combat aircraft losses over the last 40 years according to Herskovitz.
The infrared sensor package on the tip or head of a heat-seeking missile is known as the seeker head. Generally the entire seeker assembly is mounted on a gimbal system that allows it to track the target through wide angles, and the angle between the seeker and the missile aircraft is used to produce guidance corrections.
Countermeasures
There are two primary ways to defeat IR seekers, using flares or IR jammers.
Chaff and flares are defensive mechanisms employed from military aircraft to avoid detection and/or attack by adversary air defense systems. Flares are high-temperature heat sources ejected from aircraft that mislead heat-sensitive or heat-seeking targeting systems and decoy them away corn the aircraft. Self-protection flares are magnesium pellets that, when ignited, burn for a short period of time (less than 10 seconds) at 2,000 degrees Fahrenheit. The burn temperature is hotter than the exhaust of an aircraft and therefore attracts and decoys heat-seeking weapons targeted on the aircraft. Flares ejected from the target with an object of decoying a hostile missile have been extensively utilized for IR countermeasures because even a two-pound unit can provide up to 10-seconds protection against IR-seeking missiles.
Aircraft, especially military aircraft, often carry pyrotechnic decoy flares as countermeasures for luring incoming anti-aircraft missiles away from the aircraft. The decoy flares typically are ejected from the aircraft and remotely or automatically ignited in flight. More sophisticated flares contain a propulsion system for propelling the flare over a flight path similar to, but divergent in direction from, the path of the aircraft. The propulsion system is designed to confuse anti-aircraft missiles that can discriminate between a free-falling flare and a propulsion-powered object, e.g., the aircraft. If the decoy flares function correctly, the anti-aircraft missile will lock into and follow the decoy flare, and cease pursuit of the aircraft, allowing the aircraft to proceed unharmed by the missile.
One solution to the flare problem is to use a dual-frequency seeker. Early seekers used a single detector that was sensitive to very hot portions of the aircraft and to the jet exhaust, making them suitable for tail-chase scenarios. To allow the missile to track from any angle, new detectors were added that were much more sensitive and in other frequencies as well. This presented a way to distinguish flares; the two seekers saw different locations for the target aircraft – the aircraft itself as opposed to its exhaust – but a flare appeared at the same point at both frequencies. These could then be eliminated. Modern imaging systems, which image directly instead of scanning, and have the further capability of eliminating small targets by measuring their angular size directly.
Jammers
Early seeker systems determined the angle to the target through timing of the reception of the signal. Jammers typically use an infrared laser shining on a rapidly rotating mirror. As the beam paints the seeker it causes a flash of light to appear out of sequence, disrupting the timing pattern used to calculate angle. When successful, IR jammers cause the missile to fly about randomly.
IR jammers are far less successful against modern imaging seekers, because they do not rely on timing for their measurements. In these cases, the jammer may be detrimental, as it provides additional signal at the same location as the target. Some modern systems now locate their jammers on towed countermeasures pods, relying on the missile homing on the strong signal, but modern image processing systems can make this ineffective and may require the pod to look as much as possible like the original aircraft, further complicating the design.
A more modern laser-based technique uses some other form of detection to identify the missile and aim the laser directly at it. This blinds the seeker continually, and is useful against even modern imaging seekers. These directional infrared countermeasures’ (DIRCMs) are very effective, they are also very expensive and generally only suitable for aircraft that are not maneuvering, like cargo aircraft and helicopters. Their implementation is further complicated by placing filters in front of the imager to remove any off-frequency signals, requiring the laser to tune itself to the frequency of the seeker or sweep through a range. Some work has even been put into systems with enough power to optically damage the nose cone or filters within the missile, but this remains beyond the current state of the art.
Imaging Infrared Seeker technology
Modern heat-seeking missiles utilise imaging infrared (IIR), where the IR/UV sensor is a focal plane array which is able to produce an image in infra-red of the whole field of view, much like the CCD in a digital camera. This complete image of the field of view made it is possible to use more complex features of the objects seen, in more sophisticated algorithms, and thus have a higher probability of success in differentiating between the countermeasures and the platform they protect.
In addition to being more flare-resistant, newer seekers are also less likely to be fooled into locking onto the sun, another common trick for avoiding heat-seeking missiles. “The target identification process starts with pre-processing or filtering to suppress noise and enhance spatial discontinuities or edges. This is followed by segmentation, a process that isolates the blobs that correspond to the objects present in the scene. Once this has been done, features are extracted from the blobs, care being taken to use the smallest possible number of features, which have to be selected for their strong discriminating power. A classifier is then trained with some data and then tested on new data it had never seen before,” write G. Labonté , W.C. Deck Department of Mathematics and Computer Science, Royal Military College of Canada.
By using the advanced image processing techniques, the target shape can be used to find its most vulnerable part toward which the missile is then steered. All western Short-range air-to-air missile such as AIM-9X Sidewinder and ASRAAM, Chinese PL-10 SRAAM and Israeli Python-5 use imaging infrared seekers, while Russian R-73 still uses infrared seeker.
Single Color Vs Dual Color IIR Seekers
There are three wavelength windows in the atmosphere that are transparent to the IR radiation, namely 1-2 micrometer short wavelength IR (SWIR), 3-5 micrometer medium wavelength IR (MWIR) and 8 -14 micrometer long wavelength IR (LWIR). The IR emission from objects in typical ambient temperature of about 300 K peaks around 10 micrometer.
Early infrared seekers were most effective in detecting infrared radiation with shorter wavelengths, such as the 4.2 micrometre emissions of the carbon dioxide efflux of a jet engine. This made them useful primarily in tail-chase scenarios, where the exhaust was visible and the missile’s approach toward it was carrying to toward the aircraft as well.
In combat these proved extremely ineffective as pilots attempted to make shots as soon as the seeker saw the target, launching at angles where the target’s engines were quickly obscured or flew out of the missile’s field of view. Such seekers, which are most sensitive to the 3 to 5 micrometre range, are now called single-color seekers.
This led to new seekers sensitive to both the exhaust as well as the longer 8 to 13 micrometer wavelength range, which is less absorbed by the atmosphere and thus allows dimmer sources like the fuselage itself to be detected. Such designs are known as “all-aspect” missiles. Modern seekers combine several detectors and are called two-color systems.
Dual code seeker can spectrally discriminate between the decoy and the aircraft exhaust plume, by comparing their luminosity at two different wave lengths simultaneously. A typical flare burns at 2000 deGC while an aircraft engine is in the 600-800 degC range; this allows the seeker to recognize its aircraft target.
Uncooled Vs Cooled
All-aspect seekers also tend to require cooling to give them the high degree of sensitivity required to lock onto the lower level signals coming from the front and sides of an aircraft. Background heat from inside the sensor, or the aerodynamically heated sensor window, can overpower the weak signal entering the sensor from the target. Therefore, the seeker head of an active missile is cooled up to minus 160 degrees Celsius in order to establish optimal sensitivity.
Earlier Missile seekers used nitrogen cooling from a bottle whose cooling time depended on size of the bottle (6 litre bottle could provide 2.5 hours of cooling time). Modern all-aspect missiles like the AIM-9M Sidewinder and Stinger use compressed gas like argon to cool their sensors in order to lock onto the target at longer ranges and all aspects. Some such as the AIM-9J and early-model R-60 used a peltier thermoelectric cooler for unlimited seeker cooling time as long as the missile has power.
Intelligent Seekers
The goal of the latest development in infrared seeker heads is to make the seeker heads “intelligent” and thus to make them immune to conventional infrared fake targets, i.e., to design them in such a manner that they respond to the object signature, in particular the aircraft signature. The development of tiny but powerful computer chips, together with accurate optics, has made the missiles smarter.
The last generation of infrared imaging aircraft seekers and trackers uses pattern recognition algorithms to find and keep a lock on an aircraft in the presence of decoy flares. These algorithms identify targets, based on the features of the various objects in the missile’s field of view. The tracking algorithm uses feature extraction and machine learning to discriminate between a desired target ship and decoys within the seeker’s field of view. The target features are then used as inputs to a neural network, which performs the task of target selection, to determine the seeker’s aim-point. The neural network is trained on these features to recognise the difference between targets and decoys.
Another technology incorporates heat “signatures,” which recognizes that the exhaust from a jet engine is different than that of a flare. A method to eliminate false targets consists of a frequency analysis by means of the seeker head, which can distinguish between the radiation characteristics of the infrared radiators (for example aircraft engines) of the target that exhibit a comparatively low temperature and the radiation characteristics of a hot fake target cloud.
Still others add UV sampling as a way to tell real target from fake. Yet others may combine radar guidance with IR, UV and optics to generate guidance data. Thus, in summary the known infrared fake target clouds are not in a position to defend an object against missiles equipped with intelligent seeker heads. Of course, as the missiles get smarter, so do the dummies.
China’s next long-range air-to-air missile may have a periscopic IR seeker
A diagram from a research paper written by the China Airborne Missile Academy suggests using separate aperture compound seeker for the country’s next generation long-range air-to-air missile. The authors proposed using the same approach as the Israeli Stunner missile but they suggest housing the infrared (IR) seeker behind the radar seeker and the IR sensor uses a periscope to detect its target.
They argued that such an approach is more ideal as the IR sensor is less affected by the heat generated on the nose cone as the missile speed towards its target. The seeker will only be extended out into the air stream as the missile closes in on its target. This is the drawback as the weapon cannot rely on the passive seeker during mid-course. A common aperture compound seeker solution was also explored but the disadvantage was that the nose cone is spherical which is not aerodynamically efficient.
Small Diameter Bomb II’s Tri-Mode Seeker Can Destroy Moving Targets in Adverse Weather
The Air Force is engineering and testing a new air-dropped weapon able to destroy moving targets in all kinds of weather conditions at ranges greater than 40-miles, Air Force and Raytheon officials said. The seeker works in three different modes to provide maximum operational flexibility: a millimeter wave radar to detect and track targets through weather, an imaging infrared for enhanced target discrimination and a semi-active laser that enables the weapon to track an airborne laser designator or one on the ground.
Raytheon’s GBU-53/B SDB-II is 7″ in diameter around the tri-mode (laser, IIR, radar) seeker, with a clamshell protective door that comes off when the bomb is dropped. A GPS receiver adds a 4th targeting mode. Range is expected to be up to 40 nautical miles when launched at altitude, thanks to a high lift-to-drag ratio in the design. Since SDB-II is an unpowered glide bomb, its actual range will always depend on launching altitude and circumstances. An F-22A would be able to extend that range significantly by launching at supercruise speeds of Mach 1.5, for instance, as long as the bomb proves safe and stable at those launch speeds.
This powerful, integrated seeker seamlessly shares targeting information among all three modes, enabling weapons to engage fixed or moving targets at any time of day and in all-weather conditions. SDB II’s tri-mode seeker can also peer through battlefield dust and debris, giving the warfighter a capability that’s unaffected by conditions on the ground or in the air.
SDB II can fly more than 45 miles to strike mobile targets, reducing aircrews’ time in harm’s way. The weapon’s small size allows fewer aircraft to take out the same number of targets as previous, larger weapons that required multiple jets. SDB II’s size has broader implications for the warfighter and taxpayers, as it means fewer attacks with less time spent flying dangerous missions. The U.S. Air Force and the U.S. Navy have begun SDB II integration activities on the F-35 Joint Strike Fighter and the F/A-18E/F Super Hornet aircraft.
Rafael’s Spike Family of Missiles
Rafael’s SPIKE family of multi-purpose, tactical missiles have sophisticated electro-optic (EO) CCD and IIR sensors for day/night all-weather operation. SPIKE NLOS is an EO guided multipurpose missile for ranges of up to 30km with pinpoint accuracy and midcourse navigation. The weapon system can be launched from land, air and naval platforms.
The SPIKE-ER Multi-Purpose Missile System for Combat Vehicles, Helicopters and Naval Vessels is an extended-range, multi-purpose anti-armour missile system designed for operation from various platforms, including helicopters, fast boats and combat vehicles. It can also be mounted on a tripod for ground operation. SPIKE-ER is capable of defeating targets at a range of up to eight kilometers
SPIKE-MR and SPIKE-LR are lightweight, 4th generation, man-portable missile systems that share many of the same features that combine to meet the challenges presented by enemy forces. High hit probability against stationary and moving targets is provided by a state-of-the-art CCD / IIR seeker, for operation during day and night and in adverse weather conditions, and an advanced tracker and precision guidance system.
Conclusion
With the increasing of the complexity of mission and environment, precision guided weapons make stricter demand for infrared imaging seeker. To meet this demand Researchers are developing imaging seekers with high detection sensitivity, large dynamic range, having better target recognition capability, having better anti-jamming capability, infuse them with more intelligence and better environment adaptability.
