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Advanced hostile fire detection (HFI) technologies to protect aircraft and combat vehicles against small arms, and RAMs

Compact, low cost, accurate hostile fire detection (HFI) solutions are essential to effectively protect aircraft and combat vehicles against small arms, RPGs, mortars, anti-aircraft artillery, and surface to air missiles. Detecting and classifying these types of threats is complicated by many factors including the need for full 360 coverage, requirement for high probability of detection with low false alarms, as well as variances in real world signatures.


The US Army is engineering a new Hostile Fire Detection system for its fleet of armored combat vehicles to identify, track and target enemy small arms fire. The ability of a tank team to identify the location of enemy fire, be it small arms or Anti Tank missiles, is invaluable. This ability, given its connection to other C2 system, can give fighting forces a great advantage in today’s battlefield, where threats comes from all directions, even from underground tunnels. Enabling counterattack is a fundamental element of this, because being able to identify enemy fire would enable vehicle crews to attack targets from beneath the protection of an armored hatch.


The Army currently deploys a targeting and attack system called Common Remotely Operated Weapons System, or CROWS; using a display screen, targeting sensors and controls operating externally mounted weapons,  CROWS enables soldiers to attack from beneath the protection of armor.


Much of the emerging technology tied to these sensors can be understood in the context of artificial intelligence, or AI. Computer automation, using advanced algorithms and various forms of analytics, can quickly process incoming sensor data to ID a hostile fire signature. “AI also takes other information into account and helps reduce false alarms,” Gene Klager, Deputy Director, Ground Combat Systems Division, Night Vision and Electronic Sensors Directorate, explained.


AI developers often explain that computers are able to much more efficiently organize information and perform key procedural functions such as performing checklists or identifying points of relevance; however, many of those same experts also add that human cognition, as something uniquely suited to solving dynamic problems and weighing multiple variables in real time, is nonetheless something still indispensable to most combat operations.


This system, integrated onto Apache Attack helicopters, uses infrared sensors to ID a heat signature from an enemy weapon. The location of enemy fire could then be determined by a gateway processor on board the helicopter able to quickly geolocate the attack.


While Klager said there are, without question, similarities between air-combat Hostile Fire Detection technologies and those emerging for ground combat vehicles, he did point to some distinct differences.


“From ground to ground, you have a lot more moving objects,” he said.


Potential integration between Hostile Fire Detection and Active Protection Systems is also part of the calculus, Klager explained. APS technology, now being assessed on Army Abrams tanks, Bradleys and Strykers, uses sensors, fire control technology and interceptors to ID and knock out incoming RPGs and ATGMs, among other things. While APS, in concept and application, involves threats larger or more substantial than things like small arms fire, there is great combat utility in syncing APS to Hostile Fire Detection

Hostile fire detection systems

Technology that can determine the source of enemy fire is very useful to rapidly neutralize threats. However, most existing technologies have significant limitations.


Acoustic Detection systems

Acoustic gunshot detection systems utilizing microphone arrays are capable of establishing the approximate point of origin and trajectory of small arms fire, and meet the low SWAP requirements. However the performance of Acoustics-based systems  are easily confused in multi-path reflection environments, including mountainous regions and urban canyons.


This is because of multi-path reflections and reverberations of sound waves, and because buildings can obscure the muzzle blast waves. Also, the accuracy of estimating the direction of arrival is limited by the distance between microphones and the geometry of the setting. Moreover, relying on the detection of shock-waves from passing projectiles makes it impossible for acoustics-based systems to provide situation awareness.


An alternative to acoustics is optical detection systems designed to detect the unique signature of the muzzle flash.



Infrared Detection Systems

Since visible-spectrum flash light is barely seen in daylight, optical systems exploit the mid- and long-IR spectrum, detecting the thermal radiation created by the flash. Two types of detectors are in common use,  either cryogenically cooled detectors (large, heavy and power-hungry) or uncooled micro-bolometer detectors. Uncooled detectors offer low SWAP, but have two significant shortcomings: low imaging speed (limiting the maneuvers and speed of the carrying platform), and low specificity (discriminating between muzzle blast flash signals and naturally occurring event signals). They thus have limited potential.


Dismounted Marines typically utilize uncooled Long-wave Infrared (LWIR) imagers, such as the AN/PAS-28 Medium Range Thermal Imager and AN/PAS-30 Mini Thermal Imager , due to their low cost (less than $10,000), low power (less than three Watts), and near-instant start-up time, but these systems have only demonstrated reliable imaging of relatively large or slow projectiles, such as grenades.


Handheld MWIR imagers are available in the USMC inventory, but their high cost (greater than $20,000) and cooling needs (up to eight Watts and greater as ambient temperature increases, cool-down times measured in minutes) are accepted for only the longest range (over 2,500 meter) imaging applications. The currently fielded AN/PAS-22 Long Range Thermal Imager  is an MWIR device, but has a restricted field of view, low resolution, and insufficient imaging frame rate to resolve small, high speed projectiles perpendicular to observer.


Uncooled SWIR sensors operating in the 1.0μm–1.7μm band are very effective at detecting the black body radiance emitted by a muzzle flash (with typical effective temperatures in the range of 1000K–2000K). The short wavelength and very high speed of SWIR-sensor focal-plane arrays allow for high spatial resolution and high specificity in isolating and detecting very fast transient events of the muzzle flash (which are typically shorter than 1ms).


Typically Accuracy of points of origin have an average azimuth error of less than five degrees from the observer’s point of view, and an average range estimation error of less than 20%, during conditions of no-obstructing terrain between the source and observer.


Under ideal viewing angle (parallel, but not perpendicular, to line of sight) and environmental conditions, these optical  devices are capable of briefly perceiving small arms projectiles in flight, either directly or indirectly via their wake, but without sufficient detail to reliably track to the point of origin. Though solutions based on Mid-Wave Infrared (MWIR) sensors are feasible, they are impractical for widespread use due to cost. The mountainous and urban environments can constrain the use of line of sight based gunfire location techniques, such as muzzle flash detection.


The use of hostile fire indication systems (HFIs) to patrol ground and airborne forces is challenging in dense urban or civilian areas. Countermeasures against sparse and asymmetric forces with light firearms require precise retaliation and careful maneuvers using multiple HFI systems. A large number of vehicles and dismounted soldiers are involved, therefore the technology need to be affordable and capable of meeting requirements for low size, weight, and power (SWAP).



Radar Detection Systems

A counter-battery radar (alternatively weapon tracking radar) is a radar system that detects artillery projectiles fired by one or more guns, howitzers, mortars or rocket launchers and, from their trajectories, locates the position on the ground of the weapon that fired it.[1]:5–18 Such radars are a subclass of the wider class of target acquisition radars.


HFL-CS is a radar-based solution that provides automatic hostile fire detection and localization of the source of small arms fire, from 5.56mm caliber up to rocket propelled grenades. It calculates a projectile’s trajectory and in less than 3 seconds is able to locate the direction and range of the point of origin, as well as classify the type of threat by its caliber.


As HFL-CS is radar based, it overcomes some of the limitations of alternative acoustic solutions. It is unaffected by high background noise or acoustic echoes in urban environments and can be operated at night and in low visibility conditions such as smoke, dust, rain and fog.


The fusion of various sensors are also preferable for example integration of the SWIR sensor with a tactical radar sensor. This enables fine angular resolution of the optics combined with the range and velocity measurements of radar. It allows us to gauge the precise relative location of a missile launcher or gun that has fired a projectile at the platform carrying the HFI system. Once the defending platform has been located  (using other measurement means) we can accurately estimate the location of the launcher. The close integration of the two technologies also allows for significant power savings in the radar transmitter design, simplifying its operation and reducing the probability of radio- frequency signal intercept.


Serenity Payload

The Serenity payload, developed jointly by Aviation and Missile Research Development and Engineering Center and the Army Research Lab has been deployed to theater in the Middle East . It provides 360-degree hemispherical surveillance coverage, has self-contained power and is deployed on the Lockheed Martin Persistent Threat Detection System aerostat.


“Once the Serenity locates hostile fire, the system then communicates the shooter point of origin in Universal Transverse Mecator coordinates to the tactical Operations Center using cursor-on-target protocol,” said Timothy Edwards, PhD., chief scientist for force protection technologies. “Threat cueing and classification information allows the Soldier manning an AMRDEC-developed Containerized Weapon System or other defensive system to give prompt attention to the hostile threat.”


A dual-sensor system, Serenity combines two electro-optical (EO) pods with an acoustic sensor, developed by Hyperion Technology Group, to help friendly forces better react to incoming enemy fire by locating its point of origin. Serenity’s optical sensor picks up the flash of enemy mortar or rocket fire and then calculates its point of origin and range from the sensor. The system notes the respective time delays of their patterns as received by the microphones. The system’s acoustic sensor will then validate that enemy fire has indeed been detected by measuring the lag time between flash and bang.


Working together, these two Serenity sensors dramatically reduce the false alarms typically associated with other hostile fire detection systems. “There are legacy hostile fire detection systems out there, and while they’ve proven the concept, Serenity reduces false positives and does a better job pinpointing sources of enemy fire,” Plew says.


Serenity weighs less than 80 pounds, light enough to go on unmanned aerial vehicles. It can transmit its fused optical/acoustic data to available full motion video cameras or wide-area motion imagery (WAMI) sensors.




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