Overview
The term "optoelectronic sensor" covers a wide range of sensing technologies and applications. Fundamentally, an optoelectronic sensor converts light or changes in light into electronic signals and analyzes those signals to trigger predefined responses.
The performance of any optoelectronic sensor depends on balancing two basic limits: the combination of resolution and sensitivity, and pixelation. Resolution defines the smallest object that can be effectively seen, sensitivity indicates how dim a signal can be before it is lost in environmental noise, and pixelation refers to sampling of the sensor image.
The relative importance of each element depends on state-of-the-art technology and the task the sensor must accomplish. For example, missile launch detection systems used in ballistic missile defense search for extremely bright points in cluttered backgrounds.
Why Infrared Matters
MDA deputy director J.D. Syring told the House Armed Services Committee in June 2017 that as nations extend missile ranges and adopt countermeasures for ballistic missile defense, missiles are becoming more complex, survivable, reliable, and accurate, making the ballistic missile threat increasingly complicated.
Threat control continues to evolve and be deployed. Although concepts for hypersonic glide vehicles and nonballistic missiles date back to World War II, recent technology advances have made these systems feasible. In 2016 Russia and China announced successful launches of hypersonic glide vehicles.
Because missile launch detection systems may be land-, sea-, air-, or space-based, many missile defense advocates call for more orbital sensors to provide day-and-night, all-weather global surveillance that other sensor types cannot match, particularly when launch centers are located in remote, denied regions.
Importance of infrared
Thomas Karako, senior fellow and director of the missile defense program at an international security research center, said that ballistic missile defense tasks are detection, tracking, and identification. Infrared plays a major role in tracking threat clouds because distinguishing targets requires different techniques to identify the most threatening objects so interceptors can collide with them and avoid nonthreatening pieces. That is the identification challenge.
Karako added that, given the current threat environment, it is important to deploy practical missile detection sensors before next-generation technologies arrive. From a policy perspective, the priority should not be to delay development while waiting for the best possible technology. Acquiring space-based sensors is not primarily a technical problem but a political and cost issue. Current technology is sufficient to implement a robust space-based missile-defense layer without waiting for more advanced technologies.
He said a space-based sensor layer is a key step for ballistic missile defense in the 2020s, addressing the evolving missile era in terms of altitude and range. A space-based layer provides better vantage points, greater persistence, and a different perspective on the threat cloud produced by midcourse launches.
Low Earth orbit (LEO) infrared satellites can view threat clouds from the side, observing not only radar cross sections of warheads and other objects but also thermal signatures that contrast with the cold space background. Combining ground radar with optoelectronic sensors in orbit greatly helps interceptors locate targets.
Karako observed that decisions and full deployment will take many years, so there is even more reason to start now. Current systems are heavily dependent on radar, so improving system performance is necessary to buy time for acquiring additional capabilities to address a range of threats, including large intercontinental ballistic missiles (ICBMs).
Various sensor combinations will be needed. Today we are constrained by an overreliance on ground-based radar, Karako said. As threats change, we should increase technological diversity and responsiveness so a range of capabilities can meet current and future threats. No single system can do it alone; combinations of systems are required.
Multitarget Kill and Program Efforts
In April 2017, a U.S. Navy team demonstrated unmanned aerial system mission control concepts that modeled future MQ-25 operations.
Multitarget kill
Bruce Jurcevich, advanced intercept systems program lead at a missile fire control company, said one current effort aims to reduce technical risk for the Missile Defense Agency's multitarget kill vehicle (MOKV) program, which will use advanced optoelectronic technologies to improve system reliability while lowering total cost.
Jurcevich said that advances in optoelectronic sensors could shorten multisource data fusion response times and improve survivability and reliability in missile defense applications.
Contractors including Lockheed Martin, Raytheon Missile Systems, and Boeing Defense were contracted by the MDA to define an MOKV proof-of-concept demonstrator, show risk-reduction steps, assess technical maturity, and prioritize technologies that minimize design risk.
Jurcevich noted that materials and manufacturability for optoelectronic sensors have improved over the past decade: longer wavelengths, better spectral response, and lower cost. He expects continued focus on reducing sensor cost, and on improvements in artificial intelligence, multisensor data processing, machine learning, sensor fusion, autonomous system management, and broader spectral capacity.
He described state-of-the-art infrared sensors as available in multiple format sizes and materials, operable across short-wave and long-wave bands with very low background noise. Ultraviolet sensors now use various advanced substrate materials and employ photodiodes, photovoltaic cells, and photomultiplier detectors optimized for spectral response, noise, and dark signal for irradiance measurements.
A 2017 report from the international security research center titled "Missile Defense 2020" noted that there is no better defensive capability than sensors and command-and-control systems that determine threat location and guide removal. While interceptors often capture the imagination, sensors are an underestimated pillar of missile defense operations. Sensors are required throughout the intercept cycle: early warning, tracking, fire control, identification, and battle damage assessment. Homeland missile defense relies on sensor inputs from multiple land- and sea-based radars and high-altitude satellites.
Command and Control
Ballistic missile defense sensors provide target information to the ground midcourse defense (GMD) fire control element at Schriever Air Force Base in Colorado Springs. Under command, control, battle management, and communications (C2BMC) software, the system integrates that information and transmits it to in-flight ground-based interceptors via in-flight interceptor communications system (IFICS) data terminals (IDT).
U.S. space-based missile defense has included decades of infrared satellites in geosynchronous orbit through programs such as the Defense Support Program (DSP) and the Space-Based Infrared System (SBIRS). SBIRS was launched into high Earth orbit in 2006 and geosynchronous orbit in 2011 to replace DSP and the LEO variable-band Space Tracking and Surveillance System Demonstrator (STSS-D), with the aim of providing persistent deep missile-sensor coverage from space. STSS-D was launched in 2009 but has not been fully integrated.
In May 2017, a mix of space-, land-, and sea-based systems using optoelectronic sensors demonstrated detection and tracking of a target intercontinental ballistic missile launched from a test range between Hawaii and Guam. A ground-based interceptor (GBI) was launched from Vandenberg Air Force Base guided initially by fused sensor data. Once in space, the GBI released an exoatmospheric kill vehicle (EKV).
That test was the first full intercept test of EKV Module 1 Block 2 using its own onboard optoelectronic sensor to locate and collide with the target. This was the first intercept of an ICBM by a GBI or any other ballistic missile defense system (BMDS) component using onboard optoelectronic targeting.
MDA research includes space-based kill assessment experiments to determine the feasibility of placing sensors on commercial satellites. Sensors developed by Johns Hopkins Applied Physics Laboratory and hosted on commercial satellite buses will assess intercept probability and deliver information to warfighters to improve engagement decisions and avoid expending interceptors on already neutralized threats.
The U.S. Army deployed "Red Falcon Eye" optical nanosatellites on the International Space Station. Optoelectronic nanosatellites aim to enable tactical leaders to act in near-real time and maintain timely situational awareness.
More Sensors Is Better
The research center's report states that long-standing demand is to deploy as many sensors as possible from the most advantageous positions, equipped with as many technologies or phenomenologies as possible, then integrate their input data effectively and interpret it through centralized command-and-control networks. Sensor improvements can increase lethality, effective magazine capacity, and overall defensive robustness. Redundancy in sensors provides another layer of defense.
Without space-based sensors at high altitude, homeland missile defense depends solely on ground radars for tracking and identification. That places greater burden on the sensor network, making it harder to identify objects within an incoming threat cloud and requiring engagement of more objects within the cloud to ensure warheads are destroyed. This in turn requires launching more interceptors against a single threat cloud, reducing the effective capacity of ground-based interceptor stockpiles.
For more than half a century, ballistic missiles have been a principal threat and a significant challenge for optoelectronic sensors.
Richard Matlock, MDA advanced technology program lead, said the U.S. currently depends on SBIRS in highly elliptical and geosynchronous orbits and on shipborne X-band radars, complemented by ground sensors, to detect ballistic missile launches. This approach does not provide the persistent global sensor coverage needed to address evolving, more mobile, and more complex threats. MDA calls for a global persistent space-based sensor array including radar and optoelectronic sensors to detect, track, and discriminate missiles from launch through termination.
Matlock said it is important to shift over the coming years from primarily ground-based systems to systems that operate principally from space. The MDA advanced technology projects office is pursuing research to improve optoelectronic sensors for ballistic missile defense, including:
- Discrimination technologies: Near-term goals include adding high-altitude airborne or spaceborne optoelectronic sensors to the BMDS architecture to acquire, track, and discriminate ballistic missile targets. The agency is developing and testing sensors currently deployed on unmanned aerial vehicles.
- Advanced concept and performance assessment: An "intelligent buyer" approach using model-based engineering tools and techniques to assess emerging missile defense needs, analyze alternative concepts and technologies, understand requirements, reduce risk, and ensure cost-effective mission solutions.
- Academic research programs: Contracts to colleges and universities to develop next-generation technologies that may be fielded in BMDS, including research on reducing debris effects, rapid-response architecture optimization, propulsion, optoelectronic sensors, and material characterization.
- Small Business Innovation Research (SBIR): Funding early-stage R&D in small technology companies to stimulate technology innovation and expand commercialization under federal R&D support, increasing small-business participation in federally funded R&D and promoting participation by minority and disadvantaged firms.
- Small Business Technology Transfer (STTR): Similar to SBIR, STTR funds cooperative R&D projects between small businesses and research institutions to transfer ideas from national labs to the marketplace for civilian and military customers.
MDA requested $17 million from Congress to continue space-based kill assessment experiments using fast-frame infrared sensors as part of an integrated post-intercept battle damage assessment capability. Syring confirmed that space-based kill-assessment payloads began integration on commercial host satellites in December 2016 and the overall network was expected in orbit in 2018.
Syring told Congress that investments in radar and advanced optoelectronic sensors aim to achieve a diversified sensor architecture that can eventually provide high-fidelity midcourse tracking and discrimination. Syring cited test results from the Terminal High Altitude Area Defense (THAAD) system, which achieved 15 successful intercepts in 15 tests, demonstrating that it can defeat ballistic missiles from the air. However, the next-generation missile defense system needs to go further by using new radar technologies and sensor algorithms so weapons systems can not only detect threats but also understand exact threat nature.
Size, Weight, Power, and Cost
Size, weight, power, and cost (SWaP-C) also influence the future development and deployment of optoelectronic sensors, with miniaturization and continued development of longer-lasting, smaller, lighter power systems being key.
The multitarget kill vehicle (MOKV) program intends to destroy multiple incoming warheads with a single missile. Up to six MOKVs could be launched on a single booster. Deployed near the edge of space, they would maneuver and use separate kinetic kill vehicles to defeat multiple incoming warheads and decoys.
Chris Dobbins, infrared and optical technology researcher at a U.S. Army aviation and missile R&D center, said that regardless of other prerequisites, SWaP-C is always a driver because final volume, weight, and power limit sensor performance and thus system capability. Cost is always a designer concern because it ultimately determines whether a project is feasible on a cost-benefit basis.
Dobbins added that infrared sensor technology has changed dramatically over the past decade. Reduced detector pixel size is no longer a design constraint. Arrays have grown into the megapixel domain, and detector operating temperatures have increased with improvements in high-operating-temperature (HOT) materials.
Optoelectronic sensors typically combine light-detection hardware with software controlling multiple light sources to illuminate targets from different angles with varied waveforms. The reflected light is then analyzed to identify detected target features. Syring told Congress that MDA is focused on designing sensor algorithms that discriminate lethal from nonlethal threats.
Research published at the 3rd International Conference on Control, Automation and Robotics (ICCAR 2017) by two systems engineering researchers at the Naval Postgraduate School assumed the use of unmanned aerial vehicles equipped with optoelectronic sensors to detect and track launches of large ICBMs. The paper on rocket launch detection and tracking using optoelectronic sensors aimed to assess ascent parameters and provide targeting information so intercepting UAVs could be positioned for collision intercepts with kinetic kill interceptors.
Airborne and Unmanned Applications
Optoelectronic sensors on manned aircraft can also serve new missile-defense functions. In April 2017, Northrop Grumman Systems won a contract to supply a large aircraft infrared countermeasures (LAIRCM) program for U.S. naval aviation. LAIRCM will install laser-based optoelectronic missile defense systems on various U.S. Navy and Air Force aircraft to automatically detect missile launches, assess whether they are threats, and activate high-power laser countermeasures to track and defeat incoming missiles.
Planned LAIRCM aircraft include transport and VIP jets, C-130 variants, CV-22 tiltrotors, KC-46 tankers, P-8A maritime patrol aircraft, and some large military helicopters. Early interim versions for some Air Force aircraft will include ultraviolet sensors, countermeasure processors, and small laser turret components.
The MOKV program leverages recent optoelectronics advances to improve system reliability and lower total cost.
Future Infrared Systems
Future infrared systems will use smaller laser turrets to provide better resolution and performance in optical clutter and increase detection range. LAIRCM-style systems are seen as forerunners for aircraft protection against infrared-guided threats by detecting incoming missiles, classifying them, and then applying tailored countermeasure energy to defeat them.
The threat of missile attacks against Guam prompted new uses for the MQ-9 Reaper unmanned aerial system. After improvements to a multispectral targeting sensor, Reapers could scan the airspace between North Korea and Guam. Using optoelectronic infrared sensors to detect booster-stage heat and combining data from two or more Reapers, the system could triangulate a missile plume to create 3D target information for naval interceptors.
General Atomics said it is working to improve Reaper tracking capabilities during the early boost phase. The company also plans to repurpose the Avenger platform and other designs to host large, long-range sensors that cannot be carried on smaller UAS. Avenger modifications would accommodate large multispectral sensor suites such as the long-range MS-177A multispectral sensor currently flying on competing platforms.
General Atomics noted plans for future long-range sensors with very large apertures to enable deep sensing when UAS operate along foreign borders.
Multispectral Targeting and Widely Deployed Systems
One of the most widely deployed optoelectronic systems is the Raytheon multispectral targeting system (MTS), installed on more than 500 U.S. Navy aircraft over the past 15 years.
The system's full-motion electro-optical imaging provides long-range surveillance and high-altitude collection, tracking, and laser designation. The MTS-C variant includes a long-wave infrared detector for tracking cold bodies such as post-boost missiles and warheads, and for observing exhaust plumes during boost phases of rockets and missiles.
Artificial Intelligence, Data Processing, and Autonomy
As with many technologies, especially where rapid military understanding and decision-making are required, the role of artificial intelligence in missile-defense sensor environments remains to be proven at scale.
Dobbins said that when infrared sensors first appeared, information was typically fed directly to displays with minimal processing so users could decide how to act. Today, very large-scale information and array formats require pre-processing before a user can effectively use the data.
He added that a post-launch locked interceptor will never let a human eye see the target image directly. A missile is a state machine following a set of operations based on presented inputs. In simple terms, algorithms tell the system where to look, open its eyes, find the target, and engage.
