Electronic Warfare Sensors

Transcription

1 Electronic Warfare Sensors Background 1. Fast, automatic communications intelligence and electronic support measures system integrates functions to save time. The speed and effectiveness of tactical forces on the battlefield depend on advances in electronics for command, control, communications and reconnaissance. This fact strengthens the growing importance of electronic warfare in an environment where a large number and variety of military systems radiate electromagnetic energy into free space. The rapid improvements in electronics and information systems create not only new capabilities, but also new vulnerabilities. This free propagation of electromagnetic waves offers to anyone, including an enemy, the possibility of exploiting transmissions for their own purposes. To the degree that communications and electronics are indispensable for one side, they are important sources of information and targets of disruption and destruction to the other. 2. The principles of electronic warfare (EW) encompass a variety of techniques and technologies in areas such as electronic support measures (ESM), electronic countermeasures (ECM) and electronic counter countermeasures (ECCM). ESM measures are used to search for, intercept, locate, record and analyze radiated electromagnetic energy emitted in support of military operations. Just as ESM systems are valuable for ground-based operations, they also are important for use on maritime platforms. ESM also provides information for ECM and ECCM for detecting, warning and avoiding threats, and for targeting and homing. Electronic combat is another concept that closely integrates electronic warfare technologies with reconnaissance and fire support to neutralize, disrupt or destroy hostile command, control and communications, while protecting friendly systems. A generally accepted military principle is that victory in any future war will go to the side that can best control and manage the electromagnetic spectrum. Fig1: Sensors for use in Space Combat and Defence

2 2 Active Vs Passive Sensors 3. To detect small, widely spaced objects at a distance in three-dimensional space it is most efficient to emit a pulse of radiation and record the resulting reflections. Active sensors are phased-array radars or lidar systems. They are used for high-resolution target identification. Active sensors are used to track vessels whose drives are not active, and so are the primary sensors system at shorter ranges. The decision to 'go active' is a major tactical choice as the sensor system becomes visible at a considerably greater range at which it can detect targets. Active sensor emitters can be the target of EMP or other beam weapons; it is much more difficult to locate and destroy a widely spread cloud of passive sensors. In a civilized system, the chances are you won't be all that far away from a sensor array of some sort; any polity or entity which has any interest in defending itself and the resources of a solar system will have the outer system seeded with millions of passive sensor platforms. These could be relatively cheap, stealthy and very good at spotting a ship that is radiating at all in any frequency. Elector-Optics (EO) 4. It is defined as the interaction between optics and electronics leading to the transformation of electrical energy into light, or vice-versa, using suitable devices. EO equipments convert either the infrared (IR), visible or ultraviolet (UV) portions of the electromagnetic (EM) spectrum into a form that may either be displayed to a user, or interpreted automatically by a computer. During the Vietnam War the introduction of LASER guided bombs permitted precision attacks against point targets and by the mid- 1970s many forces were deploying LASER-based weapon and ranging systems. During the Falklands War (1982) British forces exploited their superiority in night vision systems to maintain their attacks against Argentinean positions throughout the hours of darkness. The 1991 Gulf War brought EO sensors into the spotlight as TV screens across the World showed the Coalition Forces carrying out attacks against the by day as well as night. 5. EO is the broad field of science dealing with the use of the infrared (IR), visible and ultraviolet (UV) portions of the EM spectrum. Unlike IR and visible wavelengths, exploitation of UV radiation is limited by atmospheric ozone, which absorbs nearly all radiation in the UV portion of the spectrum. In defining the use of a radar we normally state its operating frequency. However, because of the high frequencies associated with the EO portion of the EM spectrum, it is more convenient to use wavelength to identify EO systems. The systems or sub-systems considered in EO are: - (a) Infrared: Infrared (IR) systems operate in the non-visible portion of the EO spectrum. (b) LASERs: LASERs operate at a single wavelength, which may be selected from a broad section of the EO spectrum. (c) Imaging Systems: Imaging systems are used to convert EM radiation from any part of the EO spectrum and display it at visible wavelengths.

3 3 Infrared 6. Infra comes from the Latin, meaning below. Infrared (IR) therefore means below red in the visible spectrum. Infrared lies between millimetric radar and visible light i.e. wavelengths from 1 to 30 micro (micrometers or microns, 10-6 m). Infrared is electromagnetic radiation which is given off by a substance which has a temperature above absolute zero i.e. 273 O C or 0 O K. There are many military applications of infrared rays. Infrared radiations are a means of seeing in the dark without being seen; of detecting targets by their emissions and of conducting secure communications that are very difficult to intercept. At such short wavelengths, much of the radiating IR energy is absorbed by gases in the atmosphere particularly carbon dioxide (CO 2 ), oxygen (O 2 ) and water vapour (H 2 0). These gases totally absorb IR energy at certain wavelengths leading to what are referred to as IR Windows. LASER 7. A LASER (Light Amplification by Stimulated Emission of Radiation) is a device that produces a very intense and concentrated beam of light that contains radiation in a very narrow bandwidth. LASERs represent the fulfilment of a long-time technological goal that of developing a light beam intense enough to burn through any material. Because the LASER beam travels at the speed of light and has such awesome power, the potential for a death ray type weapon exists. LASER devices are technically optical MASERs (Microwave Amplification by Stimulated Emission of Radiation). Rather than using moving electric charges to obtain electronic amplification and oscillation, (as do transistors and vacuum tubes) these devices use the internal resonance of atoms, or the transitions between quantum energy levels in atoms to obtain amplification and oscillation. The devices are called masers. Masers that oscillate at optical frequencies are called LASERs. LASER Applications. 8. Perhaps the best-known military use of LASERs is as a designator for LASER guided weapons. In this application, a LASER beam is pointed at the target, which then reflects the LASER light. A LASER seeker mounted on the nose of the bomb detects the reflected energy and sends error signals to a guidance package causing the weapon to strike the LASER-designated target. A pulsed LASER can be used as a radar and range finder device. As with radar, the distance to an object is determined by the time required for a light pulse to reach and return from a target. The narrowness of the LASER beam permits sharp resolution of targets. LASER beams can also be used for communications. LASERs can be modulated in a number of ways to encode very complex signals; the beam is then transmitted, detected, and demodulated to recover the information. Due to the narrow beam width, this type of communication is very secure since a receiver must be on-axis to detect the LASER. In theory, due to the high optical frequencies, one LASER beam can carry as much information as all existing radio channels. However, since LASER beams are very highly attenuated by rain, fog, or snow, LASER communication on the earth s surface is not yet entirely reliable.

4 4 Electro Optic (EO) Sensors 9. EO sensors detect either reflected or emitted radiation just as in optical sighting wherein the light reflected off an object or body is detected by the human eye. However, as we go further into the IR, beyond 3.5 mm, the thermal emitted radiation exceeds the reflected radiation. Thus television systems, which operate in the visible and near IR, are dependent almost entirely on reflected light and therefore require scene illumination, IR photography is rarely capable of results at wavelengths greater than 1.3 mm and usually requires some illumination, which may not be visible to the naked eye. Thermal sensors operate mainly in the 3.5 to 25 mm band therefore do not require any illumination. IR Guided Systems 10. IR guided system consists of:- IR Missile Guidance: In the nose of every heat-seeking missile is a tracking system with a Field-of-view (FOV) of about 1-5 degrees. This tracking system is mounted in gimbals that allow the seeker to be steered through an angle of about degrees. The infrared seeker has two distinct roles. The first is the more obvious one of acquiring and tracking the target. To do this, the seeker must be sensitive to infrared radiation from the target. It must be able to reject clutter from the spurious background sources such as clouds. Having extracted the target from the background clutter, the seeker must then be able to discern the target direction relative to the centre of the FOV and generate error signals, which will result in the tracker being steered towards the target. The second role of the seeker is to provide guidance signals for the missile control surfaces. IR Flares. The simplest countermeasure against IR guided weapons has been the IR flare. These pyrotechnics are simple and cheap. The aim of the IR flare is to seduce the missile away from the target onto itself. To be effective, the flare must ignite and rapidly produce 2-5 times the energy of the target in the spectral band(s) to which the seeker is sensitive. The flare must be fired within the missiles FOV and burn long enough to ensure the missile does not re-acquire the target. The drawback in use of IR flares is that the missiles give no warning until it is visibly seen. Early IR missiles were must sensitive to radiation in mm band corresponding to engine exhaust. Subsequent seekers have extended this usable spectrum to 12 mm. Flares to protect ships should have outputs in 8-12 mm band. An effective method of rejecting flares based on their spectrum is to use a multi-spectral sensor. Multi-spectral sensing is difficult for the flare to overcome, although its effect may be minimised by firing flares in a ripple.

5 5 IR Jammers. Another approach to reducing the lethality of homing missiles is IR jammers. Active IR jammers are generally designed to add spurious, modulated IR energy to the target s IR signal in the missile seeker to degrade the missile and cause the seeker to lose lock-on. Other approaches could involve attempts to saturate the seeker detector with a very strong IR signal or to damage the detector, optics or IR-dome with a power LASER. Most existing jammers use relatively low power modulated radiation. A variety of techniques are used in missile seekers to generate tracking information from the IR radiation emanating from the target. The majority of IR homing missiles use one of a number of forms of reticules to modulate the received IR signal in such a way as to generate information on the angle and distance of the target image from the missile bore sight. The repetition frequency of the modulated signal is known as the carrier frequency. Depending on the seeker design, the final output after amplification and processing of the modulated signal pulses is an AM or FM signal that is compared to a reference waveform to derive the seeker error signals. The jamming technique is to add its own modulated IR energy to distort the tracking signal. Modern seekers use more complex scans, for example Rosette scans, or employ focal plane imaging arrays which are much more resistant to jamming. IMAGING EO SYSTEMS 11. Imaging Electro Optic System comprises of:- Target Acquisition and Tracking: Radar and visual (aided or unaided) techniques are the two basic methods commonly employed for target detection and acquisition. Electro-optical systems are normally used in conjunction with radar to improve the total weapon system capability. The optical system not only gives the weapon system an additional dimension, but also enhances the potential of the radar mode. Most long-range optical devices inherently have narrow fields of view because of physical properties. A narrow field of view limits a system s usefulness as a detection or acquisition device; it normally requires cuing from some external source. This cuing source is normally radar, and is one of the primary reasons these two systems have been mated. If countermeasures tactics such as jamming or chaff are successful in degrading the radar s ability to engage a hostile aircraft, the operator need only switch modes of operation and track the intruder optically. This dual capability gives the weapon system greater flexibility, and severely complicates the electronic warfare officer s problem as both the radar and optical threats must be countered simultaneously. Imaging IR Systems: Imaging IR systems are also being rapidly perfected as an instrument of modern warfare. This system differs from TV in that imaging IR senses different radiation, while TV displays contrast. An IR system will outperform a TV system and has the

6 6 additional capability to operate in any light conditions. The systems so far discussed have been presented as being integrated with a radar system. It should be understood that these systems are fully capable of independent operation and their application need not be limited to use with radar; however, an inherent characteristic of optical systems is their lack of ability to accurately measure range. This is one reason optical systems are frequently teamed with another weapon system. Advancements in LASER technology have made possible the development of LASER ranging systems. These systems are capable of measuring ranges to a very exacting degree. LASER ranging principles are similar to those for radar in that they measure the time differential between pulse transmission and pulse return. The integration of an electro-optical system with a LASER range finder creates a very precise tracking system. Optical Systems: Pure optics is quite simple in that it employs standard field glasses, binoculars or night vision glasses. The optical device is physically mounted to the tracking system s antenna assembly and functions in the same manner as the other sensors. It too is restricted in that it must have ranging information from an external source. Although these systems are limited to daylight and fairly bright night conditions, they can provide very accurate information to the organic weapon system. Anti-Missile Sensors 12. Sensors and their associated systems will function as the eyes and ears of a ballistic missile defense system, providing early warning of attack, target identification, target tracking, and kill determination. New and innovative approaches to these requirements using unconventional techniques are encouraged across a broad band of the electromagnetic spectrum, from radar to gamma-rays. Passive, active, and interactive techniques for discriminating targets from decoys and other sensor-related device technology is also needed, with the intended goal of producing either a specific product or process. BMDS (Ballistic Missile Detection Sensor) sensors would provide the relevant incoming data for threat ballistic missiles. The data from these sensors would travel through the communication systems of the proposed BMDS to Command and Control (C2) where a decision would be made to employ a defensive weapon such as launching an interceptor. The BMDS sensors would provide the information needed to determine the origin and path of a threat missile to support coordinated and effective decision making against the threat. Additionally, these sensors would provide data on the effectiveness of the defense employed i.e. whether the threat has been negated. 13. BMDS sensors would be developed or enhanced to acquire, record, and process data on threat missiles and interceptor missiles; detect and track threat missiles; direct interceptor missiles or other defenses (e.g., LASERs); and assess whether a threat missile has been destroyed. These sensors (i.e., radar, infrared, optical, and LASER) would include signal processing subcomponents, which receive raw data and use hardware and software to process these data to determine the threat missile s location, direction, velocity, and altitude. The operating environments of the existing and

7 7 proposed BMDS sensors can be considered in four general categories. Land-based sensors may be fixed, located in or on a building, or mobile, located on a vehicle or trailer. Air-based sensors are located on platforms that can travel through the air such as airplanes, balloons, and airships. Sea based sensors are located on platforms that travel on water (e.g., ships or a floating platform). 14. Onboard signal and data processing is a key technology in missile detection and tracking systems. A first generation processor was developed for the Forward Acquisition System (FAS) in the 1980's. FAS was a scanning system that used, for the first time, a mosaic array of detectors instead of a single line array of detectors. A second generation processor built for the Airborne Surveillance Test bed (AST) Program, formerly known as Airborne Optical Adjunct, was introduced in 1984 as a follow-on to FAS. AST was similar to FAS, but the AST system is carried on an airplane instead of on a satellite. The AST program was a success and it is still operating and gathering data. In 1996, a third generation processor built for a satellite-based surveillance system, called the Midcourse Space Experiment (MSX), was launched into a 900 km orbit. The MSX program is a military program funded by the U.S. Ballistic Missile Defense Organization. MSX uses an onboard signal and data processor (OSDP) to acquire and track objects. 15. Fourth generation processors for missile and aircraft surveillance systems such as the Space and Missile Tracking System (SMTS, also known as Space-Based Infrared System--Low, or SBIRS-Low) will have to meet requirements exceeding the capabilities of any existing surveillance system. The SMTS will use both scanning and staring electro-optical sensors to detect and track missiles, aircraft and resident space objects (RSO's). The scanning sensor onboard the SMTS will be required to detect events of interest over a wide field of regard and then hand over the line-of-sight (LOS) positions and velocities of these events to an onboard staring sensor and to staring sensors on other platforms. Staring sensor performance over a large area is thereby achieved with small field of view staring sensors. Resulting single-platform (mono) LOS tracks are then combined with LOS tracks from other platforms to produce interceptorquality stereo tracks. To achieve such results, the scanning sensor must provide very effective below-the-horizon (BTH) clutter suppression to insure that the staring sensors are not spending most of their time servicing false alarms. The staring sensors must provide very effective suppression of BTH and above-the-horizon (ATH) clutter and focal plane pattern noise in order to detect dim targets. Functional requirements which must be met by such a system are: Operation of scanning and staring sensors in a radiation environment. Algorithms for detecting and circumventing radiation-induced spike events in the data must therefore be implemented. Very low false alarm rate operation of a scanning sensor by clutteradaptive detection, so that the staring sensors have time to handle the handover tasks. The targets of interest are boosting vehicles, RSO's and jet aircraft ("slow walkers").

8 8 Accurate, high speed calculation of scanning sensor LOS tracks. Toward this end, the accurate typing of boosting vehicles and aircraft is needed. Attack-assessment-quality midcourse calculation of launch points, impact points and parameters from a single platform (mono, three-dimensional (3- D) tracking), where the initial position and velocity of the tracking filter is obtained with booster typing information and an initial orbit determination algorithm. Accurate handover of mono 3-D tracks to a second platform to produce interceptor-quality state vectors by multi-platform (stereo) processing. Staring sensor streak detection which adapts to background clutter characteristics in the presence of focal plane pattern noise and jitter. The targets of interest are boosting vehicles, RSO's and jet aircraft. MINI-SENSORS FOR "MILITARY OMNISCIENCE" 16. Spotting insurgents is tough in today s guerilla war zones. So tough, that no single monitor can be counted on to handle the job. The Pentagon's answer: build a set of palm-sized, networked sensors that can be scattered around, and work together to detect, classify, localize, and track dismounted combatants under foliage and in urban environments. It s part of a larger Defense Department effort to establish military omniscience and ubiquitous monitoring. Fig 2 : Mini-Sensor 17. The US military has been working on gadgets for a while, now, that can be left behind in a bad neighborhood or a jihadist training site, and monitor the situation. These Camouflaged Long Endurance Nano-Sensors (CLENS) would be an order of magnitude smaller of about just 60 milimeters long, and 150 grams. Darpa, the Pentagon's far-out

9 9 Fig 3 : Camouflaged Long Endurance Nano-Sensors research arm, also wants the monitors to take up a 10,000th of the power of previous sensors. That would give the CLENS enough leaverage to keep watch over an area for up to 180 days. The way they'd keep watch would be different, too. Not as a individual sensors, but as a network of monitors, communicating with ultra wideband radios. The same frequencies could be used as a kind of radar, to track objects and people within the sensor net. 18. "The best way to learn about an adversary what he s done, what he s doing, and what he s likely to do - is through continual observation using as many observation mechanisms as possible, also known as persistent surveillance. Dr. Ted Bially, head of Darpa's Information Exploitation Office, told a conference last year. "We ve learned that occasional or periodic snapshots don t tell us enough of what we need to know. In order to really understand what s going on we have to observe our adversaries and their environment 24 hours a day, seven days a week, week-in and week-out." A New Technology 19. To meet the requirements of both precision force-proportional measurement and force-proportional motion control input devices, a revolutionary approach to sensor design has been developed, based on an innovative method of creating strain-sensitive elements--strain gauges--on the surface of application-driven stainless steel elements. The strain gauge bridge is made a permanent part of the steel's outer layer by curing or baking it into the steel at very high temperatures. With no bond line or glue line separating the strain-sensitive material from the body of the sensor, the gauges can be made more reliable, robust, miniature, and cost effective at any quantity. Because the strain-sensitive regions are all created at the same time rather than handled individually, the entire sensor can be made very compact. The relative position of each strain gauge in the bridge can be predetermined by the design of the sensing element artwork and the application tool or stencil. This allows precise and compact clustering of strain gauges. Linearizing and stabilizing the raw strain gauge required the development of a

10 10 proprietary stabilization process. After stabilization, the outputs of the strain gauge and strain gauge bridge become linear and stable over both time and operational cycles. Life test data show that even after operating for well in excess of 10 million cycles, the sensors do not show any detectable degradation in performance or change in output. Fig 4 : (A) Fig 4 : (B) Figure 4: Finite element modeling of a 3-axis force-measurement sensor shown is used to determine the correct placement of the strain gauges to form a complete Wheatsone bridge for multi-axis and multichannel measurements (A). The same principle can be used to create a single-axis sensor for specialized applications (B). Trimming Considerations 20. To eliminate variations from sensor to sensor and from bridge to bridge, an additional process of trimming the strain gauge bridge resistance and output is carried out on the mass-produced sensors. The procedure is very similar to trimming resistors in a resistor network, but in contrast to conventional resistive materials, the strain gauge materials are brittle and very sensitive to the enormous heat present during the trim process. Attempts to trim these strain gauges with the techniques used on conventional resistors can lead to stress concentrations in the strain gauge element. These stress concentrations will propagate through the strain gauge as it deflects during normal operation, creating cracks and eventually causing the resistance of the strain gauge to unbalance the bridge. 21. The problem was solved by a new technique that allows the bridge to be trimmed without creating stress concentration in the strain gauges. With this new strain gauge technology, several universal force sensor designs were created for both single- and multi-axis force measurement based on coplanar bridge architecture. Complex networks of interconnected strain-sensitive regions or strain gauges form complete Wheatstone bridges. With an independent bridge for each axis of measurement, one sensor with one force input point can measure and interpolate the input force into multiple force vectors. The locations of the strain gauges on the surface of the sensor body are selected based on the stress concentration regions determined by finite element

11 11 modeling of the sensor body Correct placement of the strain-sensitive areas and proper coupling of the different parts of the bridge allow the resulting sensing element to report an input force in terms of three distinct and independent signals that are proportional and linear with the vectors of the applied force. The force vector-proportional outputs can perform a myriad of tasks ranging from multi-axis force measurement, forceproportional closed-loop systems, and direction resolution, or they can serve as forceproportional input devices for precision motion control applications. The design of each type of sensor is dictated by the specific application rather than by the technology itself. The design engineer can therefore concentrate on application requirements such as size, force range, and cost without having to reinvent a new sensor each time. There are additional benefits: If in a given application a Wheatstone bridge made of conventional strain gauges can provide a signal that is on the order of a fraction of a millivolt/volt input, the new strain gauges can provide outputs several orders of magnitude higher. The effect is to reduce S/N problems as well as the size of the sensor. Because the strain gauges are linked in a full-bridge architecture, and all the strain-sensitive regions are co-located on the sensors, their change in resistance with temperature shift is virtually identical. This means that the output of the bridge exhibits excellent temperature stability over a wide temperature range. With slight modifications of the artwork to fit specifically designed sensor bodies, the sensors could be made to satisfy a wide range of applications and requirements Next-Generation Multifunction Electro-Optical Sensor System 22. The objective is to develop and demonstrate a highly stabilized infrared search and track (IRST) sensor and signal processing technology for air and seaborne platforms. Recent advances in large-area infrared focal plane arrays (IRFPAs) multidimensional signal processing, integrated passive/active optical apertures, and electromechanical stabilization technology (services/industry) coupled with the technology base realized from prior work form the system building blocks. On-going investigations into development of efficient plug-and-play system architectures to enable cost-effective scaling of sensor characteristics to platform and mission needs are continuing e.g. the active LASER element of the system could be integrated only in systems for platforms such as Aegis and E2C that require precision range tracking at extended ranges for fire control purposes. Such architectures are driven by the need to reduce system cost and complexity where possible and to mitigate the prohibitive cost and risk of integrating new and emerging technologies as future system improvements become necessary.

12 12 Summary 23. The introduction of sophisticated war faring systems, which include electrooptical tracking capabilities, has dictated the expansion of optical countermeasures equipments and techniques. The biggest problem associated with electro-optical operations is the inability to determine when a threat exists. The importance of IR & EO sensors and weapons continues to grow with improvements in sensor technology and the increased use of LASERs as target range finders, designators/illuminators and guidance beams. Countermeasures are required to offset these capabilities. Future CM systems will require significantly improved radiation detection and analysis performance and will make use of wavelength, coherence and the polarisation properties of the signals to a much greater extent than at present. There are advantages in using LASERs as an integral part of a defensive aids system. In some circumstances, these active components can be used to detect passive IR/EO systems and/or to analyse scanning and other properties of the threat via retro-reflection. If greater power becomes available, the LASERs can be used as jammers or destructive weapons, weapons that are now an integral part of EW.