ELECTRO-OPTIC (EO) ECM AND EPM

INTRODUCTION

1. Electro-optic electronic countermeasures (ECM) and electronic protection measures (EPM) are required to deal with threats in the Electro-optic Spectrum; this includes the Visual and IR wavebands and the threat from Laser based systems.

2. Within the Electro-optic Spectrum, the Near IR, Visual and Near UV wavelengths from 3m to below 0.4m are considered.

3. This lesson will be dealt with in two halves:

a. Part 1: The Visual and IR threat.
b. Part 2: The Laser Systems threat.

OBJECTIVE

4. The aim of this lesson is to describe the techniques and defensive measures that will prevent the enemy from exploiting the Electro-optic Spectrum.

THE VISUAL AND IR THREAT

5. The sources of IR and UV energy have been discussed in previous lessons. All matter emits IR energy; the rate of emission and frequency is subject to Plank's Law. The main source of UV energy is from the Sun; there are very few other natural sources. The range of detection of UV wavelengths is significantly less, in comparable conditions, than the range at which IR wavelengths will be received because of attenuation in the atmosphere.

6. The source of IR and UV energy that is considered as a threat is mainly associated with missiles. UV energy is generated in the launch and boost phase of flight; IR energy is generated by the missile propulsion system (shorter wavelengths) and from the missile body (natural and kinetic heating- longer wavelengths).

7. The main threat to surface vessels today is the Anti-Ship Missile. Missile flight profiles vary from high flying cruise phases with a steep terminal dive, to the sea skimming missile. The missiles can be active or passive, with a mixture of active and/or passive sensors. TV and IR sensors are being used to complement Active Radar in the terminal phase to help to discriminate against decoys. Any means of reducing the ship signature, whether visual, noise, IR or RCS based, can only serve to make the platform a smaller and therefore a more elusive target.

8. The main threat to aircraft is, once again, the missile. Today's high-tech generation of passive air-to-air missiles, IRIS-T, AIM-9X, AA11 and Python 5 almost claim immunity against the full range of countermeasures. Shoulder launched missiles, available in huge numbers around the world, have forced combat aircraft to operate at altitudes above 15,000 feet. Radar guided missiles are capable of engaging targets above 70,000 feet. Integrated defensive aids suites in aircraft consist of Radar Warning Receivers (RWR), Missile Launch Warners (MLW), Missile Approach Warners (MAWS) which utilise sensors to detect an active radar or UV/IR sources. These systems can react automatically, dispersing chaff or flares, if a threat is detected. Because of the 'no escape zone' and decoy discrimination capabilities of modern generation missiles, laser based defensive systems are in use in helicopters, transport and commercial aircraft and under development for fighter aircraft.

9. On the ground, one of the most recent threat innovations in the past decade has been the use of laser guided weapons, laser designators and range finders and anti-personnel laser weapons. Personal protection measures are available against the latter and laser warning systems are now fitted to armoured vehicles to indicate the targeting process.

10. Initial detection, with the advent of satellite and UAV surveillance platforms, is of concern; the use of IR Imaging systems, high resolution optics and synthetic aperture radar (SAR) together with real time, high data and communication exchange systems makes concealment more difficult.

COUNTERMEASURES

11. The measures taken to prevent a platform from being detected begin in the conceptual stage of design. Modelling computers for both ships and aircraft ensure that the radar cross section, IR and EM signatures are optimised before build begins.

12. Contrast reduction measures, in the form of IR Paint or Camouflage, can be taken to ensure that the platform blends in with the background as deception measure. IR Paint is used to absorb the tell-tail wavelengths and re-emit them at a different frequency which is more easily attenuated. Camouflage is conventionally considered to defeat visual detection however, Low Light/Visibility and IR systems can also be camouflaged against threats

13. The colour and pattern of camouflage is important to defeat Low Light and IR systems. In the 1991 Gulf War, American forces using desert camouflage, coloured not to contrast with the background and of a material that emitted the natural IR wavelengths, was effective concealment. The Iraqi forces used camouflage that emitted a distinctive IR wavelength which contrasted to the background and made their positions more easily detectable.

14. The use of Decoy Flares to seduce the missile seeker is considered in depth Radiation Shielding is generally used to suppress an IR signature or to reduce a ship's RCS. In addition to IR Paint, IR Screens are used to simply block EO wavelengths. The IR signature from a ship's exhaust plume can be reduced by mixing cold air with the gas turbine engine exhaust before it is vented to the atmosphere through the funnel. Different engine options have been considered; diesel engine exhausts can be discharged underwater (diesels are however noisier and do not have a surge speed capability); electric motors and hybrid systems are under evaluation. The speed at which an engine is run also generates a larger IR signature as hot spots in the hull (engine, gearbox and exhaust locations) and in the exhaust plume.

15. Similar concerns apply to aircraft. The type of engine, use of afterburners (Plank's Law) and whether operating at supersonic or sub-sonic speeds also affect the IR signature in both the amount of energy that is emitted and the frequency at which it is emitted.

16. In another lesson however, the importance of the timely deployment and rate of expenditure of the stock of flares has tactical considerations.

17. Tactically, avoiding detection may be the priority that determines the routing of a mission and the manoeuvring around known threat locations. However it must be remembered that shoulder launched IR SAMs may be anywhere in the area. Low level flying and the use of terrain masking will also make detection more difficult, or if you have air superiority you may wish to fly at medium level above IR missiles MEZ.

18. First generation IR missiles were only successful when fired directly from behind the target; they were stern aspect only missiles. More modern missiles will detect an IR signature from a number of sources on the airframe, not just the exhaust plume.

19. IR energy is attenuated by water vapour and carbon dioxide. Warships can take advantage of these characteristics by hiding within a self-generated moisture or smoke screen. Most warships have a pre-wet system, originally installed to wash away nuclear, biological and chemical agents, that would be effective in reducing the ship IR signature.

20. In addition to expendable decoys such as flares, IR Jammers are available. These operate on the same principle as a radar noise jammer and are mainly fitted to armoured vehicles, transport aircraft and helicopters.

THE LASER THREAT

21. Lasers are being used more frequently on the battlefield as laser range finders and target designators. There is a significant and growing use of lasers as intentional or unintentional dazzle weapons against personnel. Systems are available for use on the battlefield and against aircrew piloting aircraft.

22. The range of injuries that can be sustained range from Glare, a flash blindness where permanent damage is done to the eye, to Retinal damage which can be permanent or Corneal damage which is usually temporary.

23. Eyes and skin are very sensitive to the shorter wavelengths of the Near IR and Visible waveband. The eye need protection in the bandwidth from 0.4m to 0.75m (here the light is visible) and 0.75m to 1.4m (source in near IR band is invisible to the eye). An Ng:YAG Laser operating at 1.06m is dangerous. Pulsed lasers are also more dangerous than CW lasers because of the power that can be discharged. Lasers are classed by using a measure known as the Normal Ocular Hazard Distance (NOHD). This is defined as the minimum distance at which that strength of laser will do no damage. Every individual has a natural aversion instinct; this gives a person a degree of protection against the weakest classes of laser, by quickly turning away, but will not protect against stronger classes of laser.

24. Protection can be achieved by fitting protective eyewear such as laser glasses that block the light by attenuation. Indirect viewing methods should also be employed. The STINGRAY battlefield laser locates an enemy optical system using a low power system and, when the location process is complete, fires a high energy laser into the enemy optical system.


25. Laser goggle protection in the Visible bandwidth requires a discolouring filter that makes target identification difficult. Protection technology ranges from Passive - absorbs or reflects the laser light, or Active - self actuating filter or external blanking. The ideal solution would be a protective system which would stop harmful energy from reaching the eye, block energy over a narrow bandwidth, work by day or night and be cheap. Current shortcomings are that protective goggles cannot protect against frequency agile laser weapons and cannot offer unrestricted viewing.

ELECTRO-OPTIC (EO) ECM AND EPM

26. Helmet visors are being developed that overcome some of these limitations by using 'Rugate Filter' technology. This filters out all light but the primary eye colours; transmission of light is reduced but proper perception of all colours allows for the mission to be accomplished.

27. Laser light can also be reflected by applying multi-layer thin dielectric film on a curved visor; transmission can be reduced by as much as 99%.


28. Blanking technology completely blanks out all radiation energy, once a warning is provided by the LWR.

29. The use of Smoke makes a target more difficult to acquire and, depending upon the wavelength of the laser, can diffuse and absorb the laser energy. Takes time to establish and effectiveness is dependent on the prevailing wind strength.

SUMMARY

30. Protection is based on preventing light reaching the eye; the more that is cut out, the less can be seen. It is not possible to cover all wavelength without increasing the limitations. Systems must be compatible with HUD, NVG and NBC. NVGs protect against Lasers but would be damaged themselves.

ESM IN ELECTRO-OPTICS

ESM IN ELECTRO-OPTICS

INTRODUCTION

1. Laser warning Receivers have been developed to provide warning against the illumination of a platform by a potentially hostile Laser Designator, Illuminator or Dazzle weapon. Presently LWR’s are predominantly found on Tanks and other Armoured Fighting Vehicles. In future years this balance will change when in response to the increasing threat from Lasers weapons, Helicopters and fixed Wing aircraft will demand their inclusion in integrated Defensive Aids Suites (DAS).
OBJECTIVES
2. The objective for this section is to explain the design, operation and limitation of LWR’s. The following areas will be covered:
a. The types of Laser to be detected.
b. LWR’s.
c. Laser propagation.
d. System design.

TYPES OF LASER TO BE DETECTED

1. LWR’s are designed to detect pulsed Lasers. CW lasers are not considered as worth detecting, as they are usually associated with very short range weapon fuses where there is insufficient time to make any difference. CW Lasers are difficult to manufacture for long range use. Therefore LWR’s look to detect coherent, pulsed, fast rise-time radiation. Rather like a RWR the following parameters can be determined:

a. Wavelength.
b. Pulse width.
c. Pulse repetition Frequency.
d. Bearing.
e. Location. (If possible)

LASER PROPAGATION

2. Lasers produce very narrow beam widths. Although there are some sidelobes, these are generally much smaller than for conventional radar beams and are thus very rarely detected. However, the Laser beam is spread out to varying degrees by atmospheric scattering and turbulence. The resulting beam is therefore no longer a narrow and can be detected over a much wider area.


3. As well as being able to detect Lasers that are directly illuminating the target the LWR has to be able to identify and alert against signals from beam scatter which may not necessarily be aimed against the platform mounting the LWR. However, beam divergence is variable and in many cases the beam from a target designator will still not cover the whole of the target. Although direct illumination of a LWR is the ideal case, they must be able to detect the low power levels from scattered signals. LWR’s perform better in poor weather due to increased scattering.

4. Another factor that causes scattering of the Laser beam is from refection of the beam as it hits other parts of the target. This is known as Target Slash and is a function of type of surface and angle of incidence of the Laser. Dirt, Smoke and Dust in the environment will also cause further scattering. It is also possible to be illuminated by a signal reflected of a nearby friendly unit that is being illuminated. There may therefore be many false alarms.

5. What makes a laser signal stand out from other sources of light is the property of coherence where the beam is in-phase and consists of a single frequency waveform. The spread of the wavefront is usually about 1 meter with the only variation from this belonging to a few carbon dioxide doppler Lasers.

6. Background noise that makes the low power laser signal difficult to detect can originate from other low power sources such as steady state Solar reflectance or thermal self emission. Other signals causing interference or false alarms can come from the following:

a. Sun glint off water.
b. Gun flashes.
c. Fires.
d. Lightning.
e. Electro-magnetic interference.
f. Cosmic rays.
g. Shot noise and thermal heating from the Sun.


LASER WARNING SYSTEM DESIGN

7. LWR’s are usually fairly simple focal plane arrays with low angular resolution, enough only to alert the crew and enable the deployment of countermeasures such as smoke. Low frequency filters are used to remove background noise. As Lasers transmit pulses of nanosecond or microsecond duration LWR’s are optimised to detect multiple pulses of this duration. False alarms and background noise can be eliminated, as they tend to produce predominantly single pulses. Only Lightning produces similar signals.

8. Another way to eliminate background clutter is to analyse signal rise time. In the case of a Laser beam this has a very fast rise time while background noise generally has a slower rise time. Unfortunately some clutter has almost a fast rise time as a Laser signal and it takes very good processing to enable discrimination, although it is possible.


SUMMARY

9. Laser beams, by definition, are very narrow and difficult to detect. A warning receiver must be able to pick up the diffuse scattered energy which may have initially been operating at a low power setting.

10. Because of the effectiveness of laser guided weapons, there is a proliferation of systems on the battlefield and laser warning receivers are being fitted to an increasing number of platforms.

11. Countermeasures against laser targeting include the use of manoeuvre to break lock, the use of smoke, or a counter-strike.

THREATS IN THE EO SPECTRUM

INTRODUCTION

1. Electro-optics plays an important role in surveillance and target acquisition operations and delivery of ordnance on the modern battlefield covering land, sea and air scenarios.

2. Modern electro-optics technology has provided military capabilities that cannot be realised by other means, providing surgically accurate strikes without collateral damage. This was amply demonstrated in the 1991 Gulf War; and has enabled an important policy option for military planning such that electro-optics targeting was used successfully in NATO peace-keeping operations to attack hostile elements which had been placed close to civilian areas.

3. This greater precision in target acquisition is achieved because much higher frequencies and shorter wavelengths, in the optical band-width, are being used giving far superior target resolution, targeting and guidance.

4. Electro-optics can operate passively or actively. In the passive mode it has a low signature, thus reducing risks of detection and deployment of countermeasures. Unlike passive radar, optical devices can exploit the natural illumination of targets by sun, moon and starlight and can also detect the thermal emissions from the target itself. In its active mode, i.e. such as laser, the very narrow beam divergence offers covert operation and resistance to electrical jamming whilst maintaining high target designation capability.

5. The size of electro-optics systems are much smaller than that of radar enabling packaging into smaller platforms e.g. strike aircraft.


OBJECTIVE

6. The objective for this lesson is to introduce the EO Spectrum, associated Military applications and EO Electronic Surveillance Measures (ESM).


GENERAL CHARACTERISTICS OF ELECTRO-OPTICS

7. EW is not simply the application of Radar and Radio; the whole of the Electromagnetic Spectrum must be considered.


8. The EO spectrum is a part of the EM spectrum and obeys exactly the same laws. Therefore EO radiation is subject to the same properties of reflection, refraction, diffraction and polarisation as possessed by radar waves. The velocity of propagation of EO energy is the same as the rest of the EM spectrum at 3 x 108 m/sec. It is the frequency and wavelength that are different and produce the characteristic properties of the EO spectrum that allows for example visual wavelengths to be detected by the human eye. The eye in effect has the same function as the radar aerial and receiver except it is processing information from a different part of the EM spectrum.
9. Although EO energy consist of EM waves in the same manner as radar or radio waves is also sometimes considered to consist of ‘packets’ of energy known as photons. This has no effect upon the way we consider the properties of the EO radiation. While in general we tend to describe radar by frequency, the EO spectrum is normally referred to in terms of it wavelength. For example you could describe EO radiation from the IR part of the EO spectrum as having a wavelength of 2 microns. This would be written as 2.0 m. Described in a similar way the coverage of the visible part of the EO spectrum stretches from 0.75 to 0.4 microns or 0.75 m to 0.4 m.
ATMOSPHERIC TRANSMISSION
10. If the atmosphere were uniform in its structure then it would be very easy to explain and predict the propagation of energy at the various EO wavelengths. Unfortunately this is not the case and the transmission of energy is affected by the atomic structure of the various gasses and their varying distribution that constitutes the atmosphere. This produces differing transmission for the different wavelengths within the EO spectrum.
11. Additionally other particles such as dust, pollution and the presence of water vapour further complicate the situation. Meteorological conditions are particularly important in the lower atmosphere and conditions vary with weather, location and altitude.
12. The atmosphere is said to affect the propagation of EO energy through the process of absorption and scattering. Absorption is the most important form of attenuation and different gasses attenuate differing wavelengths. For example nitrogen and oxygen do not significantly attenuate IR waves while water and carbon dioxide molecules do.
13. Scattering causes radiation to be reflected, refracted and diffracted and depends upon the size of the atmospheric particles compared with the wavelength of the EO energy. For example haze and mist scatter visible light while fog and clouds scatter IR. Raindrops have less affect and the transmission of IR through fine rain is surprisingly good.
14. Selection of EO equipment is therefore very dependent upon the atmospheric conditions present in the forecast area of operation and the expected wavelengths of emissions that have to be detected. For example there is little value in expecting long range IR sensors to work to their full potential in areas of very high humidity. The same equipment may work more than satisfactory in areas of low humidity. During the Balkans campaign the targeting of Laser Guided Bombs was severely limited by poor weather over the target areas.
15. The severity of each of the mechanisms that attenuate radiation is dependant upon the wavelength of the radiation. In practice the selection of EO equipment is usually a balance or compromise determined by all of the above variable factors.

THE ENVIRONMENT

16. The effectiveness of a particular surveillance operation depends upon the characteristics of the scene illumination, the target and its background, the state of the weather and the sensitivity of the detector being employed.

LEVEL OF ILLUMINATION

17. Sunlight provides good illumination over a broad range of wavelengths in the visible and infra-red regions of the spectrum. The level of illumination is sufficiently high to activate the wavelength-sensitive cone cells in the retina of the eye giving a high degree of colour discrimination. This, together with the fact that the reflectivity of most objects changes with the wavelength of the incident light, enhances the ability of the eye to recognise objects in good light and these are important considerations in visual surveillance.

18. Because of the limitations of cost, military systems are usually monochromatic and in this respect are different to visual surveillance. Their main role is to extend the surveillance capability beyond that of the eye into low level illumination.

19. The characteristics of moonlight are similar to those of sunlight, which is to be expected, but at a much lower level of intensity. Only the wavelength-insensitive rod cells of the retina are activated at this level of illumination and colour vision is lost, removing the advantage of visual surveillance. It is at this and at lower levels of illumination that the greater sensitivity of electro-optics comes into play.

20. Sunlight is a good illuminator but it has to be considered that the energy detected by an EO device is detecting reflected sunlight energy. Any object, whose temperature is above absolute zero, will naturally emit energy. Higher wavelengths, above 3.5 microns, of emitted energy is usually greater than reflected energy. Thermal emitters that operate in the 3.5 to 25 microns are thermal emission dependant and do not require illumination.

THE USE OF ELECTRO-OPTICAL SYSTEMS

21. EO systems are in very widespread use in all branches of the armed forces. As a generalisation we can describe systems by their intended use, but in practice there is a crossover between these different areas. The divisions are as follows:
a. Surveillance, Detection, Warning, Identification
b. Tracking and Guidance.
c. Dazzle and Damage.

SURVEILLANCE
22. EO surveillance equipment is fitted to virtually all types of military platform ranging form the individual soldier to space based assets and at all levels in between. Likewise sensors can range from the human eye operating in the visual wavelength to sophisticated IR and Ultra-Violet (UV) equipment. The importance of EO surveillance was highlighted during the Gulf War when approximately 4 million images were taken by over 600 platforms.
23. As an example of surveillance in the visual wavelengths the following areas must be considered;
a) The eye.
b) Binoculars and telescopes.
c) Cameras.
d) Television.
e) Image Intensifiers.
f) Night Vision Goggles.
24. In IR wavelengths surveillance equipment has the main advantage of being able to operate at night or in low light conditions and consists of the following equipment:
a) IR cameras and Linescan.
b) Forward Looking InfraRed (FLIR).
c) Infra Red Search and Track (IRST) devices.
d) Missile Warning Systems (MWS).
25. Missile Warning Systems have been developed to detect the UV signature from incoming missiles. Additionally Laser warning Receivers (LWR) have been developed to detect the variety of differing wavelengths that could emanate from laser illuminators.

MISSILE WARNING SYSTEMS



GUIDANCE AND TRACKING
26. As well as IR guided missile such as Sidewinder or SA-7 many modern gun and close-in missile systems also employ some form of daylight or IR Television, particularly at low elevations, to back up or supplement traditional radar tracking methods. Many Anti-Ship missiles employ IR guidance or a combination of radar and IR guidance in the missile seeker heads.



Figure: Maverick IR Guided Missile.
27. EO is also the main medium used in IR target acquisition and subsequent laser semi-active homing of LGBs and missiles.
28. Lasers are increasingly being used with beam riding and semi-active missiles. In the beam riding method the operator places his laser beam on the target and the missile flies up the beam until it hits the target. In the semi-active method the missile homes in on the laser energy source reflecting back off the target. This is the same technique as used in a semi-active radar-guided missile but uses a laser at an EO wavelength rather than a radar illuminator at a radar frequency.
DAZZLE AND DAMAGE WEAPONS
29. Although a fairly secretive and highly classified area of technology, information is available from unclassified sources relating to the use and development of Laser based Energy Weapons. The technology exists or is in a state of development that enables the production of lasers of differing levels of power ranging from low power eye-safe devices through to high power systems designed to shoot down ballistic missiles.
30. Low power devices such as laser Rangefinders and illuminators can cause unintentional eye damage. Although prohibited by treaty there is some evidence to support evidence of the development of intentional dazzle weapons such as the Stingray and other systems.
31. Medium power laser devices are being developed for use in the field of Directional Infra Red Countermeasures where a laser will eventually replace low power and wider Beamwidth conventional IR jammers. An example of such a system the Nemesis shown below.
32. Perhaps the biggest development and research is in High Energy Laser systems as typified by the US Airborne Laser Programme (ABL) designed to destroy a variety of targets but primarily Ballistic missiles. The Laser system is carried in a converted Boeing 747. Laser weapons are banned by treaty from being deployed in space. An example of a land-based system is the US / Israeli Tactical High Energy Laser (THEL) developed to destroy battlefield rockets fired against Israel from the Lebanon. Development of the system is proving its capability against smaller targets than originally planned.
SUMMARY

33. An understanding of the threats and equipment that utilise the EO spectrum is essential in modern warfare. The whole of the EO spectrum is exploited and no one part can be considered in isolation. Performance is highly influenced by atmospheric conditions, which can vary locally, nationally and globally. The use or selection of a particular wavelength is normally a compromise between the EO signature of the target and the forecast atmospheric conditions.
34. In general terms EO devices are used for surveillance, guidance, and damage. There exists a wide variety of systems optimised for different tasks. The main advantage of EO is that it allows successful operation at night or very low light conditions. However most simple EO systems cannot give range.