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.

LASER DAMAGE WEAPONS

INTRODUCTION
1. Although the vast number of low powered Laser illuminators, Rangefinders, LADAR and other devices are not specifically designed to be weapons, if observed by the unprotected human eye, they can produce temporary or even permanent blindness. Although banned by international protocol, purpose built Laser Dazzle weapons have been developed and deployed in the field. Additionally, higher-powered Lasers of varying power and complexity have been developed to cause actual physical damage. It is very important to be aware of the threat posed by such weapons and their future potential.

OBJECTIVE
1. The objective for this section is to explain the operation of high and low-power military Laser systems. The following subjects will be covered:
a. Classes of Laser.
b. Dazzle Weapons.
c. High Energy Systems
(1) Land Based Systems.
(2) Airborne systems.
d. Adaptive Optics.
CLASSES OF LASER
2. Lasers can be grouped into certain classes based upon their power and potential for physical damage to the human eye or skin. A term commonly referred to is ‘Aversion’ which is man’s instinctive reaction to “blink “ when illuminated by a high power light or Laser. The Classes are shown in the table below:
Class Power Rating Hazard
1 µ watts Not Hazardous within Aversion response time
2 m watts Hazardous if viewer overcomes Adversion
3 M watts - Watts Causes injury faster than Adversion response
4 Watts Cause skin damage
4. For all Laser systems, including those not designed as Dazzle weapons, there exists what is known as the Normalised Ocular Hazard Distance (NOHD). This is defined as the range where the signal strength from a Laser has reduced to a level considered to be eye safe.
5. The NOHD depends upon the Class of Laser, its wavelength and pulse rate. Typically for a Nd YAG Laser the NOHD is 1 km. During peace time operations there are strict controls on the use of military Laser systems.
DAZZLE WEAPONS
6. It has been a natural and fairly easy step for existing Lasers systems to be developed into Dazzle weapons. A Laser Rifle developed in the US came from a medical Laser. Information about such weapons is very highly classified and difficult to obtain. Damage to the eye can be either temporary or permanent depending upon the class of Laser and type of exposure the eye has received. Brief descriptions of several Dazzle systems follow:
7. The US Army Laser Rifle was developed during the 1980’s. In 1993 it is believed that 1100 Rifles were tested and can cause either temporary or permanent damage. The Beamwidth is assessed as being 0.5 metres at 1 km. However little is known about ranges or the NOHD. The figure below shows a diagram of the Rifle.

8. A Laser weapon known as Stingray was fitted to 2 Bradley Armoured Personnel Carriers and deployed to Saudi Arabia during the Gulf War. The system designator is AN/VLQ-7. Designed to scan the battlefield with an eye safe Laser, the Stingray detects reflections from optical devices such as periscopes and Binoculars. It then illuminates these with a narrow-beam high-power Laser that blinds anyone looking through the targeted optic. The system can operate in automatic, Semi-automatic or manual modes.

9. An example of a naval system can be found in the Royal Navy Dazzler found on some UK warships. The only details available are photographs.

HIGH ENERGY LASER SYSTEMS
10. The Laser is a very attractive alternative weapon system offering several advantages over conventional weapons. These are:
a. Almost zero time of flight.

b. The beam travels in a straight line.


11. However there are also limitations:

a. Very high-energy requirements.

b. Complex technology.

c. Expensive

12. Due to the classification of many of the HEL projects it is again difficult to get reliable information about each of the various systems. However their development can be traced through unclassified sources. The first hint of any such systems can be found in the late 1060’s and 1970’s, but the first major publicity of their existence came about with the US Strategic Defence Initiative SDI of the 1980’s. Recent years have produced much more open discussion with the emphasis being placed on Lasers being used purely as a defensive weapon to shoot down SCUD missiles. It is still a very sensitive area as recently witnessed with the widespread criticism of USA proposal to use Lasers as protection against ICBM’s.
13. Initial attempts at producing HEL weapons resulted in large devices with low power and poor beam quality. The beams were also degraded by atmospheric distortion, a phenomenon not overcome for many years with the introduction of adaptive optical systems. (Covered later in these notes). Early HEL employed gas lasers while more recent systems use chemical lasers to generate more power. Two examples of this are the Mid IR Advanced Chemical Laser (MIRACL) and the Chemical Oxygen Iodine Laser (COIL), both of which are used in present day systems. We shall now look at specific examples of Land and Airborne systems.
LAND BASED HEL
14. Developed initially as a naval initiative the US has developed the SeaLite 400 kW Laser that used IR and Visible sensors to track targets. It reportedly shot down a TOW ATGM in flight. In 1980 it was used to destroy a tethered and stationary UH-1 helicopter. Eventually funding was lost for this system but it was later resurrected as the Multi Purpose Chemical Laser (MPCL) producing the 1986 10 MW LATEX Laser.

15. In 1989 the SeaLite aiming and tracking system was used in conjunction with the MIRACL to produce the highest power, 2.2 MW Deuterium Fluoride, operational Laser. This is base at the High Energy Laser Systems Test Facility. (HELSTF) based at White Sands in the USA. This system has reportedly engaged a Vandal supersonic missile and shot down 5 Firebee drones.

16. In 1996 the US Army Nautilus project used the MIRACL at low power to shoot down a short-range rocket in flight. In 1997 it was reportedly fired at a satellite. The success of this project led to the forming of a joint US / Israeli project know as the Tactical High Energy Laser (THEL). This system, which is mobile, is designed to engage tactical battlefield weapons with a Deuterium Fluoride Laser. A fixed site demonstrator has been built that has demonstrated a capability to shoot down several ‘small’ Targets. However it has proven to be delicate, unreliable and requiring too much maintenance. An updated system, now called the Mobile THEL (MTHEL), is currently under development. The range of potential targets has been expanded and it is planned that each system, presently consisting of 3 units, will be reduced to one vehicle easily transportable by C-130. This reduction in size may be achieved by the use of a new solid-state Laser.

17. Additional projects in development by the US include an Army 10 kW solid-state Laser which they hope to develop to 100 kW operational system by 2006. The USA is also developing the airborne Advanced Tactical Laser. (ATL)
AIRBORNE HEL
18. There is much activity in the development of airborne Laser applications both as high power damage systems and as lower power non-lethal systems for use in IRCM. The US Army is developing the non-lethal HELSTAR system using a COIL similar in design to the highly publicised Airborne Laser Project. (ABL). Designed for helicopters, the HELSTAR is 50 – 70 kW in power and has a range of only a few kilometres.
19. The USA first trialed airborne Laser technology in the 1980’s with the creation of the Airborne Laser Laboratory (ALL). This converted KC-135 was fitted with a 10.6 micron system that successfully shot down 5 sidewinder missiles. It did however highlight the problem of how the atmosphere makes the Laser wavefront go out -of- phase with a resultant drop in power. This effect has now been overcome by the use of adaptive optics, basically a combination of computer controlled deformable mirrors, which transmit a shaped wavefront that becomes in-phase after transmission through the atmosphere. This technique was first developed to remove star scintillation caused by the variable refractive index of the atmosphere.

20. The ABL project uses a converted Boeing-747 equipped with a COIL plus 3 other low-power Lasers for tracking of the target. The system is designed to engage ICBM’s as they break cloud cover above their launch site. The coil can engage at ranges up to 450 nm and provides for 45 seconds to destroy the target, fairly slow in this phase of its flight, which has not had time to deploy decoys or multiple re-entry vehicle warheads. If the engagement is successful, there is an added bonus of the debris falling back upon the launch area. Space based Lasers are prohibited by international treaty which this system does not require as it is fired from within the atmosphere. The project should be completed by 2003 and operational by 2007.


SUMMARY

21. As it should be evident from these notes, there is evidence of many developments in the use of Laser weapons in a wide variety of applications. The technology now exists to create a Laser of low or high-power specifically designed to match a particular requirement or function. Lasers are capable of being used as low-power dazzle weapons, medium-power for IRCM applications or high power anti-missile systems.
22. Breakthroughs in solid state laser technology are resulting in successful projects such as the mobile THEL; directed energy weapons could be installed in aircraft by 2010 and be the future of precision strikes and defence against missiles.
23. The ABL is conducting evaluation trials (Nov 2002); AAR procedures are being evaluated and the IR targeting sensors monitored a US ballistic missile launch from a range of 300nm.

LASER THEORY

LASER THEORY

INTRODUCTION

1. The use of Lasers both commercially and by the military is very wide-ranging. Commercial applications find Lasers in industrial and medical cutting equipment, CD players and Laser pointers to name only a few. In military use, Lasers are used for:
a. Range finding.
b. Target designation / illumination.
c. Missile guidance.
d. Laser radar.
e. Directed Laser Energy Weapons.
2. As the enemy is highly likely to employ an array of Laser systems against you, there is a need to understand and be able to exploit and counter such systems. Indeed one of the fastest growing areas of industry is in Laser Warning Receivers as many countries try to give their platforms some protection against such systems. The word Laser comes from Light Amplification by the Stimulated Emission of Radiation. A Laser is a device that generates and amplifies coherent light radiation in the Ultra Violet, Visible and InfraRed parts of the EO spectrum. Lasers operate at selected discrete wavelengths dependant on the selection of material used as the Lasing medium.
OBJECTIVE
3. The objective for this section is to explain the basics of Laser theory, explain how a simple system operates and different types of laser have different properties. The following areas will be covered:
a. Basic theory.
(1) Electron States.
(2) Excitation.
(3) Coherency.
(4) Divergence.
b. The requirements of a simple Laser system
c. Types of Laser system.
d. Laser propagation
BASIC LASER THEORY

4. To understand how a laser operates you need to have an understanding of molecular theory. Energy is stored in electrons that exist in differing bands around the nucleus of an atom, as shown in figure below.



Movement of electrons between the bands or levels can occur, and when an electron drops from a higher level (further away from the nucleus) to a lower level, a photon of light is given off. It is the harnessing of these photons that forms the basis of all Laser systems.

5. In its normal state the electrons of a particular substance are situated in specific bands around its nucleus. In order for the electrons to move to higher levels they must either collide with another moving particle or absorb a photon of radiated energy. This process is known as Excitation and the device that performs this in a Lasing system is called a ‘Pump’. Figure shows an electron decaying and emitting a photon of light energy.



The wavelengths of the energy given off in this manner cover from IR through Visual to the UV wavelengths. The wavelength is directly proportional to the difference between energy levels. This process does not have to be man-made and occurs naturally in some materials where electrons decay back to their ground state emitting the light in a process known as spontaneous decay. An example of this can be found in naturally fluorescent materials.

6. Another property of these materials is that, if you stimulate these materials with a stream of photons and raise the electron states to higher levels, as the electrons at a higher state decay they emit another photons with exactly the same properties as the incident photon. These then combine together and increase the strength of the light. The figure below illustrates this process for a single electron. Note how the resultant photon has the same wavelength and is in phase with the original.




7. To create a large enough flow of energy a population inversion has to be created where electrons in a higher state exceed the thermal Equilibrium State. Stimulated emissions then exceed spontaneous emissions. In a practical laser more than two energy levels are used with three or four being common. The figure below





8. shows electron energy levels E1 and E2 for a normal disposition and also an example of a population inversion.




9. The process of flooding the Lasing material with photons to create the population inversion is known as ‘pumping’. There are three methods of pumping Optical, Electrical or Chemical. Optical pumping uses a tube containing a quartz iodine compound and a Nd/YAG crystal is excited by a light from a high energy lamp or low power Laser. Electrical pumping applies a charge between two electrodes in a gaseous or semiconductor material, while in Chemical pumping a chemical reaction between two materials raises electrons to a higher state. The reaction between Hydrogen and Fluorine produces excited 2HF molecules while Carbon Monoxide and Oxygen produce excited CO2 molecules.

REQUIREMENTS FOR A SIMPLE LASER SYSTEM
10. A Laser system requires the following components:

a. A medium with a suitable energy level system.

b. A source of energy to produce the required population inversion.

c. An optical resonator to amplify the signal and produce the output beam.

11. Making the emitted photons travel parallel to an optical axis within a device called a Cavity Resonator can create a Laser beam. The parallel photons are then made to travel back and forth as they are reflected by two mirrors, one of which is only 50% reflecting, at either end of an optical cavity. The beam continues to build in strength until it is of sufficient to pass through the 50% mirror and thus is emitted as the Laser beam as shown in figure below.


1. Alternately, instead of mirrors, some systems use the polished ends of a crystal (Ruby Laser), the two faces of a semiconductor or rotating rather than fixed mirrors. The use of a rotating mirror to replace the 50% transmission mirror allows for the laser signal to be pulsed.

LASER PROPAGATION

12. One of the main features of Lasers is that they produce a narrow parallel beam with very little divergence or spreading out of the shape. This is known as a collimated beam. If this beam travels thorough Space or a vacuum then there is very little if any divergence and absorption of the beam.

13. However this is not the case through the Earth’s atmosphere where the beam will be absorbed and scattered. The main losses are caused by:

a. Absorption and Scattering.

b. Beam Jitter caused by inconsistent output and vibration from servo-controlled tracking systems.

c. Atmospheric Turbulence produces variable refractive indexes that cause beam steering problems.

d. Beam Divergence (small).

TYPES OF LASER
14. There are five main types of Laser:

a. Solid state.

b. Gas.

c. Semiconductor or diode.

d. Liquid.

e. Chemical.

15. Solid state lasers use a solid rod made up of a crystal or a special glass that contains atoms of the lasing medium. Characteristics of the laser depend on the active material used as well as the substrate or host material. Materials include chromium, erbium, titanium and neodymium.

16. Nd-YAG has widespread applications including range finders and target designators. They can be battery powered, small and light. They are rugged and resistant to temperature variations.

17. Gas Lasers use a pure gas or a mixture of gasses as the lasing medium. to produce the different wavelengths as shown in the table below. CO2 used for research in HEL systems. Helium-neon lasers are small, cheap, simple and can be used for long periods. They are used by the military for ring gyros. They can use the IR, UV and visible bands as shown below.

GAS COLOUR WAVELENTH
Helium / Neon Visible Red 0.628 μm
Argon Visible Green 0.55 μm
Carbon Dioxide IR 10.6 μm

19. Liquid Lasers use an organic dye solvents as the lasing medium. These low powered lasers are widely used in medical research. They can be tuned to produce Lasers with wavelengths from 0.34 to 1.17 µm.

20. Semi-conductor/Diode lasers. An example of a Semiconductor Laser is one made from Gallium Arsenide. The beam produced is rectangular and diverges and expands rapidly making it ideal for use with fibre optic communications. Also used in CD players and DVDs.

21. Chemical lasers. Energy required provided by a chemical reaction between 2 elements. Suitable for use in HELs. Elements commonly used Hydrogen fluoride ( 2.5 – 3.0 ), Deuterium fluoride ( 3.5 –4.0 ) and Oxygen iodide ( 1.315 )

22. When selecting a Laser for a particular task, there are many factors that have to be considered such as platform size, cost and complexity. However the main constraint is that as the power rises, the efficiency of the system decreases. The table below shows various different types of Laser and their power and efficiency. The ultimate challenge in Laser technology is to produce high-power high-efficiency systems.


Type of Laser Power Efficiency
Gas Watts to Kilowatts 20%
Liquid Megawatts 25%
Solid Ruby Gigawatts 1%
Solid Nd/YAG Kilowatts 2%
Solid Semiconductor Low 2%


SUMMARY

23. In order to produce a Laser you output you require to have a suitable Lasing material that can produce a satisfactory population inversion of electrons. There must be more atoms, ions or molecules in the excited state than there are in the lower ground stable state. This permits the emission of large numbers of light photons that are combined to produce the Laser beam.
24. Some form of a ‘Pump’ is normally required to achieve this population inversion and amplification of the emissions requires a cavity or optical resonator.
25. Most Lasers are inefficient and require large amounts of power. Depending upon the requirement of producing equipment for a particular role or mission, the system designer can select from Gas, Liquid or Solid- state Lasers. Generally Gas and Liquid Lasers are more efficient than Solid-state Lasers but tend to be larger and more expensive. Solid-state lasers are inefficient but tend to be cheaper and smaller, making them very suitable for low power devices.
26. The main property of a Laser is that being Collimated and Coherent, high levels of accuracy can be achieved. However the beams travel will be altered by bending and propagation losses through the Earth’s atmosphere.

laser in military application

LASER IN MILITARY APPLICATIONS

INTRODUCTION

1. In recent years the Laser has seen widespread use especially in the Ranging and Designation for Laser Guided Bombs and Missiles. The features of a Laser including its very narrow beamwidth, highly monochromatic and source brightness make it a very powerful tool for such operations where it has advantages over and complements conventional Radar.
AIM
2. The aim of this section is to explain the design and operation of military Laser systems. The following subjects will be covered:
a. Laser Rangefinders.
b. Target Designators.
c. Target Illuminators.
d. Laser Guidance / Tracking Systems.
e. Laser Communication Systems.
f. Laser Radar LADAR.
LASER RANGEFINDERS

3. A Laser rangefinder transmits an intense highly collimated beam of short pulses. The time taken for a single pulse to travel to the target and back is recorded. Because we know the sped of light, a simple calculation will provide us with the range to the Target. If c = speed of light and T = time taken back and forth then:

Range = cT / 2

4. Early Rangefinders were made from Ruby Lasers which were then replaced by Nd/YAG systems which had the advantage of being invisible, therefore could not be seen by the enemy, and less dangerous to the human eye. More recent Rangefinders use Carbon Dioxide which has better penetration in adverse weather conditions and has four times less eye hazard than Nd/YAG. This means that Rangefinding can safely be conducted in training exercises without the fear of causing unintentional eye damage. Most Rangefinders have operational ranges of 5 to 10 Km although some have ranges greater than this. The figure below shows the basic operation of a Laser Rangefinder in an airborne scenario although the same principal applies to any platform such as the man-portable system shown in figure.

TARGET DESIGNATION

5. The principle of Designation is that the target is illuminated by Laser beam and a detector in the host platform, or weapon system, homes in onto reflected light from the target. The Lasers very narrow beam width ensures very accurate and selective marking of the target at ranges of up to 10 Km. Unless the enemy has lots of Laser Warning Receivers (LWR’s) it will not know who is being targeted. In most modern systems once the returning laser signals have been acquired, automatic tracking of the signal takes place. All the operator has to do is to keep the target illuminated and the weapon should home to the correct target. Accurate targeting information is then supplied to the aircraft or weapon systems, navigation and weapon aiming systems. Examples of Targeting systems include Litening and TIALD targeting pods which both include a Laser Designator as well as various FLIR and visual TV cameras.

The basic principal of operation of a Laser Designator is shown in the figure below.


TARGET ILLUMINATION


6. Target illumination can be used to improve the performance of Image Intensifiers. Here the Designator is used in the same manner as a torch producing reflections off the target that are large enough to be picked up by the Image Intensifier.

LADAR (LASER RADAR)

7. A Laser Radar generates very narrow beam widths which greatly improves covertness and resistance to jamming.

8. Optical Radar (LIDAR) employs visible wavelengths.
Laser Radar (LADAR) generally refers to systems employing other wavelengths.

9. The characteristics of LADAR greatly improve target profiling, definition, signature and tracking accuracy.

10. When using wavelengths in the UV band the high frequencies induce target fluorescence which improves signature analysis.

11. LADAR systems are used in;

a. Atmospheric Ozone detection.

b. Submarine detection.

c. Rangefinding.

d. As a Missile Seeker head.

12. Low Cost Autonomous Attack System (LOCAAS) – utilises LADAR technology.The purpose of this development was to illustrate that an autonomous low cost attack munition could be integrated into an air vehicle powered by a miniature turbojet engine.This would be integrated on the F-22, F-22X and JSF aircraft or a UAV as an autonomous system.



13. The vehicle would consist of a Multi-Mode Warhead (expanding rod, fragmentation) coupled to a solid state radar (LADAR) seeker with Autonomous Target Recognition (ATR) and INS/GPS midcourse guidance in a manoeuvrable airframe.

14. The LADAR allows target aim point and warhead selection to be determined automatically.

15. Endurance 30 min; Range >100km; Cost $33K/unit.