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.
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