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APPLIED PHYSICS – OPTICS – LASERS THE USE OF INFRARED RADIATION FOR THERMAL SIGNATURES DETERMINATION OF GROUND TARGETS* C. PLESA, D. ŢURCANU, V. BODOC Military Equipment and Technologies Research Agency Received December 21, 2004 The choice of optimal spectral band for thermal signature determination is generally determined by a consideration of threat signature characteristics and anticipated clutter conditions as well as scenario aspects. In this mode, the studies show us that when we will desire ground target detection, the 3- to 5 µm band provides better signal-to-noise ratios than the 8- to 12µm band. This study presents some analysis of thermal signatures of ground targets obtained in 3- to 5 µm and 8- to 12 µm bands. There are also shown the advantages and the disadvantages of each band of the two above mentioned. Key words: Infrared, signature, thermal, radiance, target. 1. INTRODUCTION Infrared source can be characterized as either area sources or point sources. Conventional infrared sources generally behave as area sources. If you consider a source of infrared radiation, irradiance E(θ,φ) directly incident on a sensor receiver will be given by E ( θ, ϕ) = I ( θ, ϕ) τ R2 where I(θ,φ) is the source radiant intensity in watts per steradian, τ represents the transmission of the intervening atmosphere, and (θ,φ) are the angular coordinates of the receiver. Ground targets generate characteristic emissions in the optical bands that are inadvertent to their propulsion and vital to the detection process. The most prominent of these are associated with the combustion of fuel during boost and sustain phases [1]. Discrete frequency emissions from rotational and vibrational transitions of water vapor and carbon dioxide molecules account for much of the exhaust emission. * Paper presented at the 5th International Balkan Workshop on Applied Physics, 5–7 July 2004, Constanţa, Romania. Rom. Journ. Phys., Vol. 51, Nos. 1–2, P. 63–72, Bucharest, 2006 64 C. Plesa, D. Turcanu, V. Bodoc 2 The intensity of plume emissions varies with many factors such as angle of the target relative to the detector, altitude and velocity of the target, and so on. 2. GROUND VEHICLE AND EQUIPMENT SIGNATURES The term ground vehicles and equipment encompasses the mobile tactical equipment employed by military forces engaged in ground combat. It includes trucks, tanks, self-propelled field and air defense artillery, command and communications equipment, and portable electric power generators. Sensors that pose threats to this equipment fall into the category of visual, image intensifier equipped, television, infrared linescan mappers, and, more recently, FLIRs, imaging seekers, and terminally guided submunitions (including sensor fuzed weapons). Visual, image intensified, and television sensors are dependent on ambient illumination for signature generation. They depend both on a reflectance difference between the target and the background to create contrast and on the availability of sufficient reflected ambient illumination to create an adequate signal level. Given adequate illumination, visible and near-infrared signatures ultimately depend on the spectral reflectivity differences between the target and the background in the sensor response band. Visual sensor can use photopic color differences as a discriminant. Image intensifiers extend the visual spectrum out to approximately 0.9µm or into the near infrared. So do television sensors than can use silicon detectors with response out to approximately 1.1µm. These nearinfrared sensors can exploit the high reflectivity of live foliage and the low reflectivity of conventional paints to see a large negative contrast difference between the vehicle and its background. FLIRs and imaging seekers nominally work in the 3- to 5- or 8- to 12-µm bands. In the 3-5µm band, the sun is still a significant contributor of reflected radiation. These sensors are capable of seeing internal target detail with temperature differences less than a degree Celsius. However, they detect target al long ranges by seeing their hot-spot emissions. Self-emissions offer the possibility for long-range detection and standoff attack, day or night, by human-assisted infrared imaging equipped systems or by autonomous munitions. This fact give impetus to understanding ground vehicle infrared signature generation mechanisms. Each of the equipment items, if not protected, emits a set of signatures because of its design and configuration. Although this set of signatures is unique to the equipment type, each of signatures can be described generically to assist in devising protective techniques. A first component of the thermal signature is that caused by internally generated. Engine exhaust gases are led through a muffler system to open air. In all cases there is a resulting exhaust gas “plume” whose size and temperature varies with the size of the engine. In most cases the muffler system is exposed to the air 3 Infrared radiation for thermal signatures 65 and is in itself a detectable signature. Radiators by their nature are exposed to the air and thus also present detectable signatures, although not of the magnitude of the engine or exhaust. Most tacked vehicle and many communications systems are equipped with small power units auxiliary to the main engine, to permit low-power operation of communications equipment. Such auxiliary power units do generate thermal signature, but they are of concern primarily at night, when all other elements of the system are quiet and cool. A second component of the thermal signature is that caused by exposure to the sun. The effects are solar heat loading and diurnal variations. The solar heating phenomenon begins with the fact that most mobile tactical equipment is first, made of metal, and second, is dark in color for camouflage reasons. As a result, when such equipment is exposed to the sun, it absorbs heat quickly and retains the heat throughout exposure. The speed and degree of heating are directly related to the construction of the specific equipment. A third component of the overall thermal signature of a military unit is influence of equipment on the adjacent ground and air. Ground tracks, exhaust emissions, and dust clouds are the major considerations. As the mobile equipment items transit their area of operations, wheels and tracks impinge upon the ground and disturb the ground surface. This action results in a heated ground track, which can be detected by thermal sensors after the passage of the equipment, in addition to its availability as a classic visual cue to military activity. When the transit is made under dry condition, it is also common that the movement action generates dust, which is thrown up into the exhaust cloud and floats with it. Depending on air temperature and wind conditions, this exhaust gas/dust cloud can linger in the area and present a thermal signature after passage of the equipment. Thermal signature almost always results from the difference, or contrast, between the target and its immediate background. Imaging sensors see internal target detail and external shape detail. Therefore, target signatures are defined by their pattern features. Those features are unique only to the extent that their proprieties differ from those in the background. Thus, resolved target signatures depend on background intensity mean values as well as on clutter intensity variations on a size scale comparable to internal target detail. Background spatial, spectral, and intensity characteristics are key to target signature generation and signature suppression. 3. PROPAGATION IR RADIATION THROUGH THE ATMOSPHERE The utility of a particular emission line or band for determination of IR targets signature depends on its transmission through the atmosphere, among other factors. 66 C. Plesa, D. Turcanu, V. Bodoc 4 Where path lengths are moderate and homogeneity of the atmosphere can be assumed, it is possible to use a Beer’s law estimate to approximate atmospheric effects. Degraded atmospheric conditions can change these extinction coefficients dramatically. Table 1 [1] shows some coefficients for the 8- to 12- µm band under less than ideal weather conditions. Table 1 Extinction Coefficient Weather Condition Haze Extinction Coefficient 0.105 Light fog 1.9 Moderate fog 3.5 Heavy fog 9.2 Light rain 0.36 Moderate rain 0.69 Heavy rain 1.39 Light snow Moderate snow 0.51 2.8 Heavy snow 9.2 Very clear and dry Clear 0.05 0.08 An empirical expression [1] for atmospheric attenuation as a function of wavelength and visible band visibility a figure normally available from meteorological reports is given by −3.91 λ − q τ A = exp R , V 0.55 where V is the visibility and R the range, both in kilometers, and λ is the wavelength in micrometers. The exponent q depends on the size distribution of scattering particles; typical values are 1.6 for high visibility, 1.3 for average conditions, and 0.585 V1/3 for low visibilities (<6 km). The choice of spectral band should not be made on atmospheric transmission alone. Other factors such as target size and contrast with background enter into the considerations. Figures 1 and 2 compare [2] the signal to noise ratio for two different bands for different situations. 5 Infrared radiation for thermal signatures 67 Fig. 1 – SNR for man sized target1. The first is based on a man –size target with no aerosol in the atmosphere and short ranges. Note that the 8- to 12-µm band is batter for short ranges, but a BLIP (background-limited performance) detector in the 3- to 5-µm band could outperform the 8- to 12-µm system at ranges beyond 5km. The second figure is for a small, high-temperature target at longer ranges. It is important to note that no plume emissions are considered here, only hot blackbody radiation from a tailpipe, for example, and that the higher clutter levels in the 8- to 12-µm band are not considered. The 3- to 5-µm band is better under these conditions. However, with current detector technology, the 8- to 12-µm band is still superior in a tropical environment for all but very hot targets. Al long ranges and with hot targets the 3to 5-µm band could potentially emerge as superior with detector technology improvements. The effect of atmosphere on target to background contrast is generally the primary concern, so a more careful definition of contrast is in order. Absolute contrast at zero range, defined as the difference between target and background radiance (or temperatures) as the target can be written C A = NT − N B , where the subscripts T and B refer to target and background radiances respectively. In the case of ground targets, the background radiance is understood to be that coming from the atmosphere behind the plane of target for the following discussions. The relative contrast is: 68 C. Plesa, D. Turcanu, V. Bodoc CR = 6 NT − N B N − NB ≈ T NB (1/ 2)( N T + N B ) Fig. 2 – SNR for small hot targets [1]. The effects of atmospheric attenuation and path emission on contrast depend on which definition of contrast is involved. For relatively flat target and background spectral radiance distributions, the band-averaged atmospheric path transmittance τ can be applied to the in band radiances. In the case of absolute contrast, the emission factor cancel and the contrast is reduced by the band averaged atmospheric path transmittance factor. In the case of relative contrast, the emission term be neglected in general. If the transmitted radiances are represented by lowercase symbols and defined as nT = N T τ + N ae nB = N B τ + N ae where τ is the band averaged atmospheric transmittance and Nae is the atmospheric path emission in the same spectral band, then the two transmitted contrast can be written as c A = nT − nB = τ( N T − N B ) = τC A and 7 69 Infrared radiation for thermal signatures cR = = nT − nB τ( N T − N B ) + N ae − N ae = (1/ 2)( nT + nB ) (1/ 2) τ( N T + N B ) + N ae NT − N B NT + N B = CR N ae / τ + ( N T + N B ) / 2 N T + N B + 2 N ae / τ NB ≈ τCR N N τ + ae B In some cases, such as short horizontal paths, NBτ+Nae≈NB and we are left with cR≈τCR. In these expressions lowercase symbols refer to transmitted radinaces or contrasts, whereas symbols are zero range values. 4. SIGNATURE ESTIMATION The radiant intensity that is emitted by any ground target (i.e., its signature) is I= ∑ εi Lband i Api (1) where I = the signature (in-band radiant intensity) εi = the emissivity of each area element Lbandi = the in-band radiance of each area element Api = the projected area of the element. The many radiation source –plume, hot parts, skin, reflected skyshine, reflected earthshine, reflected clouds, etc., and their temporal and spatial variations- make an exact determination of the signature for an arbitrary target virtually impossible. For nontactical target it is possible to make several simplificatying assumptions that can give a reasonable estimation of the signature for bands α and β. The platform is assumed to radiate as a graybody. In addition, the assumption is made that the emissivity for all elements of the target is unity. The number of radiation sources is limited to the hot parts and body skin. Furthermore, the temperature is assumed to be uniform over each source. Thus, Eq. (1) can be simplified to I = L(THP )band ∑ hot parts Api + L(Tbody )band ∑ Api body where THP is the temperature of the hot parts and Tbody is the temperature of the body skin. Usind these assumptions, analysis of several ground vehicle measurements (Fig. 3) has shown that THP can be estimate as falling into the 283 to 70 C. Plesa, D. Turcanu, V. Bodoc 8 323-K range. The parameter Tbody is estimate to be within a few degrees of ambient temperature (Fig. 4). Fig. 3 – Temperature measurement. The estimation of the pattern of the radiation produced by the hot parts can be found [3] by using the following relationship: y I ( θ) = L(THP )band ( +1) πd 2 cos d θ 4 where d = the diameter of the engine exhaust port y = the distance of the turbine plate from the exhaust port θ = the angle with respect to the normal of the exhaust port. The radiance over the band is λ2 Lband = ∫ L( λ ) d λ (2) λ1 where λ1, λ2 = the wavelength limits of band. Since the band are at most 2µm in width, one can approximate Eq. (2) by Lband = (λ 2 − λ1 ) L(λ mid ), where λmid=(λ2 + λ1)/2. The radiance can be determined from the Planck function L( λ , T ) = C1 λ [exp(C2 / λT ) − 1] 5 9 Infrared radiation for thermal signatures 71 where C1=1.191 × 104 W cm-2 µm4 sr-1 and C2=1.438 × 104 µm K. Fig. 4 – Temperature and humidity air on 25.03.2004. 5. SIGNATURE MEASUREMENT A more precise estimate of the signature can be gaind from a measurement of the vehicle. Measurement values of the platform radiation are dependent on the conditions under which the measurement is performed; they are a strong function of several factors, including the background, background temperature, engine temperature, and vehicle velocity. These effects can be particularly large in the long-wavelength band β. Several types of radiometers, such as the Fourier transform radiometer (FTR) and circular variable filter radiometer (CVFR) [4], can give an accurate spectral measurement of the vehicle signature alone. The FTR uses a Michelson interferometer, as shown as in fig.5. A collimated beam is split into two parts, each part traveling a separate path to a reflecting mirror. The separate beams are recombined at the beam splitter and reflected to a detector. Fringers will occur because of interference. The amplitude of the central fringe depends on the difference in length that each portion of the beam has traversed. For a 72 C. Plesa, D. Turcanu, V. Bodoc 10 monochromatic source, the detected amplitude varies sinusoidally as one mirror is moved with respect to the other at a constant velocity. The amplitude of the oscillation is dependent on the strength of the source. The frequency of the oscillation is a function of the velocity of the mirror and the source wavelength. For a polychromatic source, the detected voltage is a complex function of time. The detected voltage is the sum of each frequency response caused by each wavelength. The Fourier transform decomposes the time response into the component frequency responses. Thus, the Fourier transform of the detector voltage is proportional to the spectrum of source. Fig. 5 – Block diagram of the Fourier transform radiometer. The CVFR moves a filter wheel in front of a detector. Each position of the wheel allows transmission about a center wavelength with a width approximately 0.05µm. Each wavelength-dependent signal is found by rotating the wheel until the corresponding wavelength position is in front of the detector. The wheel is paused for a small amount of time and the detected voltage is measurement. When the response has been found for all the filters, the spectrum is complete. Assuming the source remains constant over the measurement time, the FTR usually can provide a higher-resolution spectrum with less radiated power and in less time than a CVFR. REFERENCES 1. David H. Pollock, Countermeasure Systems, Spiee PRESS, 1993. 2. G.A. Findlay and D.R. Cutten, Comparation of 3-5 and 8-12 micron IR systems, Applied Optics 28(23), 5029(19987). 3. C. Link and M. Maas, Northrop Corporation, private communication (1990). 4. F. Grum and R. Becherer, Radiometry, Optical radiation measurements, Vol. 1, Academic Press, New York (1979).