A ship is a complex infrared source whose surfaces are essentially graybody radiators with a distribution of temperatures influenced by internal and environmental factors. For the most part, these temperatures are within a few degrees of ambient air temperature and rarely exceed fifty degrees celsius with the exception of a few hot-spot sources such as the top of a stack or a steam catapult, where internal sources can heat surfaces to one hundred degrees celsius or higher. Depending upon the environment and aspect from which a ship is viewed, its radiant spectrum will be influenced by certain sources more than be others.
In general its radiant, spectrum will be characteristic of a source near ambient temperature; but there are times, particularly at night when the absence of solar heating allows the contrast between the "skin" of a ship and the sea to vanish, and aspects of observation, where the hot-spot sources are the predominant contributors to the radiant spectrum of the ship. A normal ship will not produce a radiant spectrum similar to a black body at flame temperatures (i.e., at about 1500.degree. K.).
It might be thought that spectra so grossly different could be distinguished by measuring the slope on the distribution, (i.e., the ratio between any two narrow bands). This is not the case. Both a target ship and a nearby decoy are seen by a seeking missile through a naturally occurring and highly selective filter, namely, the atmospheric path in the line-of-sight. The atmospheric spectral attenuation for path lengths has the effect of making gross spectral differences appear subtle in bands between atmospheric opacities.
Radiation from CO.sub.2 in the three to five micron band for example, is largely absorbed by the atmosphere over path lengths longer than two kilometers. Only comparisons over a broad spectral range, such as ratios of band integrals, provide strong distinctions.
A graybody is a temperature radiator whose spectral emissivity is less than unity and the same at all wavelengths. Radiant intensity, J, is the quotient of the radiant power emitted by a source in an infinitesimal cone containing a given direction, by the solid angle of the cone, and is expressed in units of watts per steradian (W.sr.sup.-1). The numbers in parentheses following the symbol J (e.g., J (3.4-4.3) give the corresponding half band points in units of microns.
An infrared decoy is a countermeasure against heat-seeking, anti-ship missiles. In practice a decoy is deployed between the ship and the anti-ship missile during the search and acquisition phase of the missile's flight for the purpose of attracting the exclusive attention of the missile's homing guidance system. Ideally, the spectral distribution of the decoy is indistinguishable from that of the ship over the band of interest.
Assuming that the total spectral band of interest extends only from three to thirteen microns, then the ratio of the radiant intensity emitted in the atmospheric window regions of the three to five micron band to that emitted in the eight to thirteen micron band is the criterion for spectral discrimination. That ratio is denominated at R.sub.j (3-5/8-13). While is it not possible to assign a single value to this ratio, its value is usually unity or less for a ship. A ratio based upon radiance, R.sub.n (3-5/8-13), rather than radiant intensity may be defined in an analogous manner.
Missiles track targets by detecting an infrared signal. Decoys for anti-air missiles (AAM) and anti-ship missiles (ASM) have been used to defeat this tracking. An infrared decoy is a countermeasure against heat-seeking, anti-ship missiles. In practice a decoy is deployed between the ship and the anti-ship missile during the search and acquisition phase of the missile's flight for the purpose of attracting the exclusive attention of the missile's homing guidance system. The decoys emit an infrared (IR) radiation, which may be deployed immediately after launch through the time it touches ground or floats on the water.
Liquid fueled, IR radiating decoys have been used that produce an IR plume, or signature after they have been launched, entered the water, and floated back to the surface. Because these decoys do not produce an IR decoy plume immediately after launch, a finite time passes while the decoy is launched, flies through the air, impacts the water, sinks, and then is buoyed back to the surface before it begins to produce its decoying IR plume. Consequently, such decoys do not provide adequate ship protection because during the interval while the decoy is in the air and underwater, the ship is vulnerable to an incoming IR radiation-seeking anti-ship missile (ASM).
Some ASM decoy systems use activated metals to produce IR signatures immediately upon launch. However, these decoys create only short bursts of IR radiation that rapidly fade as the expelled metal diffuses in the air and/or the chemical reaction wanes. Since the activated metal IR radiating decoys do not produce a constant IR plume over a prolonged period, successive IR radiating decoys have to be launched in a properly spaced sequence while the ship is moving. A more serious consequence of using successive IR radiating decoys is that they may actually draw an ASM seeker back to the targeted ship after the IR cloud of a previous burst has already decoyed the missile away.
Presently a floating pyrotechnic flare burning magnesium-teflon is used to provide a decoy for ships against low flying, heat-seeking missiles. As this type of flare floats directly on the sea :surface and projects a flame only on the order of one foot, it is subject to extensive shadowing by waves occurring between it and a low flying missile. Another disadvantage is that the radiant spectrum of magnesium-teflon matches that of a ship only in the three to five micron band; in the eight to fourteen micron band the intensity of the flare is too weak by at least one order of magnitude. Additionally, the recent emergence of tri-metal quantum infrared detectors means that it is now practical to deploy missiles responsive to the eight to fourteen micron band.
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