Obscurants and How They Work
Obscurants are particles suspended in the air that block or attenuate a portion (or portions) of the electromagnetic spectrum. The six types of obscurants are natural obscurants (such as fog); by-product obscurants (such as dust); visual smoke (such as WP); and bispectral multispectral and special obscurants. This appendix describes the general characteristics of obscurants, how they work, and what obscurants the United States has in its inventory.
Obscuration occurs when there is a decreased level of energy avail able for the function of seekers, trackers, vision enhancement devices, or the human eye. Battlefield visibility can be practically defined as the distance at which a potential target can be seen and identified against any background. Reduction of visibility on a battlefield by any cause reduces the amount of smoke needed to obscure a target or objective.
Obscuration generally is not associated with combat power because it is not a lethal tool on the battlefield. However, the deliberate use of smoke and the inadvertent or planned use of dust and/or adverse weather conditions on the battlefield have always been of value to units in the field.
In general, smokes are composed of many small particles suspended in the air. These particles scatter and absorb (attenuate) different spectra of electromagnetic radiation. This absorption reduces transmittance of that radiation through the smoke. When the density (concentration) of smoke material between the observer or EO device and an object exceeds a certain minimum threshold value (Cl), the object is considered effectively obscured.
Smoke, placed between a target and viewer, degrades the effectiveness of that viewer by interfering with the reflected electromagnetic radiations. The amount of smoke required to defeat that viewer is highly dependent upon meteorological conditions, terrain relief, available natural light, visibility, and the absorption effect of natural particles in the atmosphere. Other factors include smoke from battlefield fires and dust raised from maneuvering vehicles and weapon fire.
The ability to detect and identify a target concealed by such a smoke cloud is a function of target-to-background contrast. Smoke clouds reduce target-to-background contrast, making the target more difficult to detect.
The effectiveness of obscuration depends primarily upon characteristics such as the number, size, and color of the smoke particles. In the visible range, dark or black smoke absorbs a large proportion of the electromagnetic waves striking individual smoke particles. Dining bright sunlight you need a higher concentration of black smoke to effectively obscure a target because black smoke particles are nonscattering. At night or in limited visibility, considerably less black smoke is needed.
Grayish or white smoke obscures in the visible range by reflecting or scattering light, producing a glare. During bright sunlight you need a lower concentration than with black smoke to effectively obscure a target. At night or in limited visibility, considerably more than black smoke is needed.
Years of experience with white smoke technology have shown it to be superior to black smoke for most applications. Available white smoke producers include WP and RP compounds, HC, and fog oil (SGF2). WP, RP, and HC are hydroscopic (that is, they absorb water from the atmosphere). This increases particle diameters and makes them more efficient in scattering light. Fog oils are nonhydroscopic and depend upon vaporization techniques to produce extremely small diameter droplets that absorb and scatter light.
Smoke produced by a smoke generator unit or from a series of smoke pots has four distinct phases: streamer, build-up, uniform, and terminal (see Figure 19, below).
Streamer phase is the smoke cloud formed by a single smoke device before it begins to blend with the smoke from other sources.
Build-up phase is the stage of smoke cloud production when individual streamers begin to merge.
Uniform phase is a uniform smoke cloud that occurs after individual smoke streamers have merged. This is the phase commanders want over the target area.
Terminal phase is the stage of a smoke cloud in which the smoke has dispersed and concealment is no longer effective.
The diffusion of smoke particles into the atmosphere just above the earth's surface obeys physical laws. Wind speed, turbulence, atmospheric stability, and terrain all govern diffusion of smoke. Smoke diffusion on the battlefield originates from four basic smoke source configurations:
- Continuous point sources (such as smoke release from a smoke generator or smoke pot).
- Instantaneous point sources (such as bursting of a WP projectile).
- Continuous line sources (such as a series of smoke generators set up crosswind).
- Area sources (such as munitions that scatter smoke-generating sub-munitions like the armored vehicle smoke grenade launchers).
Natural obscurants are produced by nature and are therefore no drain on our assets. However, they are uncontrollable and may aid the enemy as much as friendly forces. We can use natural obscurants to our advantage if we accurately predict the weather and if there is a firm understanding of the impact of that weather on the battlefield. Natural obscurants will create large recognition and identification problems. Examples of natural obscurants are darkness, fog, sandstorms, and precipitation.
Darkness is the most common form of obscuration found on the battlefield. Darkness will degrade visual observation and target-acquisition devices that are not equipped with active infrared, image intensification, or thermal imaging. Systems equipped with these devices can operate at near-normal efficiency during periods of reduced visibility or darkness.
Fog can be an effective form of obscuration for use on the battlefield. Fog has the capability of providing a good obscurant on the battlefield because it will attenuate visual and near infrared signals in the same manner as visual smoke. Ice fog can also be a very effective obscurant because it degrades systems that operate by the use of a longer wavelength such as thermal imagers. Fog also degrades laser range finders and target designators.
Sandstorms are encountered in arid and semiarid regions and can have a dramatic effect on military operations. These storms will usually effectively obscure all observation and target acquisition devices with the possible exception of ground surveillance radars and other related devices operating in the microwave region of the electromagnetic spectrum.
Precipitation can definitely obscure battlefield viewers depending on the concentration. Rain, mist, sleet, or snow will degrade battlefield visibility greatly. When these elements are present in heavy concentration, there is no need to produce smoke. These elements can reduce visibility by themselves. The use of image intensifiers, active infrared systems, thermal imagers, laser range finders, and ground surveillance radars can be degraded and possibly defeated when the concentration of precipitation is heavy.
By-product obscurants that produce concealment are a result of other activities associated with battlefield operations. They are often inadvertent; however, when understood, they may be planned and used to the advantage of friendly forces. Examples of by-product obscurants are smoke from burning vehicles and buildings and dust caused by vehicular movement and artillery/mortar fire.
Smoke produced by fire on the battlefield will obscure viewers. This fire can be man-made or naturally produced by elements such as lightning. Other methods of generating fires that may result from a man-made device are fires produced by mortar or artillery rounds. Whether naturally produced or man-made, this obscurant will decrease visibility on the battlefield.
Battlefield dust is like the proverbial two-edged sword: its presence and use can cut both ways. For example: dust can be used for --
- Concealing details of military forces and movement. Dust is often an indicator of movement of troops and equipment. If the amount of dust generated is large (perhaps deliberately so), details of troop movement can be obscured. If no dust is desired, a simple expedient is to keep the road wet, which can be done if sufficient equipment and ample water are available.
- Blinding enemy observation points to deprive him of the opportunity to adjust fire. Artillery volleys or naval salvos can be used to temporarily obscure a narrow field of view for a short period of time. HE dust clouds are generally only effective as obscurants for several seconds but may be effective up to a minute or more.
- Degrading performance of precision-guided munitions and EO sensors. HE dust can be used to interfere with the target acquisition sequence or to break "lock-on" of an acquired target.
Dust, depending on how it is produced, can obscure different portions of the electromagnetic spectrum, in either the visible, infrared millimeter wave, or radar portions.
Dust is often produced inadvertently by bombing, gunfire, and vehicular movement. However, we can plan and use dust to the advantage of friendly forces. Dust degrades the performance of sensors and precision-guided munitions.
When HE munitions are used, dust will be produced. The amount produced depends on the size of the munition, its point of detonation (above or below the surface), and the state of the soil. The initial explosion throws up a variety of crater materials. From small clumps down to individual soil particles, obscuration will occur at all frequency bands of the electromagnetic spectrum (assuming the explosion is on or near the line of sight). Obscuration times are generally 3 to 10 seconds in the millimeter wave portion of the spectrum; this is the amount of time required for the small clumps and large particles to fall back to the ground. The remaining airborne dust that forms the drifting dust cloud continues to provide obscuration in the visible and infrared portions of the spectrum.
As a rule of thumb for drier soils, dust generally has less effect on IR sensors than on visual sensors such as the eye. For moist or very sandy soils, the two sensors are often affected equally, and under some conditions the IR sensors are obscured more than the visual sensors. In general, infrared sensors will usually offer some advantage over visible- radiation sensors when looking through dust.
Figure 20, below, shows the phases of a munition dust cloud. The initial phase lasts only a few seconds and quickly blends into the rise phase that lasts about 10 seconds or less. The degree and time of obscuration depend on the dust cloud drift and dissipation phase of the dust cloud with respect to the line of sight and the weather conditions. Dust clouds created by HE have three successive phases: impact, rise, and drill and dissipation.
- Impact phase. Upon munition impact, two parts of a dust cloud are created instantaneously. One part is the hot dust or fire ball, which has an initial size of 4 to 6 meters and is close to the surface. The dust or fire ball is initially several hundred degrees hotter than its surroundings. Most of the dirt and dust are contained in this initial dust or fireball. The second part is the dust skirt, which has a greater horizontal extent of 6 to 10 meters high, and has nearly the same temperature as its surroundings.
- Rise phase. The initial dust or fireball begins to rise and expand, cooling as it rises. The dust cloud top may reach heights of 10 to 30 meters in less than 10 seconds. The dust skirt does not rise but will continue to diffuse outward.
- Drift and dissipation phase. The entire dust cloud, both the buoyant part and the nonbuoyant dust skirt, begin to drift. Wind causes the upper portion to move out ahead while the lower dust skirt lags behind. As the dust cloud drifts, it diffuses, becoming thinner and gradually dissipating.
The amount of dust produced by vehicular traffic depends on the weight of the vehicle, the number of wheels (or tread area), the speed of the vehicle, and the state of the soil. Because vehicles kick up the smaller particles present on the soil surface, vehicular dust does not effectively attenuate the radar or the millimeter wave portions of the spectrum. However, vehicular dust clouds can provide effective obscuration in the visible and infrared portions of the spectrum. Vehicular dust can be divided into two phases: generation and drift and dissipation (Figure 21).
- Generation phase. In this phase, the dust is thrown up or lifted off the surface by the vehicle's wheels or treads and is swept up in the turbulent air under and behind the vehicle. The total amount of dust produced increases with the speed of the vehicle.
- Drift and dissipation phase. After the dust has been swept up behind the vehicle, it begins to drift and diffuse with the wind. As before, the degree and duration of obscuration depend on the position of the dust trail with respect to a line of sight and the weather conditions.
We cannot control the behavior of natural and by-product obscurants with the degree of certainty required to defeat enemy RSTA efforts. While natural and byproduct obscurants block or attenuate portions of the electromagnetic spectrum, we must produce obscurants artificially to attack enemy electro-optical systems. We classify US obscurants as visual, bispectral, multispectral, and special..
While 98 percent of all current battelfield viewers operate in the visual portion of the spectrum, future systems will acquire and engage, using IR and millimeter wave technologies. This will require integration of each class of US obscurant to attack and defeat these systems. The following portions of this appendix describe the militarily significant, artificially producted obscurants.
Many years of experience with smoke technology has shown white smoke to be superior to black smoke for most applications. Currently we have no black smoke production agents, although the US Navy does have black smoke production capability. The three principle agents for producing white smoke are oils (SGF2 and diesel), HC, and phosphorous.
We make oil smoke by vaporizing fuel oils in mechanical smoke generators or engine exhausts. The generator or engine exhaust vaporizes either SGF2 or diesel fuel and forces into the air where it condenses into a dense white smoke. This smoke can produce effective obscuration of the visual through near-infrared portions of the electromagnetic spectrum.
HC is a pyrotechnic composition of hexachloroethane, zinc oxide, and aluminum powder. A pyrotechnic starter mixture usually ignites the burning reaction. The smoke produced is zinc chloride during burning. This zinc chloride reacts with the moisture in the air to form a zinc chloride solution in tiny droplets: smoke. When first produced, HC smoke is very hot but cools rapidly and has little tendency thereafter to rise. HC munitions generally have definite burn times, which are useful for planning purposes.
HC is carcinogenic. Soldiers must wear respiratory protection (for example, a protective mask) while in HC smoke.
Phosphorus is a flammable solid that burns to form solid particles of phosphorous pentoxide in the air: smoke. The phosphorous pentoxide then reacts with moisture in the air to form phosphoric acid. We use phosphorous smokes in instantaneous-burst munitions (for example, artillery and rifle grenades), with the showers of burning phosphorous particles being highly incendiary. This makes phosphorous smoke excellent for harassing enemy personnel and starting fires, as well as its having excellent smoke properties.
Phosphorous smoke burns so hot that it tends to form a pillar of smoke, which rises rapidly. While this pillaring reduces the efficiency of phosphorous smoke, the by-product of the heat is that it obscures from the visual through the far-infrared portions of the electromagnetic spectrum. The three phosphorous smokes are WP, PWP, and RP.
Phosphorous smoke produces phosphoric acid. Soldiers must wear respiratory protection, such as protective masks, if exposed to phosphorous smoke.
WP is a spontaneously flammable natural element. It ignites on contact with air and is relatively unstable in storage. WP burns at 5,000 degrees Fahrenheit, making it the most effective smoke agent to defeat thermal imagery systems.
PWP is a formulation of white phosphorus and some other agents (for example, butyl rubber) to stabilize the smoke agent fill and slow the burning. This slowed burning tends to produce a more coherent smoke cloud with less pillaring.
RP is not spontaneously flammable, requiring ignition to burn and make smoke. RP burns at a lower temperature - 4,000 degrees Fahrenheit - which produces a more coherent smoke cloud with less pillaring. It is less incendiary than either WP or PWP, making it safer for use in smaller cartridges (for example, 40-millimeter grenades). Some munitions such as the M825 155-millimeter howitzer cartridge use felt wedges saturated with RP to produce an even distribution of smoke agent around the point of burst.
Bispectral obscurants defeat or degrade two portions of the electromagnetic spectrum simultaneously. As previously stated, phosphorous smokes defeat both the visual and infrared portions of the spectrum. Other bispectral capabilities include type III IR obscurant, which is a micropulverized metal compound. Currently we use this bispectral obscurant in self-defense systems only (for example, the M76 smoke grenade for armored vehicle grenade launchers). In the near term we will have and use a large-area bispectral obscurant capability.
As implied by the name, multispectral obscurants will defeat or degrade multiple portions of the electromagnetic spectrum. Challenges associated with this technology include preventing the inadvertent suppression of friendly force EO systems. In the mid-term we will have and use multispectral obscurants.
Special obscurants will defeat specific portions of the electromagnetic spectrum.
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