A larger RCS indicates that an object is more easily detected. An object reflects a limited amount of radar energy back to the source. The factors that influence this include:. While important in detecting targets, strength of emitter and distance are not factors that affect the calculation of a RCS because the RCS is a property of the target reflectivity.
Radar cross-section is used to detect planes in a wide variation of ranges. For example, a stealth aircraft which is designed to have low detectability will have design features that give it a low RCS such as absorbent paint, flat surfaces, surfaces specifically angled to reflect signal somewhere other than towards the source , as opposed to a passenger airliner that will have a high RCS bare metal, rounded surfaces effectively guaranteed to reflect some signal back to the source, lots of bumps like the engines, antennas, etc.
RCS is integral to the development of radar stealth technology , particularly in applications involving aircraft and ballistic missiles.
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RCS data for current military aircraft is most highly classified. In some cases, it is of interest to look at an area on the ground that includes many objects. Informally, the RCS of an object is the cross-sectional area of a perfectly reflecting sphere that would produce the same strength reflection as would the object in question.
Bigger sizes of this imaginary sphere would produce stronger reflections. Thus, RCS is an abstraction: The radar cross-sectional area of an object does not necessarily bear a direct relationship with the physical cross-sectional area of that object but depends upon other factors. Somewhat less informally, the RCS of a radar target is an effective area that intercepts the transmitted radar power and then scatters that power isotropically back to the radar receiver.
More precisely, the RCS of a radar target is the hypothetical area required to intercept the transmitted power density at the target such that if the total intercepted power were re-radiated isotropically, the power density actually observed at the receiver is produced. The term in the radar equation represents the power density watts per meter squared that the radar transmitter produces at the target.
This power density is intercepted by the target with radar cross-section , which has units of area meters squared.
Thus, the product has the dimensions of power watts , and represents a hypothetical total power intercepted by the radar target. The second term represents isotropic spreading of this intercepted power from the target back to the radar receiver. Thus, the product represents the reflected power density at the radar receiver again watts per meter squared. The receiver antenna then collects this power density with effective area , yielding the power received by the radar watts as given by the radar equation above. The scattering of incident radar power by a radar target is never isotropic even for a spherical target , and the RCS is a hypothetical area.
In this light, RCS can be viewed simply as a correction factor that makes the radar equation "work out right" for the experimentally observed ratio of. However, RCS is an extremely valuable concept because it is a property of the target alone and may be measured or calculated. Thus, RCS allows the performance of a radar system with a given target to be analysed independent of the radar and engagement parameters.
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In general, RCS is a strong function of the orientation of the radar and target, or, for the bistatic radar transmitter and receiver not co-located , a function of the transmitter-target and receiver-target orientations. A target's RCS depends on its size, reflectivity of its surface, and the directivity of the radar reflection caused by the target's geometric shape.
As a rule, the larger an object, the stronger its radar reflection and thus the greater its RCS. Also, radar of one band may not even detect certain size objects. For example. Materials such as metal are strongly radar reflective and tend to produce strong signals. Wood and cloth such as portions of planes and balloons used to be commonly made or plastic and fibreglass are less reflective or indeed transparent to radar making them suitable for radomes.
Even a very thin layer of metal can make an object strongly radar reflective. Chaff is often made from metallised plastic or glass in a similar manner to metallised foils on food stuffs with microscopically thin layers of metal. Also, some devices are designed to be Radar active, such as radar antennas and this will increase RCS. The SR Blackbird and other planes were painted with a special " iron ball paint " that consisted of small metallic-coated balls. Radar energy received is converted to heat rather than being reflected. The surfaces of the FA are designed to be flat and very angled.
This has the effect that radar will be incident at a large angle to the normal ray that will then bounce off at a similarly high reflected angle; it is forward-scattered. The edges are sharp to prevent there being rounded surfaces. Rounded surfaces will often have some portion of the surface normal to the Radar source. Radar waves penetrating the skin get trapped in these structures, reflecting off the internal faces and losing energy.
The most efficient way to reflect radar waves back to the emitting radar is with orthogonal metal plates, forming a corner reflector consisting of either a dihedral two plates or a trihedral three orthogonal plates. This configuration occurs in the tail of a conventional aircraft, where the vertical and horizontal components of the tail are set at right angles. Stealth aircraft such as the F use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them.
A more radical method is to omit the tail, as in the B-2 Spirit.
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The B-2's clean, low-drag flying wing configuration gives it exceptional range and reduces its radar profile. In addition to altering the tail, stealth design must bury the engines within the wing or fuselage , or in some cases where stealth is applied to an extant aircraft, install baffles in the air intakes, so that the compressor blades are not visible to radar.
A stealthy shape must be devoid of complex bumps or protrusions of any kind, meaning that weapons, fuel tanks, and other stores must not be carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch opens. Parallel alignment of edges or even surfaces is also often used in stealth designs. The technique involves using a small number of edge orientations in the shape of the structure. For example, on the FA Raptor , the leading edges of the wing and the tail planes are set at the same angle.
Other smaller structures, such as the air intake bypass doors and the air refueling aperture, also use the same angles. The effect of this is to return a narrow radar signal in a very specific direction away from the radar emitter rather than returning a diffuse signal detectable at many angles. The effect is sometimes called "glitter" after the very brief signal seen when the reflected beam passes across a detector. It can be difficult for the radar operator to distinguish between a glitter event and a digital glitch in the processing system.
Stealth airframes sometimes display distinctive serrations on some exposed edges, such as the engine ports. The YF has such serrations on the exhaust ports. This is another example in the parallel alignment of features, this time on the external airframe. The shaping requirements detracted greatly from the F 's aerodynamic properties. It is inherently unstable , and cannot be flown without a fly-by-wire control system. Similarly, coating the cockpit canopy with a thin film transparent conductor vapor-deposited gold or indium tin oxide helps to reduce the aircraft's radar profile, because radar waves would normally enter the cockpit, reflect off objects the inside of a cockpit has a complex shape, with a pilot helmet alone forming a sizeable return , and possibly return to the radar, but the conductive coating creates a controlled shape that deflects the incoming radar waves away from the radar.
The coating is thin enough that it has no adverse effect on pilot vision. Ships have also adopted similar methods. Though the earlier Arleigh Burke-class destroyer incorporated some signature-reduction features. Dielectric composite materials are more transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites may also contain ferrites to optimize the dielectric and magnetic properties of a material for its application.
Radiation-absorbent material RAM , often as paints, are used especially on the edges of metal surfaces. While the material and thickness of RAM coatings can vary, the way they work is the same: absorb radiated energy from a ground or air based radar station into the coating and convert it to heat rather than reflect it back. Paint comprises depositing pyramid like colonies on the reflecting superficies with the gaps filled with ferrite-based RAM.
The pyramidal structure deflects the incident radar energy in the maze of RAM. One commonly used material is called iron ball paint. FSS are planar periodic structures that behave like filters to electromagnetic energy. The considered frequency selective surfaces are composed of conducting patch elements pasted on the ferrite layer.
FSS are used for filtration and microwave absorption. Shaping offers far fewer stealth advantages against low-frequency radar. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. However, low-frequency radar is limited by lack of available frequencies many are heavily used by other systems , by lack of accuracy of the diffraction-limited systems given their long wavelengths, and by the radar's size, making it difficult to transport.
A long-wave radar may detect a target and roughly locate it, but not provide enough information to identify it, target it with weapons, or even to guide a fighter to it. Much of the stealth comes in directions different than a direct return. Thus, detection can be better achieved if emitters are separate from receivers. One emitter separate from one receiver is termed bistatic radar ; one or more emitters separate from more than one receiver is termed multistatic radar. Proposals exist to use reflections from emitters such as civilian radio transmitters , including cellular telephone radio towers.
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By Moore's law the processing power behind radar systems is rising over time. This will eventually erode the ability of physical stealth to hide vehicles. Synthetic aperture sidescan radars can be used to detect the location and heading of ships from their wake patterns. Acoustic stealth plays a primary role for submarines and ground vehicles. Submarines use extensive rubber mountings to isolate, damp, and avoid mechanical noises that can reveal locations to underwater passive sonar arrays.
Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy troops below. Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. The presence of supersonic and jet-powered stealth aircraft such as the SR Blackbird indicates that acoustic signature is not always a major driver in aircraft design, as the Blackbird relied more on its very high speed and altitude.
One method to reduce helicopter rotor noise is modulated blade spacing. Using varied spacing between the blades spreads the noise or acoustic signature of the rotor over a greater range of frequencies. The simplest technology is visual camouflage ; the use of paint or other materials to color and break up the lines of a vehicle or person. Most stealth aircraft use matte paint and dark colors, and operate only at night.
Lately, interest in daylight Stealth especially by the USAF has emphasized the use of gray paint in disruptive schemes , and it is assumed that Yehudi lights could be used in the future to hide the airframe against the background of the sky, including at night, aircraft of any colour appear dark  or as a sort of active camouflage.
The original B-2 design had wing tanks for a contrail -inhibiting chemical, alleged by some to be chlorofluorosulfonic acid,  but this was replaced in the final design with a contrail sensor that alerts the pilot when he should change altitude  and mission planning also considers altitudes where the probability of their formation is minimized. In space, mirrored surfaces can be employed to reflect views of empty space toward known or suspected observers; this approach is compatible with several radar stealth schemes.