Editing Radar cross section (section)

==Reduction==
[[File:B-2 Spirit 3.jpg|thumb|right|The [[B-2 Spirit]] was one of the first aircraft to successfully become 'invisible' to radar.]]
[[File:J-20 fighter (cropped).jpg|thumb|upright|A [[Chengdu J-20]] incorporating [[stealth technology]]]]
[[Image:Cheminee tribord du forbin.JPG|thumb|upright|Detail of the ''[[French frigate Forbin (D620)|Forbin]]'', a modern [[frigate]] of the [[French navy]]. The faceted appearance reduces radar cross-section for [[Stealth ship|stealth]].]]

{{Main|Stealth technology}}

RCS reduction is chiefly important in stealth technology for aircraft, missiles, ships, and other military vehicles. With smaller RCS, vehicles can better evade radar detection, whether it be from land-based installations, guided weapons or other vehicles. Reduced signature design also improves platforms' overall survivability through the improved effectiveness of its radar counter-measures.<ref name=":0" />

Several methods exist. The distance at which a target can be detected for a given radar configuration varies with the fourth root of its RCS.<ref name="Sweetman">{{cite book | last = Sweetman | first = Bill | title = YF-22 and YF-23 Advanced Tactical Fighters: Stealth, Speed and Agility for Air Superiority | year = 1991 | publisher = Motorbooks International | location = Osceola, Wisconsin, United States | isbn = 978-0-87938-505-7}}</ref> Therefore, in order to cut the detection distance to one tenth, the RCS should be reduced by a factor of 10,000. While this degree of improvement is challenging, it is often possible when influencing platforms during the concept/design stage and using experts and advanced computer code simulations to implement the control options described below.

===Purpose shaping===
With purpose shaping, the shape of the target's reflecting surfaces is designed such that they reflect energy away from the source. The aim is usually to create a "cone-of-silence" about the target's direction of motion. Due to the energy reflection, this method is defeated by using [[Passive radar|passive (multistatic) radar]]s.

Purpose-shaping can be seen in the design of surface faceting on the [[F-117 Nighthawk|F-117A Nighthawk]] stealth attack aircraft. This aircraft, designed in the late 1970s though only revealed to the public in 1988, uses a multitude of flat surfaces to reflect incident radar energy away from the source. Yue suggests<ref>{{cite web | author= The Tech | year= 2001 | title= Detection of the B-2 Stealth Bomber And a Brief History on "Stealth" | url= http://tech.mit.edu/V121/N63/Stealth.63f.html | access-date= 2016-02-01 | archive-date= 2009-06-10 | archive-url= https://web.archive.org/web/20090610041304/http://tech.mit.edu/V121/N63/Stealth.63f.html | url-status= dead }}</ref> that limited available computing power for the design phase kept the number of surfaces to a minimum. The [[B-2 Spirit]] stealth bomber benefited from increased computing power, enabling its contoured shapes and further reduction in RCS. The [[F-22 Raptor]] and [[F-35 Lightning II]] continue the trend in purpose shaping and promise to have even smaller monostatic RCS.

===Redirecting scattered energy without shaping===
This technique is relatively new compared to other techniques chiefly after the invention of [[Electromagnetic metasurface|metasurfaces]].<ref name="A. Modi 19 2">A. Y. Modi; M. A. Alyahya; C. A. Balanis; C. R. Birtcher, "Metasurface-Based Method for Broadband RCS Reduction of Dihedral Corner Reflectors with Multiple Bounces," in IEEE Transactions on Antennas and Propagation, vol.67, no.12, pp. -, Dec. 2019. {{doi| 10.1109/TAP.2019.2940494}}</ref><ref name="A. Modi 19">A. Y. Modi; C. A. Balanis; C. R. Birtcher; H. Shaman, "New Class of RCS-Reduction Metasurfaces Based on Scattering Cancellation Using Array Theory," in IEEE Transactions on Antennas and Propagation, vol. 67, no. 1, pp. 298-308, Jan. 2019. {{doi| 10.1109/TAP.2018.2878641}}</ref><ref name="A. Modi 17">A. Y. Modi; C. A. Balanis; C. R. Birtcher; H. Shaman, "Novel Design of Ultra-Broadband Radar Cross Section Reduction Surfaces using Artificial Magnetic Conductors," in IEEE Transactions on Antennas and Propagation, vol. 65, no. 10, pp. 5406-5417, Oct. 2017. {{doi| 10.1109/TAP.2017.2734069}}</ref> As mentioned earlier, the primary objective in geometry alteration is to redirect scattered waves away from the backscattered direction (or the source). However, it may compromise performance in terms of aerodynamics.<ref name="A. Modi 19 2"/><ref name="A. Modi 19"/><ref>Appl. Phys. Lett. 104, 221110 (2014). {{doi| 10.1063/1.4881935}}</ref> One feasible solution, which has extensively been explored in recent time, is to utilize metasurfaces which can redirect scattered waves without altering the geometry of the target.<ref name="A. Modi 19"/><ref name="A. Modi 17"/> Such metasurfaces can primarily be classified in two categories: (i) Checkerboard metasurfaces, (ii) Gradient index metasurfaces.

===Active cancellation===
With active cancellation, the target generates a radar signal equal in intensity but opposite in phase to the predicted reflection of an incident radar signal (similarly to noise canceling ear phones). This creates [[destructive interference]] between the reflected and generated signals, resulting in reduced RCS. To incorporate active cancellation techniques, the precise characteristics of the waveform and angle of arrival of the illuminating radar signal must be known, since they define the nature of generated energy required for cancellation. Except against simple or low frequency radar systems, the implementation of active cancellation techniques is extremely difficult due to the complex processing requirements and the difficulty of predicting the exact nature of the reflected radar signal over a broad aspect of an aircraft, missile or other target.

===Radar absorbent material===
{{Main|Radar-absorbent material}}
Radar absorbent material (RAM)<ref name=":0" /> can be used in the original construction, or as an addition to highly reflective surfaces. There are at least three types of RAM: resonant, non-resonant magnetic and non-resonant large volume. 
*Resonant but somewhat 'lossy' materials are applied to the reflecting surfaces of the target. The thickness of the material corresponds to one-quarter wavelength of the expected illuminating radar-wave (a [[Salisbury screen]]). The incident radar energy is reflected from the outside and inside surfaces of the RAM to create a destructive wave interference pattern. This results in the cancellation of the reflected energy. Deviation from the expected frequency will cause losses in radar absorption, so this type of RAM is only useful against radar with a single, common, and unchanging frequency.
*Non-resonant magnetic RAM uses [[Ferrite (magnet)|ferrite]] particles suspended in epoxy or paint to reduce the reflectivity of the surface to incident radar waves. Because the non-resonant RAM dissipates incident radar energy over a larger surface area, it usually results in a trivial increase in surface temperature, thus reducing RCS without an increase in infrared signature. A major advantage of non-resonant RAM is that it can be effective over a wide range of frequencies, whereas resonant RAM is limited to a narrow range of design frequencies.
*Large volume RAM is usually [[Electrical resistance|resistive]] [[carbon]] loading added to [[fiberglass]] hexagonal cell aircraft structures or other non-conducting components. Fins of resistive materials can also be added. Thin resistive sheets spaced by foam or [[aerogel]] may be suitable for spacecraft.

Thin coatings made of only dielectrics and conductors have very limited absorbing bandwidth, so magnetic materials are used when weight and cost permit, either in resonant RAM or as non-resonant RAM.

=== Optimization methods ===
Thin non-resonant or broad resonance coatings can be modeled with a [[Mikhail Leontovich|Leontovich]] [[Electromagnetic impedance|impedance]] [[boundary condition]] (see also [[Electrical impedance]]).  This is the ratio of the tangential electric field to the tangential magnetic field on the surface, and ignores fields propagating along the surface within the coating.  This is particularly convenient when using [[boundary element method]] calculations.  The surface impedance can be calculated and tested separately.
For an [[isotropic]] surface the ideal surface impedance is equal to the 377 [[Ohm (unit)|ohm]] [[impedance of free space]].
For non-isotropic ([[anisotropic]]) coatings, the optimal coating depends on the shape of the target and the radar direction, but duality, the symmetry of Maxwell's equations between the electric and magnetic fields,  tells one that optimal coatings have η<sub>0</sub> × η<sub>1</sub> = 377<sup>2</sup> Ω<sup>2</sup>, where η<sub>0</sub> and η<sub>1</sub> are perpendicular components of the anisotropic surface impedance, aligned with edges and/or the radar direction.

A perfect electric conductor has more back scatter from a leading edge for the linear polarization with the electric field parallel to the edge and more from a trailing edge with the electric field perpendicular to the edge, so the high surface impedance should be parallel to leading edges and perpendicular to trailing edges, for the greatest radar threat direction, with some sort of smooth transition between.

To calculate the radar cross-section of such a stealth body, one would typically do one-dimensional reflection calculations to calculate the surface impedance, then two dimensional [[numerical analysis|numerical calculations]] to calculate the diffraction coefficients of edges and small three dimensional calculations to calculate the diffraction coefficients of corners and points. The cross section can then be calculated, using the diffraction coefficients, with the physical theory of diffraction or other high frequency method, combined with [[physical optics]] to include the contributions from illuminated smooth surfaces and [[Fock space|Fock]] calculations to calculate [[creeping waves]] circling around any smooth shadowed parts.

Optimization is in the reverse order.  First one does high frequency calculations to optimize the shape and find the most important features, then small calculations to find the best surface impedances in the problem areas, then reflection calculations to design coatings. Large numerical calculations can run too slowly for numerical optimization or can distract workers from the physics, even when massive computing power is available.