Editing Stealth technology (section)

==Radar cross-section (RCS) reductions==
{{Main|Radar cross-section}}
Almost since the invention of radar, various methods have been tried to minimize detection. Rapid development of radar during World War II led to equally rapid development of [[List of World War II electronic warfare equipment|numerous counter radar measures]] during the period; a notable example of this was the use of chaff. Modern methods include [[radar jamming and deception]].

The term ''stealth'' in reference to reduced radar signature aircraft became popular during the late 1980s when the Lockheed Martin F-117 stealth fighter became widely known. The first large scale (and public) use of the F-117 was during the [[Gulf War]] in 1991. However, F-117A stealth fighters were used for the first time in combat during [[Operation Just Cause]], the [[United States invasion of Panama]] in 1989.<ref>{{cite book |last=Crocker |first=H. W. III |title=Don't Tread on Me |publisher=Crown Forum |location=New York |year=2006 |page=[https://archive.org/details/donttreadonme40000croc/page/382 382] |isbn=978-1-4000-5363-6 |url-access=registration |url=https://archive.org/details/donttreadonme40000croc/page/382}}</ref> Stealth aircraft are often designed to have radar cross sections that are orders of magnitude smaller than conventional aircraft. The [[Radar#Radar range equation|radar range equation]] meant that all else being equal, detection range is proportional to the fourth root of RCS; thus, reducing detection range by a factor of 10 requires a reduction of RCS by a factor of 10,000.

===Vehicle shape===

====Aircraft====
{{Main|Aircraft design process}}
[[File:JSF F35 P1230144.jpg|thumb|The [[F-35 Lightning II]] offers better stealthy features (such as this landing gear door) than prior American multi-role fighters, such as the [[F-16 Fighting Falcon]]]]
The possibility of designing aircraft in such a manner as to reduce their radar cross-section was recognized in the late 1930s, when the first radar tracking systems were employed, and it has been known since at least the 1960s that aircraft shape makes a significant difference in detectability. The [[Avro Vulcan]], a British [[bomber]] of the 1960s, had a remarkably small appearance on radar despite its large size, and occasionally disappeared from radar screens entirely. It is now known that it had a fortuitously stealthy shape apart from the vertical element of the tail. Despite being designed before a low RCS and other stealth factors were ever a consideration,<ref name="newscientist82">Sweetman, Bill. [https://books.google.com/books?id=HVJyHCXAOtsC&dq=Vulcan+Bomber&pg=PA566 "The Bomber that radar cannot see."] ''New Scientist'', 4 March 1982.</ref> a Royal Aircraft Establishment technical note of 1957 stated that of all the aircraft so far studied, the Vulcan appeared by far the simplest radar echoing object, due to its shape: only one or two components contributing significantly to the echo at any aspect (one of them being the [[vertical stabilizer]], which is especially relevant for side aspect RCS), compared with three or more on most other types.<ref>Dawson 1957, p. 3.</ref>{{#tag:ref|Writing for the American Institute of Aeronautics and Astronautics, J. Seddon and E. L. Goldsmith noted that "Due to its all-wing shape, small vertical fin, and buried engines, at some angles [The Avro Vulcan] was nearly invisible to radar".<ref>Seddon and Goldsmith 1999, p. 343.</ref>}} While writing about radar systems, authors Simon Kingsley and Shaun Quegan singled out the Vulcan's shape as acting to reduce the RCS.<ref>Kingsley and Quegan 1999, p. 293.</ref> In contrast, the [[Tupolev Tu-95]] Russian long-range bomber ([[NATO reporting name]] 'Bear') was conspicuous on radar. It is now known that [[Propeller (aircraft)|propellers]] and jet turbine blades produce a bright radar image;{{Citation needed|date=March 2011}} the Bear has four pairs of large {{convert|5.6|m|ft|order=flip|adj=on}} diameter [[contra-rotating propellers]].

Another important factor is internal construction. Some stealth aircraft have skin that is radar transparent or absorbing, behind which are structures termed [[Concave polygon|reentrant triangles]]. Radar waves penetrating the skin get trapped in these structures, reflecting off the internal faces and losing energy. This method was first used on the Blackbird series: A-12, [[Lockheed YF-12A|YF-12A]], [[Lockheed SR-71 Blackbird]].

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-117 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. The B-2's clean, low-drag [[flying wing]] configuration gives it exceptional range and reduces its radar profile.<ref name="croddy 341-2">Croddy and Wirtz 2005, pp. 341–342.</ref><ref>Siuru 1993, pp. 114–115.</ref> The flying wing design most closely resembles a so-called infinite flat plate (as vertical control surfaces dramatically increase RCS), the perfect stealth shape, as it would have no angles to reflect back radar waves.<ref>{{cite web |url=https://www.northropgrumman.com/wp-content/uploads/B-2-Spirit-of-Innovation.pdf |title=B-2: The Spirit of Innovation |website=Northrop Grumman Corporation |access-date=15 October 2023 }}</ref>
[[File:Northrop McDonnell Douglas YF-23A PAV-1 87-0800 Black Widow II LEngineIntake R&D NMUSAF 25Sep09 (14414042127).jpg|thumb|right|YF-23 [[S-duct]] engine air intake conceals engine from probing radar waves]]
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 [[Lockheed Martin F-22 Raptor|F-22A 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 reflection|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 [[airframe]]s sometimes display distinctive [[serration]]s on some exposed edges, such as the engine ports. The [[YF-23 Black Widow II|YF-23]] 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-117's [[Aerodynamics|aerodynamic]] properties. It is [[relaxed stability|inherently unstable]], and cannot be flown without a [[Aircraft flight control system#Fly-by-wire control systems|fly-by-wire control system]].

Similarly, coating the [[cockpit (aviation)|cockpit]] canopy with a [[thin film]] [[transparent conductor]] ([[physical vapor deposition|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.

[[File:K32 HMS Helsingborg Anchored-of-Gotska-Sandoen cropped.jpg|right|thumb|{{HSwMS|Helsingborg|K32|6}}, a stealth ship]]

====Ships====
{{main|Naval architecture}}
Ships have also adopted similar methods. Though the earlier American {{sclass|Arleigh Burke|destroyer|1}}s incorporated some signature-reduction features.<ref>{{Cite web |title=DDG-51 Arleigh Burke-class |work=FAS website |publisher=Federation of American Scientists |url=https://fas.org/programs/ssp/man/uswpns/navy/surfacewarfare/ddg51_arleighburke.html |access-date=2 February 2011 |archive-url=https://web.archive.org/web/20131224113129/https://fas.org/programs/ssp/man/uswpns/navy/surfacewarfare/ddg51_arleighburke.html |archive-date=24 December 2013}}</ref><ref>{{Cite web |last=Benson |first=Robert |title=The Arleigh Burke: Linchpin of the Navy |work=Asia-Pacific Defense Forum |publisher=Federation of American Scientists |date=November 1998 |url=https://fas.org/man/dod-101/sys/ship/docs/ArleighB.htm |access-date=2 February 2011}}</ref> the Norwegian {{sclass|Skjold|corvette|1}}s was the first coastal defence and the French {{sclass|La Fayette|frigate|1}}s the first ocean-going [[stealth ship]]s to enter service. Other examples are the Dutch {{sclass|De Zeven Provinciën|frigate|1}}s, the Taiwanese {{sclass|Tuo Chiang|corvette|1}}s, German {{sclass|Sachsen|frigate|1}}s, the Swedish {{sclass|Visby|corvette|1}}, the American {{sclass|San Antonio|amphibious transport dock|1}}s, and most modern [[warship]] designs.

===Materials===

====Non-metallic airframe====
[[Dielectric]] [[composite material]]s are more transparent to radar, whereas electrically conductive materials such as metals and [[carbon fiber]]s reflect electromagnetic energy incident on the material's surface. Composites may also contain [[Ferrite (magnet)|ferrites]] to optimize the dielectric and magnetic properties of a material for its application.

====Radar-absorbent material====
{{Main|Radiation-absorbent material}}
[[File:B-2 Stealth Bomber Skin, 2009 - Museum of Science and Industry (Chicago) - DSC06496.JPG|thumb|Skin of a [[Northrop Grumman B-2 Spirit|B-2 bomber]].]]
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.<ref>{{cite journal |url=http://www.researchinventy.com/papers/v3i12/D0312015019.pdf |title=Stealth Technology And Counter Stealth Radars: A Review |first1=Swayam |last1=Arora |first2=Ramanpreet |last2=KaurResearch |journal=Inventy: International Journal of Engineering and Science |volume=3 |issue=12 |date=December 2013 |pages=15–19 |eissn=2278-4721 |issn=2319-6483}}</ref> Current technologies include dielectric composites and metal fibers containing ferrite isotopes. Ceramic composite coating is a new type of material systems which can sustain at higher temperatures with better sand erosion resistance and thermal resistance.<ref>{{cite web |url=https://news.ncsu.edu/2021/05/tougher-skin-for-stealth-aircraft/ |title=How a Tougher Skin Could Change the Shape of Stealth Aircraft |date=18 May 2021}}</ref> 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''.<ref>{{cite web |url=http://www.livescience.com/32943-how-stealth-planes-evade-enemy.html |title=How Do Stealth Planes Evade the Enemy? |last=Wolchover |first=Natalie |date=21 January 2011 |website=Live Science |place=Bath, England |access-date=1 July 2019}}</ref> It contains microscopic iron spheres that resonate in tune with incoming radio waves and dissipate most of their energy as heat, leaving little to reflect back to detectors. 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.

===Radar stealth countermeasures and limits===

====Low-frequency radar====

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 system]]s 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.<ref>[https://www.defense.gov/transcripts/transcript.aspx?transcriptid=597 Maj. Gen. Carlsons briefing on Stealth Fighters, Tuesday, April 20, 1999]</ref>

====Multiple emitters====

Stealth aircraft attempt to minimize all radar reflections, but are specifically designed to avoid reflecting radar waves back in the direction they came from (since in most cases a radar emitter and receiver are in the same location). They are less able to minimize radar reflections in other directions. Thus, detection can be better achieved if emitters are in different locations 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 [[transmitter]]s, including [[cell site|cellular telephone radio towers]].<ref>[http://tech.mit.edu/V121/N63/Stealth.63f.html MIT's "The Tech – online edition"] article ''Detection of the B-2 Stealth Bomber and a Brief History on "Stealth"'' by Tao Yue published 30 November 2001 in (Volume 121, Issue 63)</ref>

====Moore's law====

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.<ref>[http://www.aviationweek.com/aw/generic/story.jsp?id=news/FIGHT030409.xml&headline=Global%20Opposition%20Movement%20Challenges%20JSF&channel=defense Global Opposition Movement Challenges JSF]</ref><ref>[https://books.google.com/books?id=4S3h8j_NEmkC&pg=PR10 The Naval Institute guide to world naval weapon systems By Norman Friedman, Introduction page x]</ref>

====Ship wakes and spray====

Synthetic aperture sidescan radars can be used to detect the location and heading of ships from their wake patterns.<ref>{{Cite journal |last1=Reed |first1=Arthur M. |last2=Milgram |first2=Jerome H. |date=1 January 2002 |title=Ship Wakes and Their Radar Images |url=https://zenodo.org/record/1234971 |journal=Annual Review of Fluid Mechanics |volume=34 |issue=34 |pages=469–502 |bibcode=2002AnRFM..34..469R |doi=10.1146/annurev.fluid.34.090101.190252}}</ref> These are detectable from orbit.<ref>{{Cite journal |doi=10.3390/rs9111107 |title=Performance Analysis of Ship Wake Detection on Sentinel-1 SAR Images |journal=Remote Sensing |volume=9 |issue=11 |pages=1107 |year=2017 |last1=Graziano |first1=Maria |last2=Grasso |first2=Marco |last3=d'Errico |first3=Marco |bibcode=2017RemS....9.1107G|doi-access=free}}</ref> When a ship moves through a seaway it throws up a cloud of spray which can be detected by radar.<ref>{{cite book |chapter-url=https://www.researchgate.net/publication/271464016 |doi=10.1109/IGARSS.2013.6723723|chapter=Radar backscattering from sea foam and spray |title=2013 IEEE International Geoscience and Remote Sensing Symposium - IGARSS |year=2013 |last1=Raizer |first1=Victor |pages=4054–4057 |isbn=978-1-4799-1114-1 |s2cid=32858575}}</ref>