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64-480: Sea Shadow ( IX-529 ) was an experimental stealth ship built by Lockheed for the United States Navy to determine how a low radar profile might be achieved and to test high-stability hull configurations that have been used in oceanographic ships. Sea Shadow was built in 1984 to examine the application of stealth technology on naval vessels and was used in secret until a public debut in 1993. In addition,

128-427: A Leontovich 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

192-409: A SWATH (small-waterplane-area twin hull) design. Below the water were submerged twin hulls , each with a propeller, aft stabilizer, and inboard hydrofoil . The portion of the ship above water was connected to the hulls via the two angled struts. The SWATH design helped the ship remain stable in rough water up to sea state 6 (wave height of 18 feet (5.5 m) or "very rough" sea). The shape of

256-415: A low RCS ; noise reduction plays a role in naval stealth because sound is conducted better in water than air. Some of the techniques used include muffled exhaust systems, modified propeller shapes, and pump-jets . The shape of the hull can also have a great effect on the reduction of the noise from a ship. Another major element is signal emission control. Modern warships emit much electromagnetic radiation in

320-406: A radar system with a given target to be analysed independent of the radar and engagement parameters. In general, RCS is a function of the orientation of the radar and target. A target's RCS depends on its size, reflectivity of its surface, and the directivity of the radar return caused by the target's geometric shape. As a rule, the larger an object, the stronger its radar reflection and thus

384-436: A range eliminates the need for placing radar absorbers behind the target, however multi-path interactions with the ground must be mitigated. An anechoic chamber is also commonly used. In such a room, the target is placed on a rotating pillar in the center, and the walls, floors and ceiling are covered by stacks of radar absorbing material. These absorbers prevent corruption of the measurement due to reflections. A compact range

448-508: A related quantity called the normalized radar cross-section ( NRCS ), also known as differential scattering coefficient or radar backscatter coefficient , denoted σ or σ 0 ("sigma nought"), which is the average radar cross-section of a set of objects per unit area: where: The NRCS has units of area per area, or ⁠ m / m ⁠ in MKS units. Informally, the RCS of an object

512-450: A ship have existed for centuries or even millennia. In designing a ship with a reduced radar signature, the main concerns are radar beams originating near or slightly above the horizon (as seen from the ship) coming from distant patrol aircraft, other ships, or sea-skimming anti-ship missiles with active radar seekers . Therefore, the shape of the ship avoids vertical surfaces, which are effective at reflecting such beams directly back to

576-400: A ship stand out in the ocean, making it easier to spot. Because it is possible to see infrared emissions through features that would normally hide a ship such as fog, or a smoke screen, many detection platforms like patrol aircraft, UAVs, and satellites often have the ability to see multiple bands in the infrared spectrum. This necessitates the control of thermal emissions. The most common method

640-412: A spherical target), and the RCS is a hypothetical area. In this light, RCS can be viewed as a correction factor that makes the radar equation "work out right" for the experimentally observed ratio of P r / P t {\textstyle P_{r}/P_{t}} . However, RCS is a property of the target alone and may be measured or calculated. Thus, RCS allows the performance of

704-751: A stealth ship is visual camouflage . This area is probably the oldest form of stealth, with records going back almost as far as the writing of ancient mariners using visual tricks to make their ships harder to spot. Although still relevant, this area has taken on lesser importance with the advent of long-range radar. Just like choices in shaping, the choice of materials affects the RCS of a ship. Composites like fiberglass and carbon fiber are effective blockers of radar and give smaller vessels an advantage in further RCS reductions. However, composites are fragile and often unsuited to larger ships or ships that expect to take fire, although new laminates can partially negate some of

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768-425: A surface could contain indentations that act as corner reflectors which would increase RCS from many orientations. This could arise from open bomb-bays , engine intakes, ordnance pylons, joints between constructed sections, etc. Also, it can be impractical to coat these surfaces with radar-absorbent materials . The size of a target's image on radar is measured by the radar cross section or RCS, often represented by

832-409: A target can be detected for a given radar configuration varies with the fourth root of its RCS. 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

896-569: A variety of analytic and numerical methods, but changing levels of military interest and the need for secrecy have made the field challenging, nonetheless. The field of solving Maxwell's equations through numerical algorithms is called computational electromagnetics , and many effective analysis methods have been applied to the RCS prediction problem. RCS prediction software are often run on large supercomputers and employ high-resolution CAD models of real radar targets. High frequency approximations such as geometric optics , physical optics ,

960-492: 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-71 Blackbird and other aircraft were painted with a special " iron ball paint " that consisted of small metallic-coated balls. Radar energy received

1024-448: Is a sandwich construction comprising a PVC core with carbon fiber and vinyl laminate. Avoidance of right angles in the design results in a smaller radar signature, reducing the ship's detection range. The Royal Navy 's Type 45 destroyer has similarities to the Visby class, but is much more conventional, employing traditional steel instead of carbon fiber. Like Visby , its design reduces

1088-443: Is an anechoic chamber with a reflector to simulate far field conditions. Typical values for a centimeter wave radar are: Quantitatively, RCS is calculated in three-dimensions as Where σ {\displaystyle \sigma } is the RCS, S i {\displaystyle S_{i}} is the incident power density measured at the target, and S s {\displaystyle S_{s}}

1152-399: Is by radar . 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 an RCS because RCS is a property of the target's reflectivity. Radar cross-section

1216-428: 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. Several methods exist. The distance at which

1280-470: Is converted to heat rather than being reflected. The surfaces of the F-117A 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 rounded surfaces which are normal at some point to the radar source. As any ray incident along

1344-704: 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 (RAM) 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. 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. Thin non-resonant or broad resonance coatings can be modeled with

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1408-453: 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,

1472-409: Is the scattered power density seen at a distance r {\displaystyle r} away from the target. In electromagnetic analysis this is also commonly written as where E s {\displaystyle E_{s}} and E i {\displaystyle E_{i}} are the far field scattered and incident electric field intensities, respectively. In

1536-420: Is to mix any hot gasses emitted by the main source of heat—the engines' exhaust—with cold air to dilute the signature and make it harder to pick out the ship from the background warmth. Another method vents the exhaust into the water, although this increases the ship's acoustic signature. For the hull, cool water can be actively distributed across the hull of the ship. Another less crucial but still relevant part of

1600-490: Is to utilize metasurfaces which can redirect scattered waves without altering the geometry of the target. Such metasurfaces can primarily be classified in two categories: (i) Checkerboard metasurfaces, (ii) Gradient index metasurfaces. 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

1664-474: Is used to detect airplanes 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 the 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

1728-510: The Naval Vessel Register , which, as a U.S. government publication, is in the public domain . The entry can be found here . Stealth ship These techniques borrow from stealth aircraft technology, although some aspects such as wake and acoustic signature reduction ( acoustic quieting ) are unique to stealth ships' design. Although radar cross-section (RCS) reduction is a fairly new concept, many other forms of masking

1792-581: The 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 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

1856-758: The San Diego Naval Station until September 2006, when it was relocated with the Hughes Mining Barge to the Suisun Bay Reserve Fleet in Benicia, California . Until 2006, Sea Shadow and the HMB-1 were maintained and operated by Lockheed Martin for the US Navy. The vessels were available for donation to a maritime museum. The USNS Impeccable and Victorious ocean surveillance ships have inherited

1920-437: The boundary element method ( method of moments ), finite difference time domain method ( FDTD ) and finite element methods are limited by computer performance to longer wavelengths or smaller features. Though, for simple cases, the wavelength ranges of these two types of method overlap considerably, for difficult shapes and materials or very high accuracy they are combined in various sorts of hybrid method . RCS reduction

1984-457: The geometric theory of diffraction , the uniform theory of diffraction and the physical theory of diffraction are used when the wavelength is much shorter than the target feature size. Statistical models include chi-square , Rice , and the log-normal target models. These models are used to predict likely values of the RCS given an average value, and are useful when running radar Monte Carlo simulations. Purely numerical methods such as

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2048-410: 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

2112-406: The RCS relates to the two scattering phenomena that takes place at the antenna. When an electromagnetic signal falls on an antenna surface, some part of the electromagnetic energy is scattered back to the space. This is called structural mode scattering. The remaining part of the energy is absorbed due to the antenna effect. Some part of the absorbed energy is again scattered back into the space due to

2176-415: 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

2240-421: The control options described below. 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 (multistatic) radars . Purpose-shaping can be seen in the design of surface faceting on

2304-410: The design phase, it is often desirable to employ a computer to predict what the RCS will look like before fabricating an actual object. Many iterations of this prediction process can be performed in a short time at low cost, whereas use of a measurement range is often time-consuming, expensive and error-prone. The linearity of Maxwell's equations makes RCS relatively straightforward to calculate with

2368-407: 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 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

2432-630: The dimensions of power (watts), and represents a hypothetical total power intercepted by the radar target. The second 1 4 π r 2 {\textstyle {{1} \over {4\pi r^{2}}}} term represents isotropic spreading of this intercepted power from the target back to the radar receiver. Thus, the product P t G t 4 π r 2 σ 1 4 π r 2 {\textstyle {{P_{t}G_{t}} \over {4\pi r^{2}}}\sigma {{1} \over {4\pi r^{2}}}} represents

2496-639: The emitter. Retro-reflective right angles are eliminated to avoid the cat's eye effect . A stealthy ship shape can be achieved by constructing the hull and superstructure with a series of slightly protruding and retruding surfaces. Furthermore, round shapes on the ship are eliminated or covered up, examples being smokestacks and gun turrets. Also, cavities that present a horizontal face are to be eliminated since they are very visible to radar. To bypass these limitations, many ships use features such as panels that cover reflective surfaces or use alternate designs of hardware. Additionally, efforts are made to minimize gaps on

2560-425: The form of radar, radio, and bleed-off from the ship's electrical systems. These all can be used to track a ship, and thus modern stealth ships often have a mode that switches off many of the electronic emissions ( EMCON ), the downside being the ship has to rely on passive sensors and is unable to easily send messages further than the line of sight. Also of importance are thermal emissions. A heat signature can make

2624-487: The greater its RCS. Also, radar of one band may not even detect certain size objects. For example, 10 cm (S-band radar) can detect rain drops but not clouds whose droplets are too small. Materials such as metal are strongly radar reflective and tend to produce strong signals. Wood and cloth (such as portions of airplanes 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

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2688-494: 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 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

2752-416: The ideal surface impedance is equal to the 377 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 0 × η 1 = 377 Ω , where η 0 and η 1 are perpendicular components of

2816-435: The impedance mismatches, called antenna mode scattering. For the bistatic radar configuration—transmitter and receiver separated (not co-located) -- the bistatic radar cross-section ( BRCS ) is a function of both the transmitter-target orientation and the receiver-target orientation. A normalized bistatic radar cross-section ( NBRCS ) or bistatic normalized radar cross-section ( BNRCS ) may also be defined, similar to

2880-478: 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. For the case of an antenna the total RCS can be divided into two separate components as Structural Mode RCS and Antenna Mode RCS. The two components of

2944-417: The normal will reflect back along the normal, rounded surfaces make for a strong reflected signal. From the side, a fighter aircraft will present a much larger area than the same aircraft viewed from the front. All other factors being equal, the aircraft will have a stronger signal from the side than from the front; hence the orientation of the target relative to the radar station is important. The relief of

3008-415: The physical profile smaller. Rather, by reflecting much of the radiation away or by absorbing it, the target achieves a smaller radar cross section. Measurement of a target's RCS is performed at a radar reflectivity range or scattering range . The first type of range is an outdoor range where the target is positioned on a specially shaped low RCS pylon some distance down-range from the transmitters. Such

3072-412: The radar is perpendicular to the flat surface. At off-normal incident angles , energy is reflected away from the receiver, reducing the RCS. Modern stealth aircraft are said to have an RCS comparable with small birds or large insects, though this varies widely depending on aircraft and radar. If the RCS was directly related to the target's cross-sectional area, the only way to reduce it would be to make

3136-417: The radar transmitter produces at the target. This power density is intercepted by the target with radar cross-section σ {\textstyle \sigma } , which has units of area (meters squared). Thus, the product P t G t 4 π r 2 σ {\textstyle {{P_{t}G_{t}} \over {4\pi r^{2}}}\sigma } has

3200-426: The receiver is produced. This statement can be understood by examining the monostatic (radar transmitter and receiver co-located) radar equation one term at a time: where The P t G t 4 π r 2 {\textstyle {{P_{t}G_{t}} \over {4\pi r^{2}}}} term in the radar equation represents the power density (watts per meter squared) that

3264-405: 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

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3328-419: The reflected power density at the radar receiver (again watts per meter squared). The receiver antenna then collects this power density with effective area A e f f {\textstyle A_{\mathrm {eff} }} , 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

3392-490: The ship was designed to test the use of automation to reduce crew size. The ship was created by the Defense Advanced Research Projects Agency (DARPA), the U.S. Navy and Lockheed . Sea Shadow was developed and built at Lockheed's Redwood City, California , facility, inside the Hughes Mining Barge (HMB-1), which functioned as a floating drydock during construction and testing. Sea Shadow had

3456-434: The ship. Hull shapes include tumblehome hull designs, which slope inward from the waterline, and small-waterplane-area twin hulls (SWATH), which allow for better stability when using a tumblehome hull. These RCS design principles were developed by several navies independently in the 1980s using work done on aircraft RCS reduction as the starting point. Sea Shadow , which utilizes both tumblehome and SWATH features,

3520-447: The sides, along with passive cool air induction in the mack , reduces infrared signature . Overall, the destroyer's angular build makes it "50 times harder to spot on radar than an ordinary destroyer", according to Chris Johnson, a spokesman for Naval Sea Systems Command. The Swedish Navy 's Visby -class corvette is designed to elude visual detection , radar detection, acoustic detection, and infrared detection . The hull material

3584-412: The source, many protrusions like the engines, antennas, etc.). RCS is integral to the development of radar stealth technology , particularly in applications involving aircraft and ballistic missiles . RCS data for current military aircraft is mostly highly classified. In some cases, it is of interest to look at an area on the ground that includes many objects. In those situations, it is useful to use

3648-430: The stabilizer and canard method to help perform their stability-sensitive intelligence collection missions. In 2006, the U.S. Navy tried to sell Sea Shadow to the highest bidder; after the initial offering met with a lack of interest, it was listed for dismantling sale on gsaauctions.gov. The US government mandated that the buyer not sail the ship and be required to scrap it. The ship was finally sold in 2012. Sea Shadow

3712-527: The superstructure was sometimes compared to the casemate of the ironclad ram CSS  Virginia of the American Civil War . Sea Shadow had 12 bunks, one small microwave oven, a refrigerator and table. It was not intended to be mission-capable and was never commissioned, although it was listed in the Naval Vessel Register . Sea Shadow was revealed to the public in 1993 and was housed at

3776-453: The symbol σ and expressed in square meters. This does not equal geometric area. A perfectly conducting sphere of projected cross sectional area 1 m (i.e. a diameter of 1.13 m) will have an RCS of 1 m . For radar wavelengths much less than the diameter of the sphere, RCS is independent of frequency. Conversely, a square flat plate of area 1 m will have an RCS of σ = 4π A / λ (where A =area, λ =wavelength), or 139.62 m at 1 GHz if

3840-487: The trend in purpose shaping and promise to have even smaller monostatic RCS. This technique is relatively new compared to other techniques chiefly after the invention of metasurfaces . 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. One feasible solution, which has extensively been explored in recent time,

3904-455: The use of right angles. The ROC Navy 's Tuo Chiang -class corvette is a class of fast stealth multi-mission corvettes currently in service with the Republic of China (Taiwan) Navy . The ships are designed to have a low radar cross-section and evade radar detection making it difficult to detect the ship when operating closer to the coastline. Stealth technology represents more than just

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3968-425: The weaknesses. This restricts larger ships to metals like steel and aluminum alloys. To compensate, a ship may include a coating of a radar-absorbing material, although this can be expensive and may not stand up to the corrosive effects of salt water. (Year of commission) Radar cross-section Radar cross-section ( RCS ), denoted σ, also called radar signature , is a measure of how detectable an object

4032-559: Was an early US exploration of stealth ship technology. The currently-serving Zumwalt -class destroyer is a modern example of a stealth ship from the US Navy. Despite being 40% larger than an Arleigh Burke -class destroyer , its radar signature is more akin to a fishing boat, according to a spokesman for Naval Sea Systems Command ; sound levels are comparable to the Los Angeles -class submarine . The tumblehome hull and composite material deckhouse reduce radar return. Water sleeting along

4096-781: Was dismantled in 2012 by Bay Ship & Yacht Company. In the 1997 James Bond film Tomorrow Never Dies , media tycoon Elliot Carver ( Sir Jonathan Pryce ) operated a stealth ship that resembled Sea Shadow 's appearance. Christened as Sea Dolphin II in the film, the secret and stealthy floating lair was used as a plot device to attempt to initiate World War III. In the Strike series of video games, it appears in Urban Strike as an enemy unit and in Nuclear Strike as home base. [REDACTED]   This article includes information collected from

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