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49-417: (Redirected from J-12 ) J12 may refer to: Vehicles [ edit ] Aircraft [ edit ] Nanchang J-12 , a prototype Chinese fighter Automobiles [ edit ] Hispano-Suiza J12 , a French luxury car Subaru J12 , a Japanese hatchback Ships [ edit ] HSwMS  Sundsvall  (J12) , a Visby -class destroyer of

98-427: 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

147-485: A letter–number combination. If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=J12&oldid=1213431108 " Category : Letter–number combination disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Nanchang J-12 In 1969,

196-620: A lighter area ruled fuselage, and revised intake. In 1977 the development of J-12 was abandoned, due to inadequate firepower and engine thrust and also likely due to the introduction of the Chengdu J-7 , which offered superior performance and was based on the Soviet MiG-21 F. In addition, the concept of aerial guerilla warfare was by this time considered to be untenable. The J-12 prototypes had accumulated 61 hours in 135 flights by 1977. In 1990s, Lu Xiaopeng proposed upgrading

245-406: 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

294-436: 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

343-514: 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

392-412: 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

441-425: 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

490-409: 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

539-569: 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 ,

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588-492: 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

637-440: Is a measure of how detectable an object 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

686-443: 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}}

735-428: 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

784-474: 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

833-704: 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

882-453: 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,

931-409: 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

980-490: 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

1029-474: 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

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1078-589: 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

1127-437: 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

1176-457: 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

1225-507: The J-12's fighter design with a reduced Radar cross-section to make the J-12 stealthy, and suggested a modified J-12 fighter to a carrier based fighter for PLA Navy , but none of the proposals were accepted. Data from Chinese Aircraft:China's aviation industry since 1951 General characteristics Performance Armament Radar cross-section Radar cross-section ( RCS ), denoted σ, also called radar signature ,

1274-1026: The PLAAF issued an order to build a small, inexpensive, STOL (short takeoff and landing) lightweight fighter in order to replace the MiG-19 . Two designs were submitted, namely the Shenyang J-11 and the Nanchang J-12. Prototypes of the J-12 were designed by Lu Xiaopeng and built by the Nanchang Aircraft Manufacturing Company (NAMC) . The J-12 was a small single-seat jet fighter with low-set, swept wings , swept control surfaces, tubular fuselage, and nose intake with small or absent shock cone and flight testing began on 26 December 1970. Due to less than satisfactory performance, three additional J-12I prototypes were built with improvements such as simplified control surfaces ,

1323-410: 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

1372-406: 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

1421-549: The Swedish Navy Other uses [ edit ] Chanel J12 , a Swiss-made watch County Route J12 (California) Triangular bipyramid , a Johnson solid (J 12 ) Marlboro Pike Line , a bus route in the Washington Metro Area Viral pneumonia [REDACTED] Topics referred to by the same term This disambiguation page lists articles associated with the same title formed as

1470-415: 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

1519-421: 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

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1568-410: 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

1617-407: 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

1666-630: 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

1715-487: 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

1764-494: 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

1813-416: 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

1862-435: 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

1911-478: 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

1960-417: 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

2009-415: 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

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2058-412: 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

2107-417: 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

2156-426: 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

2205-405: 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

2254-419: 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

2303-412: 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

2352-453: 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

2401-487: 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,

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