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Radar cross-section (RCS) is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected. An object reflects a limited amount of radar energy. A number of different factors determine how much electromagnetic energy returns to the source such as: *material of which the target is made; *absolute size of the target; *relative size of the target (in relation to the wavelength of the illuminating radar); *the incident angle (angle at which the radar beam hits a particular portion of target which depends upon shape of target and its orientation to the radar source); *reflected angle (angle at which the reflected beam leaves the part of the target hit, it depends upon incident angle); *the polarization of transmitted and the received radiation in respect to the orientation of the target 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, smooth 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. 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. In those situations, it is useful to use a related quantity called the ''differential scattering coefficient'' (also called the ''normalized radar cross-section'' or ''backscatter coefficient'') σ0 ("sigma naught"), which is the average radar cross-section of a set of objects per unit area: : where: * RCSi is the radar cross-section of a particular object, and * Ai is the area on the ground associated with that object. ==Definition== 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.〔Skolnick, M.I., Introduction to Radar Systems, McGraw-Hill, 1980.〕 This is a complex statement that can be understood by examining the monostatic (radar transmitter and receiver co-located) radar equation one term at a time: : where * = power transmitted by the radar (watts) * = gain of the radar transmit antenna (dimensionless) * = distance from the radar to the target (meters) * = radar cross-section of the target (meters squared) * = effective area of the radar receiving antenna (meters squared) * = power received back from the target by the radar (watts) 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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Radar cross-section」の詳細全文を読む スポンサード リンク
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