Radar cross section (RCS), denoted σ, also called radar signature, 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:
the material with which the target is made; the size of the target relative to the wavelength of the illuminating radar signal; the absolute size of the target; the incident angle (angle at which the radar beam hits a particular portion of the target, which depends upon the shape of the target and its orientation to the radar source); the reflected angle (angle at which the reflected beam leaves the part of the target hit; it depends upon incident angle); the polarization of the radiation transmitted and received with 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 an RCS because RCS is a property of the target's reflectivity. Radar cross section 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 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 a related quantity called the normalized radar cross section (NRCS), also known as differential scattering coefficient or radar backscatter coefficient, denoted σ0 or σ0 ("sigma nought"), which is the average radar cross section of a set of objects per unit area:
σ 0 = ⟨ σ A ⟩ {\displaystyle \sigma ^{0}=\left\langle {\sigma \over {A}}\right\rangle }
where:
σ is the radar cross section of a particular object, and A is the area on the ground associated with that object. The NRCS has units of area per area, or m2/m2 in MKS units.
Formulation 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. This statement can be understood by examining the monostatic (radar transmitter and receiver co-located) radar equation one term at a time:
P r = P t G t 4 π r 2 σ 1 4 π r 2 A e f f {\displaystyle P_{r}={{P_{t}G_{t}} \over {4\pi r^{2}}}\sigma {{1} \over {4\pi r^{2}}}A_{\mathrm {eff} }}
where
P t {\displaystyle P_{t}} = transmitter's input power (watts)
G t {\displaystyle G_{t}} = gain of the radar transmit antenna (dimensionless)
r {\displaystyle r} = distance from the radar to the target (meters)
σ {\displaystyle \sigma } = radar cross section of the target (meters squared)
A e f f {\displaystyle A_{\mathrm {eff} }} = effective area of the radar receiving antenna (meters squared)
P r {\displaystyle P_{r}} = power received back from the target by the radar (watts) The
P t G t 4 π r 2 {\textstyle {{P_{t}G_{t}} \over {4\pi r^{2}}}}
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