Editing Radar cross section (section)

{{Short description|Strength of an object's radar echo}}
{{more footnotes needed|date=January 2014}}
[[File:Radar cross section of metal sphere from Mie theory.svg|thumb|300px|A normalized [[log–log plot]] of the radar cross section of a metallic sphere as a function of the radiation frequency. For cases in which the circumference of the sphere is less than three-fourths of the wavelength, the radar cross section is approximately proportional to  ƒ<sup>4</sup> (ƒ=the frequency of radiation). For cases in which the circumference of the sphere is greater than 20 wavelengths, the radar cross section is approximately the same as the physical cross section of the sphere. This plot is known as [[Mie scattering]].]]
[[Image:Sigma invader RCS.png|thumb|300px|A typical RCS diagram (using the [[A-26 Invader]] as the scattering object). The diagram is a [[polar plot]] of the RCS as a function of angle, with the image of the plane in the center helping to relate the angles in the diagram to physical features of the scattering object]]

'''Radar cross-section''' ('''RCS'''), denoted &sigma;, 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.<ref name="ReferenceA">{{YouTube|_GDBtD6qBeg|"Radar Cross Section, Optical Theorem, Physical Optics Approx, Radiation by Line Sources"}}</ref>

An object reflects a limited amount of radar energy back to the source. The factors that influence this include:<ref name="ReferenceA"/>
*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 [[angle of incidence (optics)|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 {{em|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 {{em|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 missile]]s.<ref name=":0" /> RCS data for current military aircraft is mostly highly classified.

{{anchor|Normlized|NRCS}}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 &sigma;<sup>0</sup> or &sigma;<sub>0</sub> ("sigma nought"), which is the average radar cross-section of a set of objects per unit area:
:<math>\sigma^0 = \left\langle {\sigma\over{A}} \right\rangle </math>
where:
* &sigma; is the radar cross-section of a particular object, and
* A is the area on the ground associated with that object.<ref>{{cite book|last=Ulaby |first=Fawwaz |title= Microwave Remote Sensing: Active and Passive, Volume 2| date=1986|publisher= Artech House, Inc. |isbn= 978-0-89006-191-6|page=463}}</ref>
The NRCS has units of area per area, or {{sfrac|m<sup>2</sup>|m<sup>2</sup>}} in [[MKS_system_of_units|MKS]] units.