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 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 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:
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 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.C. A. Balanis, "Advanced Engineering Electromagnetics", 2nd ed. New York, NY, USA: Wiley, 2012.
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 statement can be understood by examining the monostatic (radar transmitter and receiver co-located) radar equation one term at a time:
where = transmitter's input power (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 as a correction factor that makes the radar equation "work out right" for the experimentally observed ratio of . However, RCS 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 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.
Factors
Size As a rule, the larger an object, the stronger its radar reflection and thus 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.
Material 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 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.
Radar absorbent paint 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 is converted to heat rather than being reflected.
Shape, directivity and orientation 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 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.
Smooth surfaces The relief of 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.
Measurement The size of a target's image on radar is measured by the radar cross section or RCS, often represented by the symbol σ and expressed in square meters. This does not equal geometric area. A perfectly conducting sphere of projected cross sectional area 1 m2 (i.e. a diameter of 1.13 m) will have an RCS of 1 m2. For radar wavelengths much less than the diameter of the sphere, RCS is independent of frequency. Conversely, a square flat plate of area 1 m2 will have an RCS of σ = 4π A2 / λ2 (where A=area, λ=wavelength), or 139.62 m2 at 1 GHz if 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,F-22 Raptor Stealth GlobalSecurity.org 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 the physical profile smaller. Rather, by reflecting much of the radiation away or by absorbing it, the target achieves a smaller radar cross section.Bill Sweetman, Unconventional Weapon: What we learned about stealth technology from the combat career of the F-117, Smithsonian Air & Space Magazine, 01 January 2008
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 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 is an anechoic chamber with a reflector to simulate far field conditions.
Typical values for a centimeter wave radar are:Radar Cross Section Measurements (8-12 GHz)Ship RCS Table
Insect: 0.00001 m2 Bird: 0.01 m2 Stealth aircraft: <0.1 m2 (e.g. F-117A: 0.001 m2) Surface-to-air-missile: ≈0.1 m2 Human: 1 m2 small combat aircraft: 2–3 m2 large combat aircraft: 5–6 m2 Cargo aircraft: up to 100 m2 Coastal trading vessel (55 m length): 300–4000 m2 Corner reflector with 1.5 m edge length: ≈20,000 m2www.radartutorial.eu (Radar Cross Section (RCS))M. Skolnik: Introduction to radar systems. 2nd Edition, McGraw-Hill, Inc., 1980, p. 44 Frigate (103 m length): 5000–100,000 m2 Container ship (212 m length): 10,000–80,000 m2
Calculation Quantitatively, RCS is calculated in three-dimensions as
Where is the RCS, is the incident power density measured at the target, and is the scattered power density seen at a distance away from the target.
In electromagnetic analysis this is also commonly written as
where and are the far field scattered and incident electric field intensities, respectively.
In 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 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, 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.