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Radar cross section

From Academic Kids

Radar cross section (RCS) is a description how an object reflects an incident electromagnetic wave. For an arbitrary object, the RCS is highly dependent on the radar wavelength and incident direction of the radar wave.

Quantitatively, the RCS is an effective surface area that intercepts the incident wave that and scatters the energy isotropically in space. The RCS, <math>\sigma<math> is defined in three-dimensions as

<math>\sigma = 4 \pi R^{2} \frac{P_{s}}{P_{i}}<math>

Where <math>P_{i}<math> is the incident power density measured at the target, and <math>P_{s}<math> is the scattered power density seen at a distance <math>R<math> away from the target. In electromagnetic analysis this is also commonly written as

<math>\sigma = 4 \pi R^{2} \frac{|E_{s}|^{2}}{|E_{i}|^{2}}<math>

where <math>E_{i}<math> and <math>E_{s}<math> are the incident and scattered electric field intensities, respectively.

The RCS is integral to the development of radar stealth technology, particularly in applications involving aircraft and ballistic missiles. RCS data for current military aircraft are almost all highly classified.

Contents

Measurement

Measurement of a target's RCS is performed at a radar reflectivity range. The first type of range is an outdoor range where the target is positioned on the pylon some distance down-range from the transmitters. Such a range eliminates the need for placing radar absorbers behind the target, however multipath effects due to 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.

Calculation

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 peformed in a short time at low cost, whereas use of a measurement range is often time-consuming, expensive and error-prone.

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 such as the Method of Moments, Geometrical Optics, Physical Optics, and the Physical Theory of Diffraction. RCS prediction software are often run on large supercomputers and employ high-resolution CAD models of real radar targets.

Reduction

RCS reduction is chiefly important in stealth technology. With smaller RCS, aircraft may better evade radar detection, whether it be from land-based installations or other aircraft.

Purpose Shaping

Purpose shaping is an RCS reduction technique in which 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 aircraft’s direction of flight. Purpose-shaping techniques can be seen in the design of surface faceting on the F-117A Nighthawk stealth fighter. This aircraft, designed in the late l970s 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 Joint Strike Fighter continue the trend in purpose shaping and promise to have even smaller monostatic RCS.

Active Cancellation

In active cancellation techniques, the target aircraft generates a radar signal equal in intensity but opposite in phase to the predicted reflection of an incident radar signal. This creates destructive interference between 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. The implementation of active cancellation techniques 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.

RAM

The third RCS reduction technique for aircraft and missiles is the use of RAM either in the original construction or as an addition to highly reflective surfaces. There are two types of RAM: resonant; and non-resonant. Resonant or “lossy” materials are applied to the reflecting surfaces of the target. The thickness of the material corresponds to one-quarter wavelength of the expected illuminating radar-wave. The incident radar energy is reflected from the outside and inside surfaces of the RAM to create a destructive interference pattern. This results in the cancellation of the reflected energy. Non-resonant RAM uses ferrite particles suspended in epoxy or paint to reduce the reflectivity of the surface to incident radar waves. Because the non-resonant RAM dissipates incident radar energy over a larger surface area, it usually results in an increase in surface temperature, thus reducing RCS at the expense of an increase in infrared signature. A major advantage of non-resonant RAM is that it can be effective over a broad range of frequencies, whereas resonant RAM is limited to a narrow range of design frequencies.

References

  • Shaeffer, Tuley and Knott. Radar Cross Section. SciTech Publishing, 2004. ISBN 1891121251.
  • Harrington, Roger F. Time-Harmonic Electromagnetic Fields. McGraw-Hill, Inc., 1961. ISBN 070267456.
  • Balanis, Constantine A. Advanced Engineering Electromagnetics. Wiley, 1989. ISBN 0471621943.

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