Radar Cross Section (RCS) and the techniques to reduce RCS of a target

By:- Namit Ohri - 12213010 Amit Vashisht – 12213004 Nikhil Harsoor – 12213011 Seemant Meena - 12116052

Introduction
Definition
Radar cross section(RCS) is the measure of a target's ability to reflect radar signals in the direction of the radar receiver, i.e. it is a measure of the ratio of backscatter power per steradian (unit solid angle) in the direction of the radar (from the target) to the power density that is intercepted by the target.
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. A larger RCS indicates that an object is more easily detected.
The conceptual definition of RCS includes the fact that not all of the radiated energy falls on the target. A target’s RCS (F) is most easily visualized as the product of three factors:
F = Projected cross section x Reflectivity x Directivity
Where,
Reflectivity: The percent of intercepted power reradiated (scattered) by the target.
Directivity: The ratio of the power scattered back in the radar's direction to the power that would have been backscattered had the scattering been uniform in all directions (i.e. isotropically).
RCS depends upon Size, Material, Radar absorbent paint, Shape, directivity and orientation and surface of a target.
For the case of an antenna the total RCS can be divided into two separate components as Structural Mode RCS and Antenna Mode RCS. The two components of the RCS relates to the two scattering phenomena that takes place at the antenna. When an electromagnetic signal falls on an antenna surface, some part of the electromagnetic energy is scattered back to the space. This is called structural mode scattering. The remaining part of the energy is absorbed due to the antenna effect. Some part of the absorbed energy is again scattered back into the space due to the impedance mismatches, called antenna mode scattering.
RCS 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. 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. Typical RCS diagram (A-26 Invader)
Techniques to reduce Radar Cross Section
Active Cancellation
With active cancellation, the target generates a radar signal equal in intensity but opposite in phase to the predicted reflection of an incident radar signal(similarly to noise cancelling ear phones). This creates destructive interference between the reflected and generated signals, resulting in reduced RCS. To incorporate active cancellation techniques, the precise characteristics of the waveforms and angle of arrival of the illuminating radar signal must be known, since they define the nature of generated energy required for cancellation. According to Electromagnetic inverse scattering theory, if the source of radiation field distribution is known, scatter characteristics and distribution of the scattering can be known. If the radar signals are limited within a small precise angle for the EM wave cancellation, the target can be invisible to radar's receiver system.

The formal definition of the RCS is:
(1)
Where is the target RCS complex root, Ei is the electric field strength of the incident signal on the target, R is the distance between the target and the radar, êr is aligned unit vector along electric polarization of the receiver, and ĒS is the vector of the scattered field. Using active cancellation means, reducing the strength incident field on the target to reduce the reflected power to the radar receiver. A target’s RCS can be reduced by reducing the target scattering intensity. According to (1); target's RCS can be measured for many scattering directions and a radar target’s scattered field direction can be identified as in (2):
(2)
The cancellation signal can be described as

Where ΔĒ is the cancellation residual field and ĒS is the target's scattering field. Complete stealth is realized when S = 0.

Shaping
The objective of shaping is to orient the target surfaces and edges to deflect energy in directions away from the radar. This cannot be done for all viewing angles within the entire sphere of solid angles because there will always be viewing angles at which surfaces are seen at normal incidence, and there the echoes will be high. The aim is usually to create a “cone-of-silence” about the target’s direction of motion.
Reduction by shaping on Aircrafts:
Ufimtsev’s theory was first adopted by the engineers in Lockheed Martin to analyse RCS for various geometric shapes. Purpose shaping is to direct most of the reflected radar waves away from the incident direction. Hence, it will create a “cone of silence” along the direction of the aircraft’s motion.
Plane Alignment According to optics and electromagnetics theories, curved surface usually reflect radar waves in a collection of directions , while in contrast, a plane only reflect them in one direction. Though SR-71 was the first aircrafts included RCS reduction at the beginning of the project , F-117(Fig.1) is recognized as the first stealth operational military aircraft. It employed “facet” methods. Typically, there is no curvature on F-117’s surface. There are only planes and sharp transitions between planes so that there will be nothing incident to the incident radar waves. Hence, no radar wave will be reflected back to the transmitter. At the same time, the number of planes and angles are kept at minimum to reduce directions of radar signal reflection. Most of the incident radar waves are deflected away from the source to achieve radar stealth.
Flying Wing
Flying Wing is an ideal stealth shape for aircrafts. It minimizes the number of leading edges, which in turn, reduces radar echo signals. German Ho 229 is the earliest stealth plane, though it was by coincidence. Its design was a flying wing. Northrop’s B-2 “Spirit” bomber also adopted flying wing shape with some “zig-zag” shape at the tail, reducing its radar echo to as small as a 0.1 m² metal object.
Radar Absorbent Material
Radar-absorbent material, or RAM, is a class of materials used in stealth technology to disguise a vehicle or structure from radar detection. A material's absorbency at a given frequency of radar wave depends upon its composition. RAM cannot perfectly absorb radar at any frequency, but any given composition does have greater absorbency at some frequencies than others; no one RAM is suited to absorption of all radar frequencies.
We intend to introduce a class of materials called double zero (DZR) meta-materials, of which the permittivity and permeability are purely imaginary [namely Re(εr) = 0 & Re(µr) = 0]. In this paper we intend to investigate the properties of wave propagation incident onto a perfectly electric conductor, coated by DZR meta-materials. Some uncommon phenomena will appear for DZR meta-materials, which have not been observed for common materials and meta-materials. We consider a perfectly electric conductor (PEC) plate covered by several layers of DZR meta-material coatings under an oblique plane wave incidence of arbitrary polarization. Several analytical formulas are derived for the realization of zero reflection from such structures. The angle of reflection in the DZR meta-materials becomes complex, which leads to the dissociation of the constant amplitude and equiphase planes. DZR meta-materials can be used for fabrication of radar absorbing materials (RAMs) for the reduction of radar cross section (RCS) of various objects, coating the interior walls and objects inside anechoic chambers, design of antennas with low side lobe levels and protection against electromagnetic interference in high speed circuits. RAMs may be designed for operation at a single frequency or in a narrow frequency band width, which may be straightforward.
Plasma Based RCS Reduction

Plasma stealth is a proposed process to use ionized gas (plasma) to reduce the radar cross section (RCS) of an aircraft. Interactions between electromagnetic radiation and ionized gas have been extensively studied for many purposes, including concealing aircraft from radar as stealth technology. Various methods might plausibly be able to form a layer or cloud of plasma around a vehicle to deflect or absorb radar, from simpler electrostatic or radio frequency (RF) discharges to more complex laser discharges. It is theoretically possible to reduce RCS in this way, but it may be very difficult to do so in practice.
When electromagnetic waves, such as radar signals, propagate into a conductive plasma, ions and electrons are displaced as a result of the time varying electric and magnetic fields. The wave field gives energy to the particles. The particles generally return some fraction of the energy they have gained to the wave, but some energy may be permanently absorbed as heat by processes like scattering or resonant acceleration, or transferred into other wave types by mode conversion or nonlinear effects. A plasma can, at least in principle, absorb all the energy in an incoming wave, and this is the key to plasma stealth. However, plasma stealth implies a substantial reduction of an aircraft's RCS, making it more difficult (but not necessarily impossible) to detect.

References * http://en.wikipedia.org/wiki/Radar_cross-section * http://www.tscm.com/rcs.pdf * http://www.radartutorial.eu/01.basics/Radar Cross Section.en.html * http://www.ijceronline.com/papers/Vol3_issue7/part 1/D0371019024.pdf * Radar Cross Section, Second Edition - By Eugene F. Knott, John Shaeffer, Michael Tuley * http://www.homepages.ucl.ac.uk/~zcapf41/workfile/GU JIATENG'S Report.pdf * http://en.wikipedia.org/wiki/Plasma_stealth * http://www.iust.ac.ir/files/ee/6a_5.pdf