Equivalent surface radar

Equivalent surface radar (SER or RCS for radar cross-country race section in English) is a physical property inherent of the objects indicating the relative importance of the surface of reflection of an electromagnetic beam which they cause. Equivalent surface radar is function of the shape of the object, of the nature of sound (its) material (X) constitutive (S) as well as wavelength, angles of incidence and reflection of the radiation.

The SER is a parameter specific to any object (in fact a vehicle, plane or ship) influencing largely its susceptibility to be detected by a radar. Represented in a mathematical formula (equation) the SER is indicated by the Greek letter σ (sigma) expressed in square meter.

To present a weak SER is an essential quality for a military machine (aircraft) which will enable him of as much to better escape an unfavourable radar. Technologies of Furtivité were consequently developed with an aim of reducing the SER. The SER of an object is in general a military secrecy and is not revealed.

The diagram illustrating this article is derived from a diagram published by Mr. I. Skolnik (see bibliography) representing the SER σ/σ₀ measured in an experimental way on the bomber B-26 Invader for a frequency of 3 GHz.

Measure

The SER of an object is measured in a typical way using radars. That can be carried out in the open air or in a anechoic Chambre absorbing in their totality the electromagnetic waves of the emitted frequency.

Typical values measured using a radar with centimetric waves:

furtive planes : 0,01 to 0,02 m ²
anti-aircraft missiles: approximately 0,1 m ²
fighter plans: 0,1 to 1 m ²
bombers: 1 to 2 m ²
heavy transport aircraft and civil commercial aircraft: up to 50 m ².

Calculation

The SER is function many factors. Its calculation is not possible that for objects of simple form. It depends on the form of the object and wavelength, more precisely relationship between dimensions enlightened structure and wavelength. Quantitatively, the SER expresses an effective surface which reflects the incidental beam in an isotropic way (homogeneous) in space. In three dimensions the SER $\ sigma$ is defined like equal to:

\ sigma = 4 \ pi R^ {2} \ frac {P_ {\ rm S}} {P_ {\ rm I}} \; .

Where $P_ {\ rm I}$ is the density of the power on the level of the target enlightened by the radar and $P_ {\ rm S}$ the density of the power dissipated at a distance from $R$ of this target.

That can be written also in the following way:

\ sigma = 4 \ pi R^ {2} \ frac

Calculation by simulation

Instead of measuring it on a finished apparatus, it became current to today calculate the SER before thanks to simulation programs. It is thus possible to calculate and optimize with lower costs the SER of the fighters as of the phase of design.

Reduction of the SER

One can reduce the SER by modifying the shape of an object, by using materials absorbing or permeable with the radiation or even by reducing in an active way the considered power. A plane as Lockheed F-117 has an especially studied form preventing the reflection of the major part of the beam radar in direction of the transmitter. One almost exclusively obtains that by using plane surfaces and by avoiding the right angles. The right angles cause on the whole the practically total reflection of an incidental radiation in direction of its transmitter. As example, the Réflecteur S for radars (maritime Navigation) are composed of three metal surfaces assemblies with right angle.

The materials absorbing cause or the transformation of the radiation into a warming of the target or, if the wavelength is known, by eliminating it by rotation from phase.

The active reduction of the SER takes place by the emission of waves of interferences and is not used yet in practice.

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