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Chaff RCS Modelling By Neil Kruger Supervisor: Prof. KD Palmer University of Stellenbosch

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Slide 2 © CSIR Introduction Chaff Background Dipole RCS Dipole Spatial Average RCS Chaff Cloud RCS Screening Effect of Chaff GUI Tool Summary

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Slide 3 © CSIR Chaff Background Chaff consist of very thin dipoles cut to resonant length With different cut lengths a larger radar bandwidth can be covered Dispensed in the atmosphere to form a cloud of scatterers Dispensed by dropping or firing from ships or aircraft. Chaff can be used in different missions, but overall the purpose of chaff is to mask the radar target

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Slide 4 © CSIR Dipole RCS - Single Dipole Orientation Numerous factors influence the RCS a chaff cloud To model a chaff cloud as a whole, one first needs to understand the RCS behavior of a single dipole element This initial investigation was done analytically

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Slide 5 © CSIR Dipole RCS - Analytical Model

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Slide 6 © CSIR Dipole Single Orientation RCS - Analytical Results vs. FEKO

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Slide 7 © CSIR Dipole Single Orientation RCS - Analytical Results with Error Correction

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Slide 8 © CSIR Dipole Spatial Average RCS The RCS of a dipole can be calculated for any orientation, but this is limited to the resonant frequency. Literature addressing this problem by O. Einarsson is available The original Einarsson paper was obtainable but a revised paper was not, so it was decided to direct the modeling approach from a analytical to a computational approach.

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Slide 9 © CSIR Dipole Spatial Average RCS - Literature From literature the average value was found to differ between 0.15λ² and 0.28λ² depending on approach used. Further literature study grouped these values as below 0.15λ² λ² for a dipole uniformly distributed over a sphere 0.27λ² λ² for a dipole uniformly distributed over a disc 0.22λ² is the value associated with the Scattering Cross Section For SCS the polarization is not taken into account

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Slide 10 © CSIR Dipole Spatial Average RCS - Dipole Bistatic Spatial Average RCS at Resonance

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Slide 11 © CSIR Dipole Spatial Average RCS - Results

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Slide 12 © CSIR Dipole Spatial Average RCS - Results

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Slide 13 © CSIR Chaff Cloud RCS The next step is modeling the RCS of a chaff cloud. Simple mathematical equations exist to address this problem analytically. These equations are however limited to dipoles at resonance The chaff cloud modeling needs to be investigated computationally.

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Slide 14 © CSIR Chaff Cloud RCS - Back Scatter RCS Simple relationship exists for calculating the backscatter RCS of a chaff cloud: This simple equation is well known for sparsely spaced chaff clouds with negligible inter-dipole coupling and will the formulation will not be discussed

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Slide 15 © CSIR Chaff Cloud RCS - Hypothesis for Forward Scatter RCS A relationship exists between the forward scatter RCS (being coherent) and the number of dipoles, such that the forward scatter RCS is directly proportional to N²:

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Slide 16 © CSIR Chaff Cloud RCS - Modeling a Chaff Cloud Creating a sphere of randomly orientated and uniformly distributed dipoles

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Slide 17 © CSIR Chaff Cloud RCS - Modelling a Chaff Cloud A 1m³ spherical chaff cloud was simulated with an increasing dipole density, to compare analytical and computational back scatter results Results were averaged over 15 simulations to determine a statistical average The forward scatter RCS was also averaged and the proportional constant was derived as k = 0.07

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Slide 18 © CSIR Chaff Cloud RCS - Average RCS Plot over 15 Simulations

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Slide 19 © CSIR Chaff Cloud RCS - Forward Scatter and Back Scatter Results Simulations results coincide within 2dB from Analytical results

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Slide 20 © CSIR Chaff Cloud RCS - Coupling

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Slide 21 © CSIR Chaff Cloud RCS - Coupling 1 dB compression density (N/λ³) 3 dB compression density (N/λ³) Literature Back Scatter (FEKO) Forward Scatter (FEKO) These values serve as guidelines for applying the analytical formulation

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Slide 22 © CSIR Screening effect of Sparse Clouds Chaff’s primary application is as a military defense mechanism to avoid detection or attack by adversary defense systems. A chaff cloud forms the EM equivalent of a visual smoke screen that can temporarily hide the target from radar. This is known as the screening effect of chaff or “shadowing” and will be discussed.

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Slide 23 © CSIR Screening effect of Sparse Clouds - Hypothesis A relationship exists between the forward scattering of a chaff cloud and that of a solid sphere so that the E-field behind the cloud can be modeled in terms of this relationship: The formulation of the hypothesis will be explained at the hand of the following figures…

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Slide 24 © CSIR Screening effect of Sparse Clouds - Hypothesis

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Slide 25 © CSIR Screening effect of Sparse Clouds - Hypothesis

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Slide 26 © CSIR Screening effect of Sparse Clouds - Hypothesis

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Slide 27 © CSIR Screening effect of Sparse Clouds - Hypothesis

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Slide 28 © CSIR Screening effect of Sparse Clouds - Initial Simulation

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Slide 29 © CSIR Screening effect of Sparse Clouds - Initial Simulation Results

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Slide 30 © CSIR Screening effect of Sparse Clouds - Simulation investigating the Hypothesis

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Slide 31 © CSIR Screening effect of Sparse Clouds - Near field results for increasing N and constant density

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Slide 32 © CSIR Screening effect of Sparse Clouds - Proportional constants

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Slide 33 © CSIR Screening effect of Sparse Clouds - Near field results for increasing N and increasing density

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Slide 34 © CSIR Screening effect of Sparse Clouds - Near field results at high densities

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Slide 35 © CSIR Screening effect of Sparse Clouds - Near field results vs. Hypothesis

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Slide 36 © CSIR Screening effect of Sparse Clouds - Near field results vs. Hypothesis

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Slide 37 © CSIR Screening effect of Sparse Clouds - Conclusion It is thus possible to accurately model the near field behavior of a chaff cloud on small scale This is however limited to low density chaff clouds at resonance Further investigation and modeling is still possible

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Slide 38 © CSIR GUI Tool Demo

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Slide 39 © CSIR Summary

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Slide 40 RCS Screening Theory + Corr. Term σ avg, f 0 σ avg, GHz σ cloud GUI Theory + Postulate σ forward, f 0 Postulate screening, f 0 c

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Slide 41 Questions?

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Slide 42 © CSIR 2006

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Slide 44 © CSIR GUI Tool

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