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By Neil Kruger Supervisor: Prof. KD Palmer University of StellenboschChaff RCS Modelling By Neil Kruger Supervisor: Prof. KD Palmer University of Stellenbosch
Introduction Chaff background Chaff cloud characteristicsUsing Matlab & FEKO Modelling development & results Dipole RCS Characteristics Dipole RCS Simulation Conclusion © CSIR
Chaff Background Chaff was used for the first time during World War IIThough numerous research has been done on Chaff since WWII, it still is effective as a passive countermeasure against enemy radar Chaff consist of very thin dipoles cut to resonant length With different dipole lengths a larger radar bandwidth can be covered © CSIR
Chaff Background Dispensed in the atmosphere to form a cloud of scatterers Dispensed by dropping or firing from ships, aircraft and vehicles. The technique used determines the intension, but overall the purpose of chaff is to mask the radar target © CSIR
Chaff Cloud CharacteristicsA number of factors influence the RCS a chaff cloud Aerodynamics solution (coordinates): Initial conditions Method of dispersion Fall speed Electromagnetic solution (scattering): Dipole characteristics Coupling between dipoles Masking within the chaff cloud With reasonable assumptions modelling can be simplified © CSIR
Using Matlab & FEKO Matlab code previously developed was modified & expanded for the RCS modelling Chaff is generated in Matlab from which FEKO is called up for simulation, Matlab then extracts the data for processing and interpretation Symmetry is used to speed up calculation since chaff modelling is approached statistically. Due to processing capabilities of hardware and software the number of chaff elements is still quite limited © CSIR
Modelling development & resultsSpherical Chaff Cloud Top view Right view 3D Chaff Sphere Front view © CSIR
Modelling development & resultsCreating a sphere of randomly orientated and uniformly distributed dipoles © CSIR
Modelling development & resultsRCS of a sphere Fig 2.9, Skolnik, Introduction to radar systems 2πa/λ = 39 © CSIR
Modelling development & resultsDetermining average normalized RCS for a spherical chaff cloud as density increases to pieces © CSIR
Modelling development & resultsSpherical chaff cloud; Correct model vs. Error model © CSIR
Modelling development & resultsDirection of Incident Field One of the effects seen in chaff clouds are shielding within the chaff cloud self Polarization is linear and perpendicular to the direction of incidence 1000 pieces Smallest current -44.1dBA © CSIR
Modelling development & resultsDirection of Incident Field 5000 pieces pieces -56.2 dBA dBA © CSIR
Modelling development & resultsE-field line Points through the spherical region (Near-field) © CSIR
Modelling development & resultsE-field line through the spherical region (Near-field) © CSIR
Dipole RCS CharacteristicsThe average RCS of a random orientated dipole was investigated through theory and simulations From theory the average value differs 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 © CSIR
Dipole RCS Simulation Geometry of dipole in space defined by dipole length and theta & phi angle © CSIR
Dipole RCS Simulation Uniform distributed over Disc (in angle) Sphere© CSIR
Dipole RCS Simulation In determining the average RCS of a dipole the first approach led to the uniform angle distributed RCS value of 0.28λ² From a uniform spherical distributed simulation the average RCS value was found to be 0.187λ² Value was verified for incidence on the x-axis and z-axis The reason for the larger than expected value is possibly due to a finite thickness of the dipole element © CSIR
Dipole RCS Simulation Confirming the Cos^2 relationship with Ø = 90°© CSIR
Dipole RCS Simulation Ø = 0°, no cos^2 relationship© CSIR
Dipole RCS Simulation The numerical simulation data has been compared with mathematical derivations from literature The average RCS values has been found to agree quite well with the literature The individual RCS values however did not agree This is the current status of investigation… © CSIR
Conclusion The angular average RCS of a dipole has been computed and compared to literature The individual angle dipole RCS and coupling need to be investigated to determine assumptions that can simplify modelling Realistic chaff positioning and orientation must be incorporated in modelling to make it applicable Practical effects like “bird nesting” & shielding within the chaff cloud need investigation to make modelling accurate © CSIR
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