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

Introduction Chaff background Chaff cloud characteristics Using Matlab & FEKO Modelling development & results Dipole RCS Characteristics Dipole RCS Simulation Conclusion © CSIR 2006 www.csir.co.za

Chaff Background Chaff was used for the first time during World War II Though 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 2006 www.csir.co.za

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 2006 www.csir.co.za

Chaff Cloud Characteristics A 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 2006 www.csir.co.za

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 2006 www.csir.co.za

Modelling development & results Spherical Chaff Cloud Top view Right view 3D Chaff Sphere Front view © CSIR 2006 www.csir.co.za

Modelling development & results Creating a sphere of randomly orientated and uniformly distributed dipoles © CSIR 2006 www.csir.co.za

Modelling development & results RCS of a sphere Fig 2.9, Skolnik, Introduction to radar systems 2πa/λ = 39 © CSIR 2006 www.csir.co.za

Modelling development & results Determining average normalized RCS for a spherical chaff cloud as density increases to 15000 pieces © CSIR 2006 www.csir.co.za

Modelling development & results Spherical chaff cloud; Correct model vs. Error model © CSIR 2006 www.csir.co.za

Modelling development & results Direction 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 2006 www.csir.co.za

Modelling development & results Direction of Incident Field 5000 pieces 10 000 pieces -56.2 dBA -69.6 dBA © CSIR 2006 www.csir.co.za

Modelling development & results E-field line Points through the spherical region (Near-field) © CSIR 2006 www.csir.co.za

Modelling development & results E-field line through the spherical region (Near-field) © CSIR 2006 www.csir.co.za

Dipole RCS Characteristics The 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 - 0.17λ² for a dipole uniformly distributed over a sphere 0.27λ² - 0.29λ² 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 2006 www.csir.co.za

Dipole RCS Simulation Geometry of dipole in space defined by dipole length and theta & phi angle © CSIR 2006 www.csir.co.za

Dipole RCS Simulation Uniform distributed over Disc (in angle) Sphere © CSIR 2006 www.csir.co.za

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 2006 www.csir.co.za

Dipole RCS Simulation Confirming the Cos^2 relationship with Ø = 90° © CSIR 2006 www.csir.co.za

Dipole RCS Simulation Ø = 0°, no cos^2 relationship © CSIR 2006 www.csir.co.za

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 2006 www.csir.co.za

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 2006 www.csir.co.za