The plasma generation in hypersonic flight is known to affect the radar signatures of flight vehicles as well as their communications and sensing (radio blackout). Although these effects have been known since the beginning of spaceflight in the 1950s, accurate prediction is still difficult. Today, the most promising approach is to combine Computational Fluid Dynamics (CFD) and Computational Electromagnetics (CEM) to solve the plasma distribution in the flow field and derive the respective microwave response based on the two major plasma parameters: plasma and collision frequency. But dedicated experimental reference data is extremely scarce, which renders the validation of the multi-model dependent CFD-CEM approach difficult. To improve the validation, this study provides new experimental shock-tunnel radar data of the stagnation-point plasma of a 15 cm diameter sphere. To vary plasma and collision frequency independently, both the flow enthalpy and the freestream oxygen level were varied. The measurements are compared with respective CFD-CEM simulations.
An all-dielectric leaky wave antenna operating in the frequency range of 60 to 80 GHz is designed, simulated, and experimentally characterized. The antenna exhibits beamscanning radiation in the transverse plane. Despite the absence of a metallic ground plane, measurements show that more than 80% of the radiated power is directed into a single hemisphere, with simulations indicating a potential exceeding 85%. The antenna achieves a directivity greater than 21 dBi across the entire frequency range while maintaining high radiation efficiency.
Clutter poses a significant challenge to radar system performance, impacting measurement accuracy and operational efficiency. Rural clutter channels, characterized by complex and dynamic amplitude, Doppler, and phase variations, are particularly difficult to model and simulate. This study addresses these challenges by analyzing transfer functions derived from real X-band radar measurements collected in diverse rural environments, including forested and vegetated areas, with the aim of developing realistic synthetic data for enhanced radar simulation. Two generative models, Generative Adversarial Networks (GANs) and Denoising Diffusion Probabilistic Models (DDPMs), are developed to generate realistic rural clutter channels. To evaluate the models, a 1D Convolutional Neural Network classifier is used to assess the accuracy of the generated samples, alongside a statistical analysis comparing synthetic data to the original measurements. The results demonstrate that both models effectively replicate the key statistical properties of the measured data. These findings highlight the potential of generative models to enhance radar system simulations, providing a foundation for further research into clutter modeling and mitigation.
The gyrotropic and magneto-optical properties of a commercially available SrFe_12O_19 hard ferrite material are investigated for the first time in the Ka-band (26.5−40 GHz), at frequencies well below the intrinsic ferromagnetic resonance of the material. Using a vector network analyzer and quasi-optical measurement setup, the complex permeability tensor and permittivity of the ferrite are determined, including the estimation of uncertainties. The frequency dependence of the material parameters is analyzed to characterize the hard ferrite for potential applications in quasi-optical Ka-band circulators. The results reveal that while dielectric losses remain low throughout the band, permeability losses become dominant in the upper half of the Ka-band, as the frequency approaches the magnetic resonance. This results in high attenuation and a pronounced frequency dependence of the Faraday rotation, which limits the usable bandwidth at higher frequencies of circulators utilizing this ferrite. In the lower half of the Ka-band, the frequency dependence is reduced, and the material exhibits low attenuation, making it promising for broadband quasi-optical nonreciprocal devices in this frequency range. These findings highlight both the potential and the limitations of the investigated hard ferrite for Ka-band quasi-optical components operated below the ferromagnetic resonance.
Over the past few decades, Fraunhofer FHR has been involved in developing the SABBIA® system, a satellite-based passive radar for polarimetric target imaging and drone detection. Following the achievement of promising results in stationary configurations, an updated version of the system, with automatic platform motion compensation, was tested on a ship travelling along a river. The first experimental results, in the form of polarimetric range-Doppler maps, will be presented for various river targets, such as ferries and tourist ships. The achieved results confirm the ability of the SABBIA® 2.0 system to image non-cooperative targets while the system itself is on a moving platform.
