Mitigating Radio Blackout in Hypersonic Flights and Atmospheric Entries – Computational Electromagnetics and Experimental Validation

The radio blackout phenomenon disrupts essential communication, and navigation
during hypersonic flights posing significant challenges especially for atmospheric
entry missions because effective data collection is vital for emergency preparation and response. Modeling electromagnetic (EM) wave propagation is complex and costly.
Ray-tracing algorithms offer cost-effective tools for optimizing radio communication
in complex scenarios and thus for blackout modeling. During atmospheric entry,
vehicles experience high velocities and significant interactions with the atmosphere
causing plasma formation and radio blackout. Radio blackout mitigation techniques
such as aerodynamic shaping, quenchant injection, and magnetic windows have
been proposed. The magnetic windowing method is analyzed in this work as part
of the Magnetohydrodynamic Enhanced Entry Systems for Space Transportation
(MEESST) project.
This thesis researches and applies an advanced Magnetohydrodynamics (MHD)
ray-tracer to analyze the effects of magnetized plasma on signal propagation during
hypersonic flight and atmospheric re-entry, focusing on addressing the radio communication blackout caused by the ionized gas surrounding a spacecraft. The MHD ray-tracer integrates electromagnetic wave theory with high-speed fluid dynamics, offering detailed analysis of signal properties under various inhomogeneous plasma conditions.
In a first step, the analysis of electron number densities and ray trajectories in
magnetized plasma indicated that sufficiently strong magnetic fields alter the flow
field, consistent with existing literature. Cases with applied magnetic fields show an
increased radiation aperture angle, suggesting a potential to mitigate or eliminate
radio blackout.
In a second step, a new signal characterization method analyzing the electric
field along the ray path provides a deeper understanding of the physical mechanisms
behind radio blackout phenomena. Analysis of a non-magnetized plasma case reveals
that plasma increases signal intensity by bending rays towards each other, more
influential than refraction losses. Free space loss, which rises with higher frequencies, counteracts the effect of plasma on the spread factor. Validation against VKI ground experiments confirms the method’s effectiveness in explaining radio blackout physics.
In a third step, the behavior of various angles between the wave vector and the
magnetic field vector for ordinary and extraordinary waves aligned with literature and
provided important insight. An applied magnetic field can reduce electron number
density and alter the refractive index along the ray trajectory, increasing the radiation
aperture angle and reducing signal attenuation, thereby improving communication.
Magnetized plasmas introduce complexities like Faraday rotation and increased signal
absorption but offer the potential for mitigating blackouts by widening communication angles through controlled magnetic fields. The study demonstrates the functionality of the magnetic windowing method to create communication corridors for re-entry vehicles. BORAT matches very well with experimental results for non-plasma cases. For plasma cases, computed attenuations are above those measured.
The developed signal characterization method provides deeper insights into the
physical phenomena causing radio blackout. This research advances the understanding of plasma effects on signal propagation and explores mitigation techniques, contributing to safer and more successful space missions. Further insight can be gained when additional and more accurate measurements are available.