Communication

Atmospheric gas flowing by a hypersonic vehicle will be heated by the supersonic shock in front of the vehicle and then frictionally as it flows around the vehicle. At velocities near or above Mach 10, this intense heating of the gas will result in the disassociation of molecules and atoms. This transforms the gas flow into an ionized flow or so-called “plasma sheath” that surrounds the vehicle. The plasma layer can reach densities exceeding 1013cm–3.

At such high densities, the plasma frequency greatly exceeds the frequency range of conventional S, C, and X band communication signals that range from approximately 1 GHz to just over 10 GHz. Electromagnetic radiation under normal non-magnetized conditions cannot penetrate thick plasma layers where the plasma frequency is greater than the electromagnetic wave frequency. The signal is instead reflected. Even at frequencies higher than the plasma frequency, collisional damping can severely attenuate the signal. This layer of charged particles, therefore, prevents the reception of GPS signals and usually prevents and disrupts reception and transmission of communication signals between ground control and the spacecraft. These communication signals are often reflected altogether, or at least significantly attenuated. On spacecraft reentering the atmosphere, this period of flight is known as “blackout."

There is a need to develop innovative solutions for communication through the plasma sheath generated around an air vehicle at hypersonic Mach numbers. Possible approaches being considered include aerodynamic shaping, a magnetic “window,” and liquid or solid injection.