A GNSS–based technique to investigate the black-out during space vehicles’ re-entry

The development of re-entry – and hypersonic - vehicles calls for an accurate knowledge of the surrounding aero-thermodynamic field, which is strongly modified by ionization processes also responsible for the communications black-out. This paper aims to introduce a novel technique to study the plasma layer surrounding the vehicle by means of the radio-frequency signals subject to the black out during some time intervals of the descent. Signals to be considered are the ones transmitted from GNSS sources, nowadays in a large number, with stable characteristics and above all impinging on the vehicle from well-known and sparse directions. It would be possible to track these signals all along the descent, until their disappearance and then since their return after the black-out phase, to infer the properties of the ionized flow surrounding the re-entry vehicle. Such a tracking could be conveniently accomplished by sampling and recording onboard the signals received by a set of antennas, ideally providing an almost spherical coverage all around the vehicle, and then performing a detailed post-flight analysis, combined with flight data, by means of a software receiver to detect the captured or disappearing signals and evaluate their attenuation. Notice that, due to the limited request of onboard equipment, and to the likely availability of GNSS receivers in modern re-entry vehicle, the implementation of the technique looks not especially difficult nor expensive. The concept, fully original in the knowledge of the authors, is presented in the paper, together with a very preliminary example. While it is likely that this technique cannot fully substitute complex and time expensive aero-thermodynamic simulations, the exercise shows its possible usefulness in complementing and validating the numerical analyses.