Space-Time Varying Plasma Sheath Effect on Hypersonic Vehicle-Borne SAR Imaging

Abstract

Hypersonic vehicle-borne synthetic aperture radar (SAR) imaging has enormous potential in civilian and military application due to high speed and strong survivability of the hypersonic vehicle. However, owing to high temperature and high pressure, a plasma flow, called plasma sheath, covers the hypersonic vehicle. The electron density of the plasma sheath varies with space and time, and space-time varying plasma sheath will affect hypersonic vehicle-borne SAR imaging severely. In this article, the effect of space-time varying plasma sheath on hypersonic vehicle-borne SAR imaging is investigated. First, the SAR signal model coupling space-time varying plasma sheath effect is established using the transmission line method. The variation law of electron density is based on flow field theory and experiment. The double propagation for SAR signal in plasma sheath and the time sequence of electron density for transmitting and receiving period are investigated and included in the SAR signal model. Moreover, the modulation effect of SAR signal under space-time varying plasma sheath is studied, and the range compressed signal under plasma sheath is derived. Based on the SAR signal model, hypersonic vehicle-borne SAR imaging defocus phenomenon with the presence of space-time varying plasma sheath is systematically analyzed. The simulation shows that the point target imaging result under plasma sheath is composed of several azimuth defocus strips, and in each strip the range dimension defocuses. The results of point target imaging and a SAR image of an area under plasma sheath verifies theory and analysis in this article. Finally, the SAR imaging degradation under different SAR and plasma sheath parameters are studied and the condition that serious defocus occurs is analyzed. The work in this article will benefit the study of hypersonic vehicle-borne SAR imaging method under plasma sheath in the future.