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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. [ABSTRACT FROM AUTHOR]

Millimeter wave is a potential way to overcome the blackout problem. However, the plasma sheath is axially nonuniform, and it has different effects on millimeter wave at different axial positions. In this article, the influences of the spatially inhomogeneous time-varying plasma sheath on millimeter waves are investigated. The electron density model is established, the time-domain sequence of the random channel is calculated, and the channel statistical parameters at different axial positions are extracted. The results show that the millimeter wave channel is spatially selective. Millimeter waves can transmit through the plasma sheath with less attenuation, multiplicative interference, and nonlinear effects at any position of the vehicle except the nose. When the antenna position is properly selected, the attenuation and multiplicative interference intensity can be further reduced. This indicates that millimeter wave communication technology is the most promising way to overcome the blackout problem and accomplish diverse communication tasks of a hypersonic vehicle. [ABSTRACT FROM AUTHOR]

Due to its high-speed movement, the reentry object is enveloped with a plasma sheath featuring complex electromagnetic (EM) characteristics. Based on the physical process of EM wave transmission/reflection in plasma sheath, we propose a calculation method of EM wave reflection suited for plasma sheath environment. The proposed method comprehensively considers the resonance shielding surface, the critical angle of total reflection, and the damping effect. Our method is simulated and verified using typical blunt cone computational fluid dynamics (CFD) simulation data. We analyze the reflection intensity distributions of EM waves in the plasma sheath with different altitudes and speeds, revealing the reflection law of EM wave under scenarios of complex EM environment and oblique incidence of plasma sheath. Simulation/experimental results suggest that our proposed method exhibits excellent performance under the conditions of sparse data grid and grazing incidence. [ABSTRACT FROM AUTHOR]

As the reentry target passes through the atmosphere at hypersonic velocity, its surface is covered with a plasma sheath. The plasma sheath produces an amplitude-phase modulation effect on the radar echo and increases the signal component of the radar echo, which seriously affects the radar’s reliable detection of reentry target and leads to a series of abnormal radar detection problems. Through the radar echo model of the plasma-sheath-covered target, the multidomain characteristics of the echo signal are analyzed, and the electromagnetic (EM) shielding effect of the plasma sheath on EM waves is revealed. Based on the intrapulse Doppler frequency extraction technology, a design method of false target filter is proposed, which can effectively suppress false targets generated by the plasma sheath. Through the simulation analysis of single-period echo signals and multiperiod echo signals, the feasibility of the algorithm in this article is verified, and the reliability of the algorithm is verified by statistical experiment analysis, which lays a theoretical foundation for reliable radar detection of plasma-sheath-covered target. [ABSTRACT FROM AUTHOR]

During the reentry process of a spacecraft, a plasma sheath will attenuate and distort the amplitude and phase of the transmitting electromagnetic (EM) waves, influencing signal reception and measurement accuracy of the direction of wave arrival of the spacecraft-borne array antenna, by which the spacecraft communication and guidance system may be affected. To solve this problem, based on the space distribution characteristics of plasma sheath flow field, an EM simulation model of an array antenna under plasma sheath is established, and EM simulations related to field propagation across plasma sheath and radome are performed. Then, the array signal model is constructed by extracting the field strength data at the array element. Several spatial spectrum estimation methods are conducted to calculate the direction of arrival of an EM wave incident on the array antenna. By comparing the direction of the incoming wave in free space with the estimated direction after propagating through the plasma sheath, the direction-finding error of the spacecraft-borne array under the plasma sheath is obtained, and the cause of the error is analyzed. With compensation information, the results of the proposed method can enable higher target-tracking accuracy for the spacecraft guidance system. [ABSTRACT FROM AUTHOR]

Accurate atomic and diatomic partition functions are required to determine the level populations for the calculations of radiative properties in thermodynamic equilibrium and nonequilibrium plasmas produced by various atmospheric re‐entries. In this work, a reliable partition functions database was rebuilt for some atomic and diatomic species from 100 K to 50000 K. The atomic partition functions were obtained by a four‐level model, while the diatomic partition functions were predicted based upon a more rigorous approach for the computation of the energy levels. Compared with previous publications, the number of diatomic electronic states considered in our work is as large as possible. Estimates are made for the contributions of each electronic state of the diatomic molecule to the partition function. Moreover, the effect of the number of electronic states on the partition function was also evaluated. Finally, we calculated the specific heat based upon the obtained partition functions. All the results were validated by the available data in recent references and the relative errors were systematically analysed. [ABSTRACT FROM AUTHOR]

Hypersonic vehicle-borne radar target detection will suffer from time-varying plasma sheath which is formed during hypersonic flight. In this study, effect of time-varying plasma sheath on hypersonic vehicle-borne radar target detection is investigated. First, the radar echo model coupling time-varying plasma sheath effect is proposed using an analytical computation method. The regulation of electron density variation is based on flow parameters. Coherent integration result with the present of time-varying plasma sheath effect is derived. The degradations of hypersonic vehicle-borne radar target detection under time-varying plasma sheath are studied through real hypersonic vehicle flight cases. Simulation results shows that high electron density of time-varying plasma sheath will cause significant attenuation, which will lead to decrease of detection probability. In some cases, the signal-noise ratio (SNR) threshold that target can be detected increases more than 40 dB compared with situation without time-varying plasma sheath. Moreover, time-varying plasma sheath will cause time-varying amplitude and phase in pulse and between different pulses, causing intra-pulse and inter-pulse modulation phenomena. These phenomena lead to the disappearance of peak value in range-Doppler plane which will degrade the coherent accumulation as well as target detection performance. By using plasma sheath electron density diagnosis method on vehicle, compensation factor according to the electron density can be established to compensate plasma sheath effect in radar echoes. Simulations results based on real hypersonic vehicle flight flow parameters confirm the conclusions above. The work in this paper will benefit the design and application of hypersonic vehicle-borne radar in the future. [ABSTRACT FROM AUTHOR]

Performing as an important technology, microwave interferometry helps understand and analyze plasma characteristics. However, the relatively appropriate diagnosing methods being discovered for investigating fast time-varying plasma have met with limited success so far. The crucial problem of this task lies in eliminating the interference of additive noise while compensating for phase ambiguity of the initial point and of the phase jump points. In particular, randomly varying electron density and noise make it impossible to directly detect these points. Theoretical analysis has found that the time-varying electron density of plasma causes the received signal to rotate on the complex plane. Inspired by this fact, a deep learning (DL) method is proposed to track the time-varying plasma and solve the above two problems, during which the deep neural network (DNN) is utilized to extract the curve from the data, thereby eliminating the noise and detecting the initial point and the phase jump points according to the projection of the data on the curve. Experimental results suggest that the learned curve is in good consistency with the ground-truth curve. Meanwhile, the time-varying electron density and the collision frequency can be accurately diagnosed using the learned curve. [ABSTRACT FROM AUTHOR]

Modeling wideband plane-wave propagation through a stratified magnetized plasma sheath is important toward predicting the quality of communications during re-entry, scattering caused by electron density perturbations, and many other applications. When using the finite-difference time-domain (FDTD) method to solve the problem, modeling the incident field in the simulation domain is difficult if the incident field travels obliquely to the grid axes. In this article, a total-field/scattered-field (TF/SF) formulation whereby a plane-wave source is introduced into cold layered magnetized plasma has been developed for the FDTD method. Based on phase matching theory, twelve auxiliary 1-D propagators are specified to calculate the correction field values for the TF/SF boundary. A modified convolution perfect match layer (CPML) is implemented to terminate the 1-D propagators. Numerical results are also presented to show the accuracy and effectiveness of the method. Although this method is developed for magnetized plasma, the extension to other anisotropic dispersive layered media is straightforward. [ABSTRACT FROM AUTHOR]

To overcome some drawbacks of the plasma stealth technology in real-life application, a practical multilayer stealth structure composed of enclosed plasma slab and radar absorbing material (RAM) is presented in this paper. Based on a technique referred to as the transmission line analogy method, reflection coefficients of the perpendicularly polarized wave, the parallel polarized wave, and the circularly polarized wave obliquely incident upon this multilayer structure are determined, respectively. The effects of the incident angle, kinds of RAMs, and parameters of the plasma slab including electron density, collision frequency, and thickness on the stealthy effectiveness of this composite stealth structure have been studied systematically. The numerical results indicate that by a proper design, the power of reflected wave is significantly reduced over a wide frequency bandwidth, which provides some useful references to the plasma stealth technology applied to aircrafts, ships, and missiles. [ABSTRACT FROM PUBLISHER]