Linear-based low-rank and sparse models (LRSM) and nonlinear-based deep autoencoder (DAE) models have been proven to be effective for the task of anomaly detection (AD) in hyperspectral images (HSIs). The linear-based LRSM is self-explainable, while it may not characterize the complex scenes well. In contrast, the nonlinear-based DAE is able to extract the discriminative features between the background and anomaly for the complex scenes, whereas it is not self-explainable. To effectively combine the advantages of both, a dynamic low-rank and sparse priors-constrained DAEs (DLRSPs-DAEs) for hyperspectral AD (HAD), in this article, is proposed. In order to utilize the low-rank prior existing in an HSI, a low-rank prior-based DAE (DAE_LR) is designed to generate an excellent background reconstruction effect and terrible anomaly reconstruction performance. Further, to consider the sparsity reflecting the anomalies in the HSI, a DAE that is constrained by the sparse prior obtained by the decomposition of the HSI (DAE_S) is developed. Notably, to make the model more compact, the DAE_LR and DAE_S share a common encoder. To achieve global optimal performance, an end-to-end joint optimization strategy with the consideration of the interaction between the learning of the DAEs and the decomposition of the HSI is proposed. Additionally, to yield better detection performance, a nonlinear fusion strategy is exploited to comprehensively combine the detection results obtained from both the DAE_LR and DAE_S. Extensive experiments conducted on several datasets show that the proposed DLRSPs-DAEs detector achieves tremendous performance with respect to the classical and state-of-the-art detectors.

During hypersonic flight, the communication link is affected by the multipath effect and plasma sheath. As a result, envelopes and phases of the wireless signals fluctuate with time. In this paper, an integrated channel model is proposed based on geometric propagation theory. The statistical characteristics of the plasma sheath channel are carefully studied. The probability density functions (PDF), the correlation coefficient between envelope and phase, the autocorrelation function (ACF), the level-crossing rate (LCR) and the average fade duration (AFD) are derived and detailed based on electromagnetic propagation theory. Two numerical examples are also used to demonstrate the effectiveness of the model. All statistical curves agree with the theoretical curves. The proposed model comprehensively considers the multipath effect, the Doppler effect and the plasma sheath effect. It is crucial to plausibly understand the blackout problem to facilitate performance evaluations of hypersonic vehicle communication systems.

During reentry, plasma sheath is affected by many flight factors, such as trajectory, velocity, angle of attack, and fluid turbulence. As a result, the plasma parameters are time-varied and nonstationary. The nonstationary effects of reentry plasma sheath on electromagnetic (EM) wave propagation are investigated in this paper. First, the factors that affect the plasma sheath are analyzed, and the plasma sheath variation is divided into three scales. A multiscale reentry plasma sheath electron density model is established. A compatible modeling method is proposed, and this method is proven effective. Finally, we analyze the effects of plasma sheath on EM wave at 1.57542, 2.2, and 5 GHz, which are generally used in GPS and telemetry, tracking, and command systems. Results show that the statistical parameters of complex transmission coefficients are time-varied and nonstationary.

Channel capacity is the prerequisite and basis for the design of a communication system. To assess the impact of a harsh plasma environment on the communication system of reentry vehicles, Shannon's information theory is used to evaluate the channel capacity through the estimation results of the signal-to-noise ratio obtained by the communication link budget method. First, the attenuation caused by the plasma sheath is calculated with a stratified medium finite-difference time-domain method for typical S, C and Ka telemetry frequencies in a typical reentry plasma environment. Thereafter, typical telemetry transceiver parameters are considered to estimate the channel capacity. Results show that the S-band channel capacity is almost zero at the altitude of 30–40 km and the plasma attenuation at the C-band is slightly better. However, the blackout phenomenon remains obvious. The Ka-band signal can penetrate the plasma sheath layer with the smallest attenuation, thus significantly increasing the capacity of the channel and it may thus adequately meet telemetry needs.

A three-dimensional (3-D) total-field/scattered-field (TF/SF) plane-wave source was developed for the analysis of the plane wave propagation characteristics in the reentry plasma sheath. Owing to the dynamic variation in the surrounding flow field, the plasma sheath during the reentry process presents various 3-D inhomogeneous structures, such as dynamic plasma turbulence and considerable metal ablation particles. The proposed 3-D model can be efficiently applied to simulate the scattering effects of the 3-D inhomogeneous structures on the EM signals. The proposed model utilizes a 3-D TF/SF boundary to introduce the plane wave source into the simulation domain, and six pairs of twodimensional (2-D) auxiliary propagators are created to generate the incident field on the TF/SF boundary based on the phase matching and the current density convolution finite-difference time-domain (JEC-FDTD) method. The uniaxial anisotropy perfectly matched layer (UPML) is modified as the absorbing boundary condition of the 2-D propagators. Implementation issues such as stability and dispersion are also discussed. The accuracy and efficiency of the proposed scheme are validated by admirable numerical simulation results and excellent agreement with the transmission line analogy method.

A plasma sheath generated around a hypersonic vehicle can be a serious obstacle to fixed-frequency communications, especially navigation systems. This paper studies the effects of a plasma sheath on a Beidou navigation circular microstrip patch antenna. The dynamic stratified medium model method, which is based on the gradient of electron density profile curve, is employed to model the nonuniform plasma sheath. Using the algorithm calculation of the time-domain finite integration, the navigation antenna radiation pattern property is obtained, along with the changes in the reflection coefficient and input impedance properties. Results show that antenna pattern distortion, resonant frequency shift, and impedance mismatch caused by plasma are proportional to the electron density. The process of reentry blackout is also reproduced from the perspective of the antenna performance degradation. The inductor L and capacitor C (LC tuning circuit) impedance mismatch compensation scheme proposed is demonstrated to be effective and it can decrease return loss by 19.4 dB under the most serious plasma environment, thus reducing signal reflection and enhancing navigation system performance.