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In this article, a hypersonic target electromagnetic (EM) scattering echo model combined with the inhomogeneous zonal medium model (IZMM) and the classical scattering center model (SCM) is proposed with a distributed satelliteborne array radar as the detection platform. A parallel physical optics (PO) method is used for multiview inverse synthetic aperture radar (ISAR) imaging of a moving hypersonic target covered with plasma sheath based on the analysis of high-resolution range profile in the S-X ultrawideband range, reconstructing 2-D EM scattering echo data (the target) and motion compensation. The results show that the surface of the inhomogeneous plasma sheath flow field is an excitation layer with random and irregular fluctuation characteristics, which increases the false scattering centroid of the 1-D range profile of the hypersonic target and can interfere with and disrupt the radar localization of the target along the radial direction. In addition, shallow scattering of EM waves occurs in the plasma sheath, and the average signal intensity of the target can gradually reduce from 0.5 x 10⁻⁵ at 60 km and 20 Ma to 0.1 x 10⁻⁵ at 30 km and 20 Ma, with a fivefold weakening of the overall scattered echo signal. In particular, the faster the hypersonic target travels at 30-km altitude, the weaker the imaged scattered echo signal becomes, with the average intensity of the imaged signal weakening by approximately threefold from 15 to 25 Ma. This study provides considerable technical support and data assurance to establish a synthetic aperture radar (SAR) automatic target recognition (ATR) database, and the findings of this study can be used as a reference for fine-structure feature analysis of hypersonic targets for feature extraction and the classification and identification of targets.

Using the parallel physical optics method, the range profile of a hypersonic vehicle with an electrically large size covered by 1-D inhomogeneous plasma sheath was studied. In this article, under the condition of far-field approximation, the improved physical optical method of 1-D nonuniform plasma coating of the conductive target was applied to calculate the scattered electric field of the target at different frequencies; then, 1-D Fourier inversion of the scattered electric field was carried out to obtain the 1-D range profile of the target. Numerical results showed that the temperature and pressure changes inside the plasma will have an impact on the range profile of the aircraft, and the intensity of the range profile of the target is inversely proportional to the temperature and pressure. In addition, the flight Mach parameters of the target aircraft, which affect the 1-D range profile, were also analyzed. The results showed that the larger the flight Mach number, the easier it is to detect the target aircraft. Finally, it was found that the nonuniformity of the plasma significantly influenced the 1-D range profile of the coated target; the intensity value of the range profile of the target aircraft (covered by the plasma sheath) at different flight heights was significantly lower than that of the pure metal target, resulting in the so-called “stealth” effect. This research provides some methods for effectively measuring and controlling hypersonic vehicles in adjacent space and alleviating the interruption of reentry communication.

To investigate the electromagnetic scattering problem related to a foil in a turbulent hypersonic plasma environment, the scattering field formulas for hypersonic plasma turbulence were improved by applying the anisotropic power spectrum. Using the theory of electromagnetic scattering of a conducting object, the second moment of a conducting object's scattering field is obtained in the hypersonic plasma turbulence. Finally, the backscattering cross-section of the foil surrounded by the hypersonic plasma turbulence was calculated. The results showed that the fractal dimension led to an increase in the complexity of the hypersonic turbulent structure, and thus had a greater influence on the electromagnetic wave scattering. The scattering cross-section of the foil in the hypersonic turbulence decreases rapidly. Meanwhile, the anisotropic parameters in the model represent scale-dependent stretching of the turbulent eddy. As the anisotropic parameters of the turbulence increase, there will be less scattering of the hypersonic turbulence. At this point, the total scattering cross-section of the foil in hypersonic plasma turbulence increases gradually. By improving the aerodynamic shape of the aircraft, it can produce a large number of anisotropic turbulent eddies which can effectively improve the passive jamming capability when the foil is cast.

On the basis of the fractal theory and measurements of hypersonic turbulence, the spectral index, and two-dimensional (2D) power spectrum of the refractive index fluctuation are derived, and the 2D band-limited Weierstrass fractal function is applied to simulate the refractive index fluctuation in hypersonic turbulence. Thus, the impact of hypersonic turbulence on synthetic aperture radar (SAR) imaging system is simulated using the fractal phase screen method. Results show that the resolution of SAR imaging is severely deteriorated under the influence of hypersonic turbulence. The phase error caused by the turbulence microstructures in the plasma sheath becomes more apparent with the increase in the fractal dimension of the hypersonic turbulence. The larger Reynolds number associated with the hypersonic turbulence results in a more evident microstructure fluctuation and inferior imaging quality. Furthermore, when an aircraft flies at high speed, the azimuth focus is considerably sensitive to the turbulence microstructure along the direction of flow field. Consequently, hypersonic turbulence in the plasma sheath has a significantly larger effect on the azimuth than on the range in point-target imaging.

In this study, the new fractal model and power spectrum of plasma sheath turbulence are established based on the fractal dimensions in hypersonic turbulence that were measured through experiments. The expression for beam spot size, scintillation index, angle-of-arrival fluctuation, and beam wander are obtained from the power spectrum. Finally, the propagation characteristics of a Gaussian beam in a turbulent plasma sheath are simulated. The authors’ results indicate that plasma sheath turbulence can lead to amplitude and phase variations. A larger variance in the plasma sheath turbulence causes a more evident refractive index fluctuation and a greater effect on the Gaussian beam propagation characteristics. On the contrary, a larger Reynolds number reduces the effect of the plasma sheath turbulence on the Gaussian beam. This new power spectral model is fundamental to understanding Gaussian beam propagation in plasma sheath turbulence and may provide a reference for the imaging and communication system under the effect of plasma sheath turbulence.

Visible light communication (VLC) is a new type of wireless communication technology, and its applications in offshore ships and ship-shore lamp signal systems are drawing increasing attention as a supplement of communication net. In maritime environment, VLC system is affected by many factors, of which the wave fluctuation and atmospheric turbulence are the most noticeable. The turbulence will make signal intensity fluctuate randomly, and thus reducing the performance of VLC system operating in the atmosphere. To establish an effective VLC network in the actual marine environment, an effective channel transmission model needs to be established and used to study the performance of the maritime VLC link. Considering large aperture diameter receiver with the aperture averaging effect, log-normal distribution model is employed to deduce the mathematical expression of average bit error rate of maritime VLC system in atmospheric turbulence. By using time-diversity to transmit interleaved symbols with repeated coding in a maritime VLC system, it is possible to ensure that the code-word passes through multiple channels to resist the deep fade performance, and to reduce the bit error rate due to the occurrence of deep fading in a single channel. In the actual application process, in order to improve the system performance, the average signal-to-noise ratio usually increases with the transmission power increasing, but for a VLC system, there are some difficulties in making the high-power high-rate visible light transmitters. And the power will produce light pollution and even damage the naked eye. The implementation of the repetitive coding principle is simple, and in some special cases it is even better than the complex orthogonal space-time coding and other schemes, so studying the system performance of the repetitive coding scheme is of considerable value for practical application. Based on the modified Pierson-Moskowitz spectrum, the effect of wave height, transmission distance, atmospheric turbulence intensity, receiver aperture size and visibility on the average bit error rate of VLC system are analyzed. The performance of the VLC system between lighthouse and ship is affected by the fluctuations of the sea waves, and the average bit error rate changes with randomness and complexity like the sea waves in a short distance. As the wind speed increases, the marine environment becomes worse and the average bit error rate is undulate. The average bit error rate of maritime VLC increases with the increasing of transmission distance and atmospheric turbulence intensity, and with the decreasing of receiver aperture size, wavelength and average signal-to-noise ratio. Atmospheric turbulence intensity and visibility have a significant effect on the system performance, and it should be emphatically considered to take measures to reduce the influence. Increasing receiver aperture and repetitive coding are effective to a certain extent. In the present work a new model is proposed for evaluating the performance of a maritime VLC system and providing reference for practical application.

The plasma sheath is produced by high-temperature heating during the reentry of a hypersonic vehicle to the Earth atmosphere. Temperature around the vehicle rises rapidly because of severe friction with air. The vehicle temperature behind friction is high enough to excite various real gas effects including chemical reactions of air, which contains ablation particles of vehicle, free electrons, and ions. The plasma sheath greatly affects the transmission of electromagnetic waves and has very strong interference on the communication signals, which results in interrupt between the target and the ground station, namely, blackout. The electron density of plasma sheath surrounding the aircraft is inhomogeneous and varies with time. Temperature and pressure will also change at different altitudes. Therefore, it is meaningful to investigate the propagation characteristics of electromagnetic waves in temporally and spatially inhomogeneous plasma sheath. The temporally and spatially inhomogeneous plasma sheath model is introduced and the electron density data of the National Aeronautics and Space Administration (NASA) reentry vehicle is employed. The relationships among temperature, pressure, and collision frequency are obtained with the empirical formula of collision frequency. Then, the reflection coefficient and transmission coefficient of time-varying single layer plasma are calculated with the shift operator finite-difference time-domain (SO-FDTD) method. These results are compared to verify the correctness of the proposed method. Finally, the LTJEC-FDTD method is used to calculate the reflection coefficient, transmission coefficient and absorptivity at different relaxation time, temperature, and pressure in the terahertz (THz) band. The results show that the higher temperature and pressure will enable the electromagnetic wave to penetrate the plasma sheath at high relaxation time of electron density. If the incident wave frequency is lower than the cut-off frequency of plasma, the reflection of electromagnetic wave will be more obvious. However, when the incident wave frequency is in the THz band, the effects of temperature and pressure on the propagation of electromagnetic wave are obviously weakened. The absorption of electromagnetic wave by plasma will be more obvious when the relaxation time, temperature, and pressure decrease. If the relaxation time of electron density is shorter than or equal to the period of THz wave, more energy of electromagnetic wave will be absorbed by the plasma sheath. Contrarily, if the relaxation time of electron density is much longer than the period of THz wave, the absorption of electromagnetic energy will decrease. This study gives some insight into the temporally and spatially inhomogeneous plasma sheath, and provides a theoretical basis for solving the blackout problem.