Communication blackout caused by the plasma sheath surrounding hypersonic vehicles is a serious threat to the safety of hypersonic vehicles. Terahertz (THz) communication is considered to be a potential solution to the blackout problem. However, hypersonic plasma sheaths can affect not only electromagnetic wave propagation, but also the radiation performance of onboard antennae. Therefore, it is necessary to investigate the radiation performance of THz antennae in hypersonic plasma sheaths. In the present study, the impact of vehicle shapes (blunt-coned and sharp-coned vehicles) on plasma sheaths was investigated numerically. The antenna involved in the present study was a 0.14 THz array antenna. The antenna performance was compared with the aspects of mainlobe gain, mainlobe direction, and mainlobe width. The present study shows that both sharp-coned and blunt-coned plasma sheaths are inhomogeneous. Nevertheless, their structures are obviously different. Such differences yield different antenna performance in sharp-coned and blunt-coned plasma sheaths. Compared with sharp-coned plasma sheaths, blunt-coned plasma sheaths can refract antenna radiation direction, which can result in worsened communication quality in the expected direction. In addition, the phenomena are discussed in detail. Suggestions of vehicle shape design to guarantee communication quality in hypersonic flight missions are proposed.

A large number of studies have confirmed that the wake region may be a more ideal antenna installation area for the reentry vehicle communication problem, but many practical issues such as how to choose the modulation mode, carrier frequency, and antenna orientation are still pending. Based on numerical simulations, the characteristics of tail channels and the bit error rate (BER) of extremely high-frequency (EHF) communication in the wake region of the plasma sheath of hypersonic vehicle are studied. It is found that, with an increase in the angle between the tail channel and the tail of the vehicle, the attenuation of the EHF signals decreases and the phase shift fluctuates more severely. In order to obtain better communication performance, 2PSK with a carrier frequency of 140 GHz or 225 GHz, or 4QAM (QPSK) modulation with a carrier frequency of 140 GHz, and the tail channel with an angle between 50°and 60°to the tail of the vehicle can be selected. This study reveals the propagation characteristics and BER performance of EHF signals in the wake region of plasma sheath, which can provide a valuable reference for the design of the hypersonic vehicle communication system.

Many modern ionospheric studies rely on incoherent scatter radars (ISR) since this kind of radar is able to detect various ionospheric parameters over very long ranges. The performance of ISR significantly depends on its coding system. In recent decades, a new type of coding system, which is the so-called composite coding, was presented. It used to be constructed by using a certain code to modulate alternating code to achieve better detection resolution and anti-noise performance for ISRs. In the present study, a new composite coding system was presented, which is constructed based on complementary codes and alternating codes. In this paper, the performance of the new composite code will be compared with that of several traditional codes to show that the new composite code can help to improve the detection performance of the ISR. According to the analysis based on the ambiguity function, the present composite coding system helps to improve the range resolution and detection range for ISR detections. In addition, numerical tests on anti-noise performance show that the complementary composite coding system has a good anti-noise performance and helps to reduce the necessary times of incoherent integration. As a result, the composite coding system can improve the time resolution.

Communication blackout is always a serious threat to the flight tasks of modern hypersonic vehicles moving in near space. EHF communication is considered as a potential solution to the blackout problem. Nevertheless, EHF waves suffer from severe attenuation in hypersonic plasma sheaths. An external magnetic field could mitigate EHF wave attenuation in hypersonic plasma sheaths. Dipole magnetic fields, generated by coils, are feasible in realistic scenarios. In the present study, a model for EHF wave propagation in hypersonic plasma sheaths magnetized by dipole magnetic fields that are generated with coils is developed. The dissipation caused by the inhomogeneity of dipole magnetic fields and the magnetic field component of electromagnetic waves are compared with the dissipation yielded by the collision between electrons and neutral particles. The results show that collision is still the main dissipation mechanic for EHF waves. The study also found that, in the blunt-coned plasma sheath, the mitigation effect of a dipole magnetic field is weaker than that of a uniform magnetic field. The mechanics which yield the difference is analyzed. In addition, the relation between the characteristics of EHF waves and the coil parameters is investigated. Suggestions for the coil parameters and the operation frequencies of the EHF communication systems are made based on the investigation.

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