## 4 Conclusions  
This study established a model for the skywave ionosphere communication channel in hypersonic vehicles and investigated its characteristics, filling the gaps in related fields. In analyzing the above channel parameters, it was found that the main factors affecting the channel are loss, delay, and Doppler, which undergo dynamic changes. Due to the settling effect of the ionosphere at midnight, the space available for ray tracing becomes \"narrower\". The channel for hypersonic vehicles exhibits complex variations at midnight, including an increase in the number of signal paths. These paths may suddenly vanish or emerge, and phenomena such as path merging or splitting can occur. At midnight, the main path (first path), which has the minimum loss, is at a high elevation angle rather than a low elevation angle and exhibits the maximum delay, which is opposite to the noon conclusion. Importantly, most of the multipaths during re-entry involve 1-2 hop reflections at a communication frequency of 14 MHz. However, during certain periods, a cavity phenomenon may occur, and the skywave signals will not be able to reach the vehicle receiver, which could result in the failure of OTH communication. In summary, the conclusions of this study, along with the characteristic parameters of the skywave ionosphere communication channel for hypersonic vehicles, provide crucial support for the design, development, and evaluation of long-distance TT&C communication systems. In addition to the impact of the ionosphere on the communication channels of a hypersonic vehicle, the plasma sheath wrapped around the surface of the vehicle can also cause varying degrees of attenuation of shortwave signals. Although the attenuation is smaller than that of the ionosphere, it cannot be ignored. Therefore, in subsequent research, numerical methods will be used to solve the fluid dynamic equations, analyze the three-dimensional electron density and temperature distribution around a vehicle of different shapes, and perform FDTD electromagnetic calculations of shortwave propagation in the  
Figure 21: The maximum and minimum values of the channel characteristic parameters of the main path (1 st path) at different times: (**a**) loss; (**b**) delay; (**c**) Doppler. Figure 22: The maximum and minimum numbers of multipaths. plasma sheath. In addition, due to fluid disturbance, the shortwaves will exhibit dynamic and time-varying characteristics after penetrating the plasma sheath. How to establish such a channel model is also a direction that needs to be studied in the future. [PERSON], [PERSON] and [PERSON] conceived the idea and analyzed the results; [PERSON] and [PERSON] were responsible for part of the programming; [PERSON] provided suggestions and comments; [PERSON], [PERSON] and [PERSON] revised the manuscript; and [PERSON] and [PERSON] were responsible for the preliminary investigation. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (grant numbers 62371375 and 62371372), the Innovation Capability Support Program of Shaanxi (grant number no. * (17) [PERSON]; [PERSON] Flight investigation and analysis of alleviation of communications blackout by water injection during Gemini 3 reentry 1968. _Chinese Sci._**2013**, _43_, 1242-1249. * (18) [PERSON]; [PERSON] The removal of free electrons in a thermal plasma by means of rapidly evaporating liquid additives. _NASA Spec. Publ._**1970**, \(1\), 252. * (19) [PERSON]; [PERSON]; [PERSON] Plasma Windowing for Hypersonic Radio Communications. In Proceedings of the 2023 IEEE/MTT-S International Microwave Symposium--IMS 2023, San Diego, CA, USA, 11-16 June 2023; IEEE: Piscataway, NJ, USA, 2023; pp. 811-814. * (20) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Blackout mitigation in a plasma layer near a high-speed body in E\(\times\)B fields. _J. Phys. Conf. Ser._**2013**, _572_, 012061.