## 3 Results and Discussion
### Simulation Analysis of the Skywave OTH Channel Characteristics at 12:00 PM  
Overall, as shown in Figures 12-19, compared with midnight, the number of skywave OTH channel paths for the hypersonic vehicle remains at four, with dynamic changes in the loss, delay, and Doppler of each path. The paths can be divided into two groups: low-elevation-angle paths, consisting of non-ground reflection path 1 and ground reflection path 3, and high-elevation-angle paths, consisting of non-ground reflection path 2 and ground reflection path 4. When the channel is at a high elevation angle, the loss is the highest, and the corresponding delay is the smallest, which is the opposite of what occurs at midnight (00:00). In particular, as shown in Figure 17c, the fourth path shows a positive Doppler value after 142.5 s, because the angle \(\theta\) between the propagation direction of the ground reflection path in the high-elevation state and the vehicle's velocity direction exceeds 90\({}^{\circ}\). From the above analysis, two differences between 00:00 AM (midnight) and 12:00 PM (noon) can be identified. On the one hand, the main path (first path) of the midnight channel, which is the path with the lowest loss, occurs at a high elevation angle rather than a low elevation angle, and its delay is the longest. However, at noon, the main path of the channel (the first path), characterized by the lowest loss, occurs at a low elevation angle rather than a high elevation angle and exhibits the shortest delay. On the other hand, the number of  
Figure 11: Ray-tracking results in winter at 00:00 a.m.  
Figure 12: Ray-tracing results of vehicle in spring at 12:00 p.m.: (**a**) range: 1089.71 km, time: 394.6 s; (**b**) range: 1179.371 km, time: 407.3 s; (**c**) range: 1247.451 km, time: 417.4 s.  
multipath channels at midnight is significantly higher than at noon, and the variations in the multipath channels at midnight are more unstable. In fact, the underlying reason for the two phenomena mentioned above is that the electron density in the ionosphere is significantly higher at low-elevation angles at midnight than at noon, reflecting the occurrence of the ionospheric \"settling\" phenomenon during midnight. Although the maximum electron density during the day is significantly higher than at midnight, in the 60-130 km range where ionospheric absorption occurs, the electron density at noon is notably lower than at midnight, and the altitude of the maximum electron density at noon is significantly lower than that at midnight. At noon, the low electron density in the 60-130 km region has minimal impact on the channel loss, and the multipath loss is largely influenced by the path length. As the low-elevation path is shorter than the high-elevation path, its loss is consequently smaller than that of the high-elevation path. In contrast, at midnight, the electron density in the 60-130 km region is high, and the channel loss is significantly influenced. Low-elevation paths are concentrated in the absorption zone with a high electron density (high loss), whereas high-elevation paths largely avoid this region (low loss). Because high-elevation paths have longer propagation distances (resulting in greater delays), their loss is ultimately smaller than that of low-elevation paths. It is precisely due to the ionosphere's settling effect that the space available for ray tracing becomes \"narrower\" during the midnight period, leading to a greater number of channel paths and more complex variations than at noon. Figure 14: Ray-tracing results of vehicle in summer at 12:00 p.m.: (**a**) range: 1089.71 km, time: 394.6 s; (**b**) range: 1179.371 km, time: 407.3 s; (**c**) range: 1247.451 km, time: 417.4 s.  
Figure 13: Characteristic parameters of the multipath channel during re-entry in spring at 12:00 p.m.: (**a**) loss; (**b**) delay; (**c**) Doppler. Figure 20 shows the range of the characteristic parameter values for all path channels.