## 2 NS Communication, Navigation, and TT&C Integrated Frequency Selection  
Currently, the atmospheric propagation characteristics of low-frequency bands like L and S have been extensively studied in satellite communication, navigation, and TT&C [31; 32]. However, there is no complete research data on the signal design of high-frequency bands suitable for NS communication, navigation, and TT&C systems [33; 34; 35; 36]. Since the integrated system of communication, navigation, and TT&C for NS vehicles is the core of the information support for vehicles re-entering the atmosphere and high-altitude vehicles performing tasks, the selection of frequency bands will affect the equation of the entire technical solution; it is a key issue that requires comprehensive consideration, has far-reaching influence, and is of strategic significance. Therefore, selecting a reasonable and feasible frequency band is the prerequisite and foundation for carrying out the integrated signal design. In this subsection, frequency bands suitable for the NS in the future are selected from multiple aspects to provide theoretical support for the integrated signal design of communication, navigation, and TT&C. Next, according to the actual requirements of the integrated signal design of communication, navigation, and TT&C for NS vehicles, we will demonstrate the issue of frequency band selection for the integrated signals of communication, navigation, and TT&C of NS vehicles from five aspects: the standards (as shown in Table 1) of the international telecommunication union (ITU) [37; 38; 39; 40; 41; 42; 43; 44], high-speed communication, over-the-horizon relaying, plasma sheaths and blackouts, and the maturity of equipment development at the current stage. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Type** & **Applicable Conditions** \\ \hline ITU-R S.1001-2 & Standards for satellite communication & Fixed satellite service (FSS) and mobile \\ ITU-R M.IMT-2020 & Standards for satellite communication & satellite service (MSS). \\  & integration. & 5G satellite radio interface standard. \\  & & It is used for interference assessment, \\ ITU-R P.619 & Recommendations of ITU-R Series P. & aiming at the interference paths between \\  & & the space station and the ground station \\  & & such as propagation in clear sky, \\  & & scattering by rain and snow, etc. \\  & & It provides propagation models between \\ ITU-R P.452 & Recommendations of ITU-R Series P. & the near-space and the ground to support \\  & & link design and spectrum efficiency \\  & & optimization. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relevant documents for spectrum resource management in ITU. (1) ITU spectrum division  
At the world radiocommunication conference 1997 (WRC-97), it was recommended that the frequency bands of 47.2-47.5 GHz (downlink) and 47.9-48.2 GHz (uplink) be exclusively allocated for the services of NS platforms, but considering the strong rain attenuation in the 47-48 GHz frequency band, and the fact that the related devices and equipment at that time were still under development and were expensive, the WRC-2000 conference in 2000 added the frequency bands of 27.5-28.35 GHz (downlink) and 31.0-31.3 GHz (uplink) for the use of NS communication services [45]. From this regulation of the international telecommunication union (ITU), it can be found that the frequency bands allocated for the use of the NS mainly concentrate in the K (18 GHz-27 GHz), Ka (27 GHz-40 GHz), and Q/V (40 GHz-75 GHz band, with part of the lower frequency band overlapping with the Ka band) bands, that is, the millimeter-wave band. However, considering the factor of rain attenuation, the Q/V band is not the best choice at present. Therefore, the K/Ka frequency bands can be selected as the operating frequency bands for the integrated platform of communication, navigation, and TT&C of NS vehicles to carry out services. The frequency spectrum resources in this frequency band are abundant, which can enable large-capacity or broadband multimedia communication.