## 2 NS Communication, Navigation, and TT&C Integrated Frequency Selection
Therefore, to improve the reliability and convenience of the NS platform TT&C system, it is necessary to select the air-based relay mode. At present, systems such as the tracking and data relay satellite system (TDRSS) based on the Ka band, the NASA deep space network (DSN), and the European space agency (ESA) DSN provide design references for the integration of future communication, navigation, and TT&C integrated services based on K/Ka [51; 52; 53]. Moreover, the K/Ka dual-band over-the-horizon TT&C system can realize over-the-horizon communication, navigation, and TT&C services separately. This mode has  
Figure 1: Application scenarios of various frequency bands within the frequency range from 1 GHz to 40 GHz. the advantages of a large coverage area that ground relays do not have and high flexibility that satellite relays do not have. (4) Plasma sheath and black barrier problem  
When a high-dynamic NS vehicle flies in the NS at a speed of 5 to 25 Mach, the plasma sheath effect and the blackout phenomenon will occur, which may lead to the interruption of radio signals. To address this, there are two technical approaches: one is to start from the integrated technology of communication, navigation, and TT&C itself, such as increasing the transmission power and raising the operating frequency. Theoretically, when the aperture of the space-ground system, the system noise temperature, and the transmission power remain unchanged, for the K/Ka frequency band with a frequency \(f_{1}\) ranging from 18 to 40 GHz, compared with the L band with a frequency \(f_{2}\) of 1 to 2 GHz, the frequency is increased by \(N=f_{1}/f_{2}=40/1=40\) times, then the received level is increased by 20 lg\(N\). It can be obtained that the maximum gain can be increased by 32 dB; this largely compensates for the reduction in the signal-to-noise ratio (SNR) caused by rain attenuation and space transmission loss. Similarly, according to the calculation equation of the half-power beamwidth (angle) [54],  
\[\theta=70.5^{\circ}\times(\lambda/D) \tag{1}\]  
where \(\lambda\) is the operating wavelength of the antenna, and \(D\) is the aperture of the antenna. According to Equation (1), it can be known that by narrowing the beam, the power density of the signal can be increased, which is beneficial to improving the anti-interference ability of the signal. In addition, when the beam width is narrowed, it can not only greatly save the power on the satellite and reduce the volume of the equipment, but also reduce the mutual interference between ground TT&C stations. The second approach is to change the electrical characteristics of the plasma medium, such as improving the aerodynamic shape of the vehicle, adding substances to reduce the electron density, etc. [55]. Usually, changing the aerodynamic shape of the vehicle is costly and will affect the original design of the space payload. Moreover, many of these methods are still in the theoretical research stage, with complex technology and great implementation difficulty. Therefore, only the first technical approach will be discussed below. * Increasing the power of the transmitter can offset the attenuation of radio waves by the plasma to a certain extent, but increasing the power of the transmitter is limited by various factors such as devices, and it can only be used as an auxiliary means when it is effectively coordinated with the frequency selection. * By increasing the operating frequency, in the radio attenuation measurement experiment (RAM) in the United States, three antennas with frequencies of 220 MHz, 5700 MHz, and 9200 MHz were installed on the warhead at the same time, respectively. The recorded altitudes at which the radio signals were interrupted were 80 km, 54 km, and 40 km, and the altitude at which the signals recovered from the interruption was between 22 km and 23 km [56]. China and Russia have also conducted similar experiments, and the common conclusion is that increasing the frequency of radio waves has an obvious effect on reducing the altitude at which the blackout occurs.