## 5 Results
### Pointing Error Angle Simulation  
Taking a low-orbit satellite and a geostationary orbit satellite as an example, the motion scene of the low-orbit satellite and the geostationary orbit satellite affected by the perturbation force in space is simulated by STK software. The TLE information of low-orbit satellites and geostationary-orbit satellites is given in Table 3.  
Figure 8: The relationship between pointing error angle, beam radius at the transmitting end, and signal power attenuation. \begin{table}
\begin{tabular}{c c c} \hline \hline Parameter & LEO & GEO \\ \hline Semimajor axis (km) & 686 & 35797 \\ Eccentricity & \(1.06\times 10^{-3}\) & \(2.25\times 10^{-4}\) \\ Inclination (deg) & 98.1268 & 0.1129 \\ RAAN (deg) & 354.3541 & 80.757 \\ Argument of Perigee (deg) & 65.62 & 125.98 \\ True Anomaly (deg) & 0 & 19.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Satellite TLE Information. According to the satellite perturbation model in Section 1, the six Keplerian numbers are solved through the TLE data to obtain the initial position and velocity of the satellite. The number of orbital elements of the satellite at any time, and finally the azimuth angle, pitch angle, and pointing error angle of the transmitter are solved according to the inter-satellite terahertz link model. In this paper, through the co-simulation of matlab and stk, the calculated orbital elements of the satellite at any time are input into stk through matlab, and stk is used to simulate the inter-satellite terahertz communication scene. The scene parameters are set as follows: the scene time is from 04:00:00 on 12 July 2022 to 04:00:00 on 14 July 2022, see Figure 9 for the scene. The low-orbit satellites and geostationary-orbit satellites will be affected by various perturbation forces during operation, resulting in orbital deviations. If the perturbation forces are allowed to act on the satellites, the satellite orbits will change dramatically, resulting in huge pointing errors. Therefore, when the satellite passes through the ground station, the ground station will send an orbit control command to the satellite to correct the error of the satellite deviating from the orbit, so that it can return to the predetermined orbit and effectively control the pointing error. In this paper, it is assumed that every time the satellite passes through the ground station, an orbit adjustment is performed. For the uplink terahertz communication link of LEO-GEO, the antenna of the low-orbit satellite is pointed as the transmitting direction, and the antenna of the geostationary orbit satellite is responsible for receiving. For the downlink terahertz communication link of GEO-LEO, the antenna of the geostationary orbit satellite is pointed to the transmitting direction, and the low-orbit satellite antenna is responsible for receiving. According to the satellite perturbation model and the pointing error model, the pointing errors of the uplink and downlink are calculated, respectively, and the variation of the pointing error angle with time is obtained, as shown in Figure 10.  
It can be seen from Figure 10 that with the increase of time, in the case of no orbit adjustment, the pointing error angles of the uplink and the downlink have an increasing trend as a whole, and the maximum is 0.01638\({}^{\circ}\). It can also be seen that the pointing error angle of the downlink is much larger than that of the uplink, indicating that the influence of the perturbation force on the low-orbit satellite is greater than that of the geostationary orbit satellite. In addition, it is obvious from Figure 10 that the variation of the pointing error angle with time has a periodicity, which is likely to be related to the periodic motion of the low-orbit satellite, and with the increase of the number of operating cycles of the low-orbit satellite. The magnitude and variation of the pointing error angle are also gradually increasing, probably due to the cumulative effect of the perturbation force on the satellite with the increase of time.