## 4 Analysis of Sun Outage  
Inter-satellite link sun outage refers to the phenomenon that the incident angle of sunlight is within the main lobe width of the laser terminal's receiving antenna, causing inter-satellite link communication to be interrupted. We used the Satellite Tool Kit (STK) and MATLAB to jointly simulate the actual impact of sun outage on the inter-satellite link. The simulation time is from the spring equinox to the summer solstice in 2023, the constellation is the polar-orbit Walker constellation 324/9/4.05, the orbit inclination is 89\({}^{\circ}\), the right ascension of the ascending node of the first orbit is the vernal equinox and the true periapsis angle is 0\({}^{\circ}\). In the simulation, we establish the vectors from each satellite to the adjacent satellite and the satellite to the sun in STK, and determine whether the angle between the two vectors is less than 3\({}^{\circ}\). If it is less than 3\({}^{\circ}\), the link is considered to have failed. The simulation is divided into two parts. The first part simulates the occurrence of solar transits from the vernal equinox to the summer solstice. The simulation step is 10 s. After that, the failure duration and number of failures of intra-plane ISLs and inter-plane ISLs are calculated, respectively, and the inter-satellite links located within the polar area and those outside the polar area can be distinguished. In the second part, based on the results of the first part, a time period in which inter-satellite link failures outside the polar region are more frequent is selected. The simulation duration is 1 h and the step size is 1 s. The duration and sequence of inter-satellite link failures during this period are analyzed.  
Figure 5 shows the interruption situation of the inter-satellite link in the entire network during sun outage. The time taken for intra-plane ISLs to be affected by sun outage is mainly concentrated around 100 s, while the failure time for inter-plane ISLs is between 10 s and 200 s.  
Figure 4: Traffic proportion by hour in one day.  
Figure 6 shows the failure situation of the inter-satellite link in the polar region from 4.00 to 5.00 on April 21. The abscissa represents the simulation time, the ordinate represents the link destination satellite number and the length of the line segment is the failure time. The blue line segment represents the inter-plane ISL in the west direction, and the red line segment represents the inter-plane ISL in the east direction. During this period, no sun outage occurred on the intra-plane ISL outside the polar region. It can be seen from the results that sun outage on inter-satellite links is discontinuous, and there will be two inter-plane ISLs experiencing sun outage at the same time in the entire network, but they are scattered on both sides of the Earth and have little influence on each other. For the problem of inter-satellite link failure in the polar region, since it carries less traffic and the network topology changes relatively frequently, the mesh characteristics of the topology can be used to use pre-designed backup paths to deal with link failure. For inter-satellite link failures outside the polar region, we established a traffic distribution model to solve the traffic distribution problem to achieve load balancing in the failure area.