## 4 The Digital-Twin-Based Detection and Protection Framework
### Data Description in the Framework
\[RT^{i}_{p}=\left\{\left\langle DEST^{k},NEXT^{k}\right\rangle|\ k\in(0,N\times M )\ \wedge k\
eq i\right\} \tag{5}\]  
Similarly, the digital-twin information \(I_{DT}\) obtained by the ground station also has the same data structure, as shown in Equation (6). \[I_{DT}=\left( \begin{array}{c}Pos_{DT}\\ LSA_{DT}\\ RIB_{DT}\\ RT_{DT}\end{array} \right)=\left(\begin{array}{c}Pos^{1}_{DT},\ \ Pos^{2}_{DT},\ \ \ldots,\ \ Pos^{N\times M}_{DT}\\ LSA^{1}_{DT},\ \ LSA^{2}_{DT},\ \ \ldots,\  
through sensor data, satellite-ground communication data, and other information, which is convenient for detecting the abnormal status of a satellite network. Therefore, we propose a detection and protection framework for SH and GH nodes induced by SDC errors based on digital twins, as shown in Figure 14. The framework is divided into two main parts: the physical twin and the digital twin. The physical twin collects and monitors necessary data, such as satellite position and LSA. At the same time, the digital twin can enable the ground staff to obtain the operation status of the satellite network, find out the problems of the satellite network, and make adjustments in time. In addition, the secure communication protocol [4] is used to maintain communication and state updates between the digital twin and the physical twin. By mapping the process files generated by the satellite routing mechanism to digital space, the framework can do all that it can to find the SH and GH nodes before the satellites transmit data. The proposed framework has four main modules: simulation module, routing module, detection module, and calculation module. The main functions of the simulation module include receiving LSA and satellite position information and updating the digital satellite network. The main functions of the routing module include route discovery and route planning. The RIB and RT generated in this model are important files for satellite routing. The main functions of the detection module include RIB detection and routing table detection. If the framework finds problems that can cause SH nodes or GH nodes, it will call the calculation module. The main functions of the calculation module include generating RIBs and RTs, which are used to recover the satellite network after finding problems. After collecting LSA and satellite position information, the satellite network will send them to the ground station. The ground station uses the simulation module to update the digital satellite network. At the same time, the satellite network continues to execute the routing module, generate RIB and RT in stages, and send them to the ground station, respectively. It should be noted that the RIBs generated during the route discovery phase are directly sent back to the ground station (GS) without waiting for the route planning phase to complete. Therefore, after receiving the RIBs (5. in Figure 14), the GS directly calls the detection module to detect the RIBs and returns the detection result (6. in Figure 14). If the GS finds an RIB with a problem, it will call the calculation module (7. in Figure 14), calculate the RT for the affected satellite, and send it to the affected satellite (8. in Figure 14). Similarly, after receiving the RTs (9. in Figure 14), the GS calls the detection module to detect the RTs and return the detection result (10. in Figure 14). If the GS finds an RT with a problem, it will call the calculation module (11. in Figure 14), calculate the RT for the affected satellites, and send it to the affected satellites (12. in Figure 14). In the simulation scenario of this article, the faulty nodes (including SH, GH, and affected nodes) cannot find the correct next hop for forwarding data due to the wrong routing tables. Therefore, the framework proposed in this paper recovers the faulty nodes by sending the correct routing tables to the faulty nodes. Figure 14: The detection and protection framework based on digital twins.