## 3 System Model
### System Model  
We use the blockchain network, which is jointly maintained and mined by each node in the satellite network system and each satellite node holds an identical ledger. The blockchain type we use is consortium blockchain and we introduce cryptocurrency to the network. We assume that there is no interest competition among all satellites and satellites provide services to users equally. As shown in Figure 1, adjacent satellite nodes in the same orbit are divided into the same region in the network topology. When receiving a user transaction, a satellite node stores it temporarily in the local cache pool instead of broadcasting it immediately. When a mining pool obtains the right of bookkeeping, the nodes in the pool jointly create a block. In the process of block generation, each node verifies the transaction in the local cache pool, and then the nodes at both ends of the region send the information to the master node in the middle of the region, which contains the successfully verified transaction and the calculated root hash. Finally, the master node integrates all transactions and broadcasts the block. In the consensus process, accounting in turn is adopted. All mining pools generate blocks and broadcast them in turn. This is similar to the process that the leaders in the RAFT consensus algorithm take turns to keep accounts. The RAFT consensus algorithm has three statuses: leader, candidate and follower. The followers nominate the leader via an election process. The leader is responsible for sending all messages to the participating followers in the network [36]. The order of block production in each round is random. We do not use incentive mechanism and nodes do not need to consume extra computing power to compete for the right of accounting. Each new block will be generated in sequence and thus the ledger can be synchronized.  
We design an authentication method based on cryptocurrency to improve authentication speed and security. As shown in Figure 2, authentication process is divided into three stages. In registration stage, the user applies to the authentication center for purchasing cryptocurrency. After verifying the user identity information and waiting for the user to pay successfully, the authentication center transfers the corresponding amount of cryptocurrency from the authentication center address to the user address. Then the authentication center sends the transaction to a satellite node, and the satellite node stores the transaction in the blockchain. In initial authentication stage, the user applies to a satellite node to purchase services. The satellite finds the user address on the blockchain according to the user's request information. If there is enough balance in the user address, the transaction submitted by the user is stored in the blockchain, and then the service can be provided. In the switching authentication stage, the user needs to switch to another satellite and apply for identity authentication from the satellite. The satellite checks the transaction submitted by the user when purchasing the service on the blockchain. If the transaction is valid, the satellite continues to provide services. Through the above three-stage authentication process using cryptocurrency technology, we can achieve fast switch authentication.  
Figure 1: Block generation and broadcast.