# A Hierarchical Blockchain-Based Trust Measurement Method for Drone Cluster Nodes
## 2 Trust Metric Model of Unmanned Aerial Vehicle Cluster Nodes Based on Hierarchical Blockchain  
### The Proposed Model  
UAV swarms have the characteristics of flexible self-organization of network topology and high dynamics of task data changes. UAVs can replace manual tasks through mutual coordination and information sharing by forming a network. Applications are becoming more and more intelligent. UAV formations often dynamically cluster when performing tasks, and work efficiency is improved through multi-cluster cooperative execution of tasks. However, when UAV cluster nodes dynamically combine formations, the high dynamics of nodes entering and leaving the trust cluster lead to low accuracy of node trust evaluation. The typical application environment of the UAV network is shown in Figure 1. There are multiple trust management domains in the environment. All UAV nodes belonging to the same task domain form a single trust management domain. A single management domain is controlled by the trust management node \(TMN_{x}\) and other common nodes \(N_{x}\). Aiming at the problem that UAV cluster nodes are highly dynamic in and out of trust clusters, resulting in low accuracy of node trust assessment, this paper proposes a trust measurement model for UAV cluster nodes based on hierarchical blockchain, as shown in Figure 2. All nodes in a single trust management domain jointly maintain a first-level blockchain, and trust management nodes in all management domains maintain a second-level blockchain. Each drone node in the environment has a unique identifier, and for the drones in the domain, the trust relationship of nodes is maintained by the management node, and the transfer of trust between domains is guaranteed by the secondary blockchain. Based on building a layered blockchain, this model ensures the credible evaluation, sharing, and management of the trust degree of UAV nodes. A trusted trust management model based on a layered blockchain includes a demand analysis layer, evaluation layer, and management layer. The demand analysis layer selects the node trust evaluation factor according to the characteristics of the UAV group itself and flexibly selects the appropriate credibility evaluation factor and calculation method, which can meet the needs of actual application  
Figure 1: Typical application environment of UAV network. Figure 2: Trust metric model of unmanned aerial vehicle cluster nodes based on hierarchical blockchain. scenarios. This paper uses direct trust, indirect trust, and historical trust to design evaluation factors for node credibility, and evaluates the overall behavior of nodes during the monitoring cycle. The evaluation layer is the core layer of node credible trust evaluation. Based on the trust evaluation factors determined by the demand analysis layer, the direct trust degree of the node is first calculated by using the linear weighting method. Considering that the evaluation of the direct trust degree mainly comes from its own experience, which is subjective, and through the indirect adoption of other entities, experience can reduce the influence of subjective evaluation, and then use the recommendation of neighbor nodes to calculate the indirect trust degree of the node to be evaluated. Then, considering the timeliness of the historical trust degree, this paper proposes to express the timeliness of trust based on the dynamic trust decay factor, and finally calculate the comprehensive trust degree of nodes by combining the direct trust degree, indirect trust degree, and historical trust degree. The management relies on the layered blockchain mechanism to manage the trust value, which mainly includes the record of the historical trust degree, the integration of the trust degree, and the update and adjustment of the trust degree. The accuracy and reliability of the model can be improved through validation and feedback in real situations.