# A Hierarchical Blockchain-Based Trust Measurement Method for Drone Cluster Nodes
## 1 Introduction
However, the peer-to-peer broadcast approach inherent in blockchain applications can engender excessive inter-cluster bandwidth consumption. This, in turn, could potentially trigger network congestion issues at the cluster head nodes. PoW (Proof of Work), PoS (Proof of Stake), and PBFT (Practical Byzantine Fault Tolerance) are the most common consensus algorithms in the blockchain. However, these mechanisms do not take into account the resource constraints of UAVs, such as storage and computing power; therefore, developing efficient consensus algorithms is another challenge for drone clusters based on blockchain technology. In addition, UAVs cannot store blockchains that grow due to large-scale drone networks, due to storage capacity limitations. The PoW consensus mechanism can only process several transactions per second, and the block confirmation delay is about 60 min [21]. The adoption of entrusted Proof of Stake (DPoS) and PBFT consensus mechanisms can improve the consensus speed of blockchain systems. However, these consensus mechanisms require frequent interaction of devices and require a lot of communication resources. At the same time, only individual nodes are allowed to maintain the blockchain, and the distributed performance is low [22; 23]. Reference [24] adopts the structure of a two-layer blockchain consensus to achieve the purpose of verifying multiple blocks at the same time and improve the efficiency of the blockchain chain. However, the dual-layer structure adopts a PoW consensus, which requires too much computing power resources for wireless devices. The PBFT-PoW two-layer consensus structure in our paper adopts a PBFT consensus in a small range within the cluster, which can improve the efficiency of the blockchain and reduce the computing power requirements of the system. Within the framework of trust management, the blockchain functions as a reliable ledger for capturing and consolidating node perspectives. In the work of [25], the blockchain serves as a repository for recording node viewpoints contributed by miners, thus furnishing the necessary data for trust assessment. However, the scheme uses the traditional PoW consensus algorithm, and the drone cluster has high network latency and computational complexity. The literature [26] studied a blockchain-based vehicle-to-everything (BIoV) system, in which roadside units (RSUs) act as miners in the network and generate blocks to record data transmission between vehicles. To prevent data tampering between RSUs and malicious vehicles, the system selects the miners who generate the blocks through voting among all vehicles. In the context of trust management, the work presented in [27] introduces a trust management framework based on a series of blockchain timestamps, where the behavior of these unmanned aerial vehicles is periodically monitored by a group of distributed observers (DOs). DOs calculate relative trust scores for each UAV and record these scores in a transparent and secure ledger. In the context of enhancing internal security for UAVs, a blockchain-based trusted self-organizing network mechanism was devised and denoted as BC-UTSON, as outlined in Reference [28]. This pioneering work utilizes Practical Byzantine Fault Tolerance (U-PBFT) to ensure a lightweight consensus and real-time full-node trustworthiness assessment within the layered self-organizing network structure of UAV clusters. It also designs a blockchain-based multi-weight subjective logic (BMWSL) scheme to identify malicious UAV nodes based on trustworthiness assessments. Furthermore, this scheme augments the trusted path quality awareness through the Dynamic Routing mechanism, effectively thwarting data from being inadvertently routed through compromised nodes. While the blockchain serves as a credible ledger to record opinions, the evaluation outcomes could still be influenced by misleading information.