# Satellite-Aided Consensus Protocol for Scalable Blockchains
## 1 Introduction
However, the existing space blockchain solutions have not fully considered the characteristics of satellite communications, such as delay, link budget, bandwidth, on-board resources [16; 32; 33]. These solutions rely on the conventional, costly mining such as PoW, and use satellites merely as relays to forward blocks for unserved and under-served areas (e.g., BlockStream and ChainSat), or accelerate information diffusion in the mining network (e.g., [19; 20]). Especially, if satellites are deployed as full blockchain nodes (e.g., BitSat and SpaceChain), block verification and chain backup put extra stress on the limited on-board device process and storage capability, which may explain why the project BitSat stalled eventually. Moreover, the uplink is dilemmatic. The uplink has to go through specific authorized ground stations [20], e.g., Blockstream only announced the availability of one-way satellite broadcasting. Ordinary users cannot upload newly mined blocks directly, which actually hurts the decentralization of the blockchain system. Nevertheless, even if ordinary users can upload blocks, high-speed uplinks are required to reduce the transmission delay, which, however, would impose extra cost on ordinary miners and raises the barriers to entry. Even though all these above issues are tolerated within the Bitcoin blockchain, they would become bottlenecks for next-generation high-TPS blockchains in the future. Therefore, by comprehensively taking the characteristics of satellites into account, we propose a satellite-aided consensus protocol for efficient space-terrestrial blockchains. Our proposed protocol only needs low-speed, one-way multicast transmissions from space to terrestrial, which not only addresses the uplink dilemma but also vastly reduces the transmission delay. By leveraging the features of satellite multicast transmissions, the proposed protocol does not rely on the costly PoW protocol, and is thus eco-friendly and sustainable without consuming tremendous energy on meaningless hash queries. The main contributions of our work include:  
* We develop a satellite-aided permissionless consensus protocol for sustainable, scalable blockchains, which, to the best of our knowledge, has never been investigated before. * Technically, we demonstrate how the satellite-aided blockchain evolves and eventually converges despite various misbehaviors on a case-by-case basis, and show how the transmission outage and possible adversarial participants affect the chain evolution. * Using the PoW-based blockchain as a benchmark, we theoretically analyze and assess the system performance regarding security and network throughput. * Simulation results demonstrate that the satellite-aided blockchain exhibits greater scalability and can achieve higher TPS than the traditional PoW-based blockchains. This paper is organized as follows. Section 2 presents the principle and workflow of the satellite-aided consensus protocol. Section 3 illustrates the possible patterns of blockchain evolution. Sections 4 and 5 calculate the blockchain performance in terms of security and scalability, respectively. At last, Section 6 provides the simulation results, and the paper concludes with Section 7.