# Satellite-Aided Consensus Protocol for Scalable Blockchains
## 2 Satellite-Aided Consensus Protocol  
There are essentially three types of orbits classified by the satellite altitude: geostationary earth orbit (GEO), medium earth orbit (MEO), and low earth orbit (LEO) [32; 33; 34]. Among them, GEO satellites are stationary relative to the earth's surface so that the doppler shift is negligible and has a lower transmission outage probability than non-GEO satellites [32; 35; 36]. The GEO satellites work at very high altitudes (\(\approx\)35,786 km) and can offer the most extensive coverage [37]. Thanks to the low outage probability and wide coverage, GEO satellites are preferred in our proposed protocol. However, the high altitudes of GEO satellites also result in long delays and high path losses [32; 35]. Thus, we should design the consensus protocol by taking the drawbacks of GEO into account. The brief workflow of the satellite-aided consensus protocol is illustrated in Figure 1. The proposed protocol requires low-speed, one-way transmissions only, which effectively reduces the negative impact of round-trip delays. In each round, a GEO satellite periodically multicasts random oracles (typically of hundreds of bits) to the on-ground terminals based on the standards such as DVB-S2 and DVB-S2X [27; 28]. These random oracles are digitally signed by the satellite to avoid forgery and required to be included in the generated block for validation. The on-ground miners only need to capture random oracles from space through terrestrial user terminals, such as portable mobile receivers, very small aperture terminals (VSATs), among others [32]. A random oracle will point to a specific terrestrial miner and elect it as a winner of the current round. The winner is granted an opportunity for proposing a new block on the top of the existing chain and spreading it among terrestrial miners. Via satellite multicasting (and also possible terrestrial forwarding), the majority of terrestrial miners receive the random oracle and thus can verify the identity of the winner and validate the corresponding block. Therefore, the satellite-aided blockchain grows as one block is built on the top of others. Figure 1: The illustration of the proposed satellite-aided consensus protocol. We design the above workflow by comprehensively considering the characteristics of GEO satellites. The benefits of our proposed satellite-aided protocol are hence multifold. First, the protocol imposes a minimal burden on satellites with no need to process, store, verify, or even forward blocks in space. Second, since the random oracles are generated by satellites, the uplinks are unnecessary in the proposed protocol, which reduces the delay caused by GEO, solves the uplink dilemma, and further simplifies the space-terrestrial network structure. Third, we only multicast random oracles (hundreds of bits) instead of the whole blocks (typically of megabytes). On the one hand, it can reduce the influence of high path loss of GEO satellites, letting more miners at a low signal-to-noise ratio (SNR) capture the oracles. On the other hand, it lowers the link budget and is scalable for high TPS blockchains. Fourth, our solution does not require miners to perform meaningless hash queries but takes advantage of multicast transmissions to maintain the system consensus in a novel approach, which largely reduces the mining cost and is thus sustainable and green. By leveraging the characteristics of GEO satellites including its extensive coverage and low outage probability, the satellite-aided blockchain is more scalable and can achieve a higher TPS than traditional blockchains. (Please see the analysis in Section 5). In the satellite-aided consensus protocol, random oracles can be generated based on real-time physical quantities measured by the satellite, such as cosmic rays, hydromagnetic waves, transient radiations, and so forth [38; 39; 40]. Hence, the random oracle can be truly random rather than pseudo-random to guarantee mining fairness.