The terrestrial networks face the challenges of severe cost inefficiency and low feasibility to provide seamless services anytime and anywhere, especially in the extreme or hotspot areas (e.g., disaster areas, mountains, and oceans) due to limited service coverage and capacity. The integration of multi-dimensional networks consisting of space, air, and ground layers is expected to provide solutions in delivering cost-effective and ubiquitous Internet of things (IoT) services for billions of users and interconnected smart devices. Autonomous data collection, exchange, and processing across different network segments with minimal human interventions in space-air-ground IoT (SAG-IoT) can bring great convenience to consumers, however, it also suffers new attacks from intruders. Severe privacy invasion, reliability issues, and security breaches of SAG-IoT can hinder its wide deployment. The emerging blockchain holds great potentials to address the security concerns in SAG-IoT, thanks to its prominent features of decentralization, transparency, immutability, traceability, and auditability. Despite of the benefits of blockchain-empowered SAG-IoT, there exist a series of fundamental challenges in terms of efficiency and regulation due to the intrinsic characteristics of SAG-IoT (e.g., heterogeneity, time-variability, and poor interoperability) and the limitations of existing blockchain approaches (e.g., capacity and scalability). This article presents a comprehensive survey of the integration of blockchain technologies for securing SAG-IoT applications. Specifically, we first discuss the architecture, characteristics, and security threats of SAG-IoT systems. Then, we concentrate on the promising blockchain-based solutions for SAG-IoT security. Next, we discuss the critical challenges when integrating blockchain in SAG-IoT security services and review the state-of-the-art solutions. We further investigate the opportunities of blockchain in artificial intelligence and beyond 5G networks and provide open research directions for building future blockchain-empowered SAG-IoT systems.

中文翻译：

---

区块链赋能的空空地一体化网络：机遇、挑战和解决方案

地面网络面临着严重的成本低效和随时随地提供无缝服务的挑战，特别是在极端或热点地区（如灾区、山区和海洋），由于服务覆盖范围和容量有限。由空间、空中和地面层组成的多维网络的集成有望为数十亿用户和互连的智能设备提供具有成本效益和无处不在的物联网 (IoT) 服务的解决方案。空间-空-地物联网（SAG-IoT）以最少的人为干预，跨不同网段进行自主数据收集、交换和处理，可以为消费者带来极大的便利，但也面临着来自入侵者的新攻击。严重的隐私侵犯，可靠性问题，SAG-IoT 的安全漏洞可能会阻碍其广泛部署。新兴区块链在解决 SAG-IoT 中的安全问题方面具有巨大潜力，这要归功于其去中心化、透明性、不变性、可追溯性和可审计性的突出特点。尽管区块链赋能的 SAG-IoT 有诸多好处，但由于 SAG-IoT 的内在特性（例如，异构性、时变性和互操作性差）和局限性，在效率和监管方面存在一系列基本挑战现有的区块链方法（例如，容量和可扩展性）。本文对区块链技术的集成进行了全面调查，以保护 SAG-IoT 应用程序。具体来说，我们首先讨论 SAG-IoT 系统的架构、特征和安全威胁。然后，我们专注于有前途的基于区块链的 SAG-IoT 安全解决方案。接下来，我们讨论将区块链集成到 SAG-IoT 安全服务中的关键挑战，并回顾最先进的解决方案。我们进一步研究区块链在人工智能和 5G 网络之外的机会，并为构建未来区块链赋能的 SAG-IoT 系统提供开放的研究方向。