Relays are pivotal in military communication networks, expanding coverage and ensuring reliable connectivity in challenging operational environments. While traditional terrestrial relays (TR) are constrained by fixed locations and vulnerability to physical obstructions, unmanned aerial vehicle (UAV)-mounted aerial relays (AR) offer a dynamic and flexible alternative by operating above obstacles and adapting to changing battlefield conditions. This paper provides a comprehensive survey of AR systems in military communications, presenting a detailed comparison between AR and TR paradigms and examining two specific AR technologies: active aerial relays (AAR) and aerial reconfigurable intelligent surface (ARIS) relays. The survey delves into their operation, benefits, challenges, and military applications, supported by a qualitative analysis across metrics such as coverage, flexibility, security, and cost. A novel multi-dimensional metric, the mission-critical relay effectiveness score (MCRES), is introduced as a quantitative method for evaluating relay suitability based on mission-specific weights for critical attributes like mobility, jamming resilience, deployment speed, stealth, coverage, and autonomy. Furthermore, we present Algorithm 1, a decision-making framework that leverages the MCRES to guide the systematic selection of the optimal relay type, AR or TR, and subsequently AAR or ARIS, tailored to the unique demands of a given military scenario, such as dynamic battlefield operations, electronic warfare, or covert missions. Finally, the paper addresses current implementation challenges and outlines promising future research directions to advance the deployment of robust and resilient UAV-mounted relay systems in contested military environments.

翻译：中继在军事通信网络中至关重要，能够扩展覆盖范围并在具有挑战性的作战环境中确保可靠的连接性。传统地面中继受限于固定位置且易受物理障碍影响，而无人机搭载的空中中继则通过在障碍物上方运行并适应不断变化的战场条件，提供了一种动态且灵活的替代方案。本文对军事通信中的空中中继系统进行了全面综述，详细比较了空中中继与地面中继范式，并探讨了两种具体的空中中继技术：有源空中中继和空中可重构智能表面中继。本综述深入分析了它们的运行原理、优势、挑战及军事应用，并辅以覆盖范围、灵活性、安全性和成本等指标的定性分析。本文引入了一种新颖的多维指标——任务关键中继效能评分，作为一种定量方法，用于基于任务特定权重（如机动性、抗干扰性、部署速度、隐身性、覆盖范围和自主性等关键属性）评估中继的适用性。此外，我们提出了算法1，这是一个决策框架，它利用任务关键中继效能评分来指导系统性地选择最优中继类型（空中中继或地面中继），进而选择有源空中中继或空中可重构智能表面中继，以适应特定军事场景（如动态战场作战、电子战或隐蔽任务）的独特需求。最后，本文讨论了当前面临的实施挑战，并概述了有前景的未来研究方向，以推动在对抗性军事环境中部署稳健且具有韧性的无人机搭载中继系统。