Joint Communications, Sensing, and Positioning in 6G Multi-Functional Satellite Systems: Survey and Open Challenges

Satellite systems are expected to be a cornerstone of sixth-generation (6G) networks, providing ubiquitous coverage and supporting a wide range of services across communications, sensing, and positioning, navigation, and timing (PNT). Meeting these demands with current function-specific payload architectures is challenging in terms of cost, spectral use, and sustainability. This survey introduces the framework of multi-functional satellite systems (MFSS), which integrate two or more of these core services into a single payload, enabling resource sharing and functional synergy. A unified taxonomy is proposed, covering joint communications and sensing (JCAS), joint communications and PNT (JCAP), joint sensing and PNT (JSAP), and fully integrated joint communications, sensing, and PNT (JCSAP) systems. The paper reviews the state-of-the-art in each domain, examines existing payload architectures, and outlines cooperative, integrated, and joint design strategies. Key challenges, including waveform co-design, synchronization, interference mitigation, and resource management, are discussed, along with potential solutions and future research directions. By unifying diverse satellite capabilities within a single platform, MFSS can achieve higher spectral efficiency, reduced launch mass and cost, improved energy use, and enhanced service versatility, contributing to the development of sustainable and intelligent non-terrestrial networks (NTNs) for the 6G and beyond space era.