  Satellite-based Communication systems are gaining renewed momentum in
Industry and Academia, thanks to innovative services introduced by leading tech
companies and the promising impact they can deliver towards the global
connectivity objective tackled by early 6G initiatives. On the one hand, the
emergence of new manufacturing processes and radio technologies promises to
reduce service costs while guaranteeing outstanding communication latency,
available bandwidth, flexibility, and coverage range. On the other hand,
cybersecurity techniques and solutions applied in SATCOM links should be
updated to reflect the substantial advancements in attacker capabilities
characterizing the last two decades. However, business urgency and
opportunities are leading operators towards challenging system trade-offs,
resulting in an increased attack surface and a general relaxation of the
available security services. In this paper, we tackle the cited problems and
present a comprehensive survey on the link-layer security threats, solutions,
and challenges faced when deploying and operating SATCOM systems.Specifically,
we classify the literature on security for SATCOM systems into two main
branches, i.e., physical-layer security and cryptography schemes.Then, we
further identify specific research domains for each of the identified branches,
focusing on dedicated security issues, including, e.g., physical-layer
confidentiality, anti-jamming schemes, anti-spoofing strategies, and
quantum-based key distribution schemes. For each of the above domains, we
highlight the most essential techniques, peculiarities, advantages,
disadvantages, lessons learned, and future directions.Finally, we also identify
emerging research topics whose additional investigation by Academia and
Industry could further attract researchers and investors, ultimately unleashing
the full potential behind ubiquitous satellite communications.
