Eavesdropping of RIS-integrated Terahertz HAPS-enabled satellite communication

Satellite Communication (SatCom) systems are evolving rapidly to meet increasing demands for high-speed data transmission. In this context, the Terahertz (THz) frequency band has gained attention due to its untapped high bandwidth, but introduces new Physical Layer Security (PLS) challenges, especially in uplinks to Low Earth Orbit (LEO) satellites, where scattering due to atmospheric influence may enable interception outside the main beam. Integrating a Reconfigurable Intelligent Surface (RIS)-enabled High-Altitude Platform Station (HAPS) as a relay node offers a promising approach to mitigating these security risks. In this work, we propose, to the best of our knowledge, the first deterministic 2D single-scattering model specifically tailored to evaluate Non-Line-Of-Sight (NLOS) eavesdropping risks in THz-band satellite uplinks. The model includes atmospheric attenuation, Free Space Path Loss (FSPL), and single-scattering effects to analytically compute the Signal-to-Noise Ratio (SNR) and Secrecy Capacity (SC) for both legitimate and eavesdropping links under varying weather conditions. Simulations reveal non-negligible insecure spatial areas around the communication beam where the SC drops to zero. However, using a RIS-enabled HAPS reduces this area by 48%. These findings reveal key physical-layer risks in THz communication while simultaneously showing the potential of RIS-assisted HAPS in securing THz-frequency Non-Terrestrial Network (NTN) uplinks against these risks.