Galileo is developing a Signal Authentication Service (SAS) that integrates Open Service Navigation Message Authentication (OSNMA) on the E1 band with Spreading Code Authentication (SCA) on the E6 band to strengthen its resilience to spoofing attacks. As Galileo SAS is still under development, access to realistic and controllable SAS signals remains limited, hindering both the early development of compatible receivers and reproducible research on signal authentication. To bridge this gap, this paper presents GalSAS-SDR-SIM, an open-source software-defined radio (SDR) simulation platform that emulates the SAS workflow by coupling E6 code encryption with the OSNMA key-disclosure process. The platform allows flexible SAS configuration of code encryption parameters to accommodate receivers with different computational capabilities. It also supports the concurrent generation of Galileo E1, E5b, and E6 signals for user-defined locations and times, and OSNMA cross-satellite configurations. Experimental results demonstrate simultaneous verification of navigation messages and spreading codes. We further evaluate SAS authentication performance and computational resource costs under different SAS configurations. Implemented according to publicly available official specifications, GalSAS-SDR-SIM provides a practical tool for accelerating SAS-capable receiver development and supporting the research community in evaluating and improving Galileo signal-authentication techniques.

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