  The Assisted Commercial Authentication Service (ACAS) is a semi-assisted
signal authentication concept currently being defined for Galileo, based on the
E6-C encrypted signal. Leveraging the assumption that the true E6-C encrypted
signal always arrives before any inauthentic signal, we define user concepts
for signal detection, including vestigial signal search. We define three
mitigation levels, each level defending against an increasing set of threats,
incorporating the described concepts and additional checks. The concepts are
analyzed and implemented in a simulation environment, and tested in both
nominal conditions and under advanced spoofing attacks. The results suggest
that even advanced attacks can be detected and mitigated by ACAS receivers.

Signal Processing