  Intentional interference, and in particular GNSS spoofing, is currently one
of the most significant concerns of the Positioning, Navigation and Timing
(PNT) community. With the adoption of Open Service Navigation Message
Authentication (OSNMA) in Galileo, the E1B signal component will continuously
broadcast unpredictable cryptographic data. This allows GNSS receivers not only
to ensure the authenticity of data origin but also to detect replay spoofing
attacks for receivers already tracking real signals with relatively good
visibility conditions. Since the spoofer needs to estimate the unpredictable
bits introduced by OSNMA with almost zero delay in order to perform a Security
Code Estimation and Replay (SCER) attack, the spoofer unavoidably introduces a
slight distortion into the signal, which can be the basis of a spoofing
detector. In this work, we propose five detectors based on partial correlations
of GNSS signals obtained over predictable and unpredictable parts of the
signals. We evaluate them in a wide set of test cases, including different
types of receiver and spoofing conditions. The results show that one of the
detectors is consistently superior to the others, and it is able to detect SCER
attacks with a high probability even in favorable conditions for the spoofer.
Finally, we discuss some practical considerations for implementing the proposed
detector in receivers, in particular when the Galileo OSNMA message structure
is used.
