Civilian Global Navigation Satellite Systems (GNSS) vulnerabilities are a threat to a wide gamut of critical systems. GNSS receivers, as part of the encompassing platform, can leverage external information to detect GNSS attacks. Specifically, cross-checking the time produced by the GNSS receiver against multiple trusted time sources can provide robust and assured PNT. In this work, we explore the combination of secure remote, network-based time providers and local precision oscillators. This multi-layered defense mechanism detects GNSS attacks that induce even small time offsets, including attacks mounted in cold start. Our system does not require any modification to the current structure of the GNSS receiver, it is agnostic to the satellite constellation and the attacker type. This makes time-based data validation of GNSS information compatible with existing receivers and readily deployable.
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