Fallback navigation is any alternative method used when GNSS is unavailable, degraded, or untrusted. It may rely on inertial sensors, odometry, maps, terrestrial radio systems, timing backups, or manual procedures. In practice, it gives operators a way to keep moving or keep time even when satellite signals fail.
This matters because GNSS is vulnerable to jamming, which blocks reception, and spoofing, which can feed a receiver false position or time data. A system that switches to fallback navigation can avoid blind trust in corrupted GNSS outputs. In transport and critical infrastructure, layered fallback is a core defense: it limits safety risk, supports continuity of service, and buys time for monitoring tools or operators to verify whether the navigation data is genuine.