An inline hardware device interposed between a GNSS receiver and vehicle controller independently estimates motion via an inertial sensor to verify GNSS positions. By resolving estimated velocity against a drift-free, GNSS-independent directly measured heading, the device exposes slow cross-track deceptions.A leaky cumulative‑sum metric, arming post-lock,accumulates inconsistencies;at a calibrated threshold,a hardware switch isolates the receiver outside both software trust domains with bounded latency, substituting a valid dead reckoned, protocol‑valid position stream to prevent faults