GNSS Anomaly and Spoofing Response

Warning signs include an unexplained position jump, impossible course or speed, several vessels appearing displaced from radar echoes, a charted buoy shown on land, loss of time agreement, or GNSS-derived instruments changing together. Jamming may remove or degrade a fix; spoofing may present a plausible but false position. A receiver that continues to show normal-looking numbers is not necessarily healthy.

Protect the vessel before diagnosing the electronics. Disengage any route-following automation that depends on suspect position, reduce speed, create sea room, and avoid a narrowing channel or danger. Mark the time and last position considered reliable. Change from position-driven navigation to independent evidence: compass heading, log, radar ranges, transits, visual bearings, depth contours, and a carefully advanced DR or EP.

Check which systems share the suspect source and whether an independent receiver with a separate antenna, power supply, and constellation behaviour agrees. Review receiver alarms, DOP or integrity messages, antenna and network status, datum, and chart offset. Power-cycling may remove evidence and is not the first safety action. Do not restore automatic steering to a route until position integrity has been re-established by independent checks.

The command report should be concrete: what became inconsistent, when, which equipment shares the source, the navigation method now in use, the safe-water limit, and the next decision point. Report suspected interference through the appropriate coastal authority when safe to do so, while preserving a log of times, positions shown, observations, and equipment state.