During a monitoring-only offshore trial, GeoWatch observed a pattern that is easy to miss if you only look at the final track.
The vessel did not lose GNSS completely. The feed did not go blank. The reported position still looked like navigation data.
But the behaviour did not match the vessel.
The Pattern
For several minutes, the GNSS position remained fixed at the same coordinate
while speed over ground reported 0.00 kn.
Then the reported position jumped.
After the jump, the GNSS position began walking smoothly away on a southeast
track. The reported course settled into a narrow band, while speed over ground
alternated between 0.00 kn and roughly 0.8-1.1 kn.
That is the part that matters.
A bridge display could make this look like a slow drift or low-speed movement. But the onboard motion evidence told a different story.
Why It Was Suspicious
GeoWatch was also receiving independent vessel-motion data from an onboard motion sensor. During the same period, that sensor reported low, coherent vessel motion. The motion records were present and healthy.
So the system saw two different stories:
- GNSS said the vessel was moving away on a smooth track.
- The onboard motion sensor said the vessel was barely moving.
That mismatch is a useful signal. It does not prove malicious spoofing by itself, but it does mean the GNSS position should not be treated as trusted without cross-checking.
What This Could Mean
There are several possible explanations:
- local GNSS interference
- a spoofing or drift event
- receiver or antenna fault
- configuration or data-quality issue in the GNSS source
The important point is not to jump straight to attribution. The important point is to recognise when navigation data has stopped matching physical motion.
Why Monitoring Mode Matters
This was observed while alerts were disabled for live operations. That is intentional during trials.
Monitoring mode gives us time to review evidence, tune thresholds, and separate normal offshore operating patterns from genuine integrity problems. Offshore vessels can move slowly, hold station, reposition, and operate in ways that look unusual compared with ordinary port or transit behaviour.
But this case had a stronger shape than simple low-speed movement:
- fixed coordinate
- sudden jump
- smooth reported crawl
- inconsistent reported speed
- low vessel motion from an independent sensor
That is exactly why independent verification matters.
The Takeaway
GNSS integrity problems do not always look dramatic.
Sometimes the dangerous case is a position that keeps updating, looks plausible, and is quietly wrong.
For vessel operators, the practical question is not just whether GNSS is available. It is whether the reported navigation picture still agrees with independent evidence from the vessel itself.