During another monitoring-only offshore trial, GeoWatch saw a navigation pattern that looked different from ordinary drift.
The GNSS receiver did not simply report a noisy position.
It moved in steps.
The reported coordinate would hold still for several samples. Then it would jump to a new coordinate. Then it would hold again.
Many of those jumps happened while speed over ground still reported 0.00 kn.
TL;DR
This was a valid navigation-integrity anomaly.
GNSS showed a stair-step position change: frozen coordinates, advancing timestamps, and repeated zero-speed jumps. At the same time, the onboard motion sensor stayed healthy and reported low vessel motion.
That combination is the important signal. The receiver still looked healthy in ordinary GNSS fields, but the reported position no longer matched physical vessel motion.
The Pattern
The raw RMC stream first showed plausible movement. For several minutes, the reported position moved smoothly with speed over ground in the 3-4.6 kn range.
Then the shape changed.
The reported speed dropped to 0.00 kn. The position froze. After that, the GNSS output began making discrete position changes every minute or so, with each new coordinate held for several samples.
In other words, the navigation feed was not showing a continuous vessel track. It was showing a staircase.
That matters because a stair-step track can be easy to miss on a chart. When zoomed out, it may look like slow movement. In the raw data, it is more suspicious:
- fixed coordinate
- advancing timestamps
- zero reported speed
- discrete position jumps
- another fixed coordinate
Why It Was Not Just Slow Movement
Slow offshore movement can be normal. Vessels can hold station, creep, reposition, or drift under operational constraints.
That is why GeoWatch does not treat every low-speed displacement as a serious anomaly.
In this case, the important evidence came from comparing GNSS output against independent vessel-motion data.
The onboard motion sensor reported low vessel motion. The IMU status was healthy. Dynamic acceleration stayed low. Yaw rates were small. The sensor record was present and coherent.
So the system saw a mismatch:
- GNSS position changed by more than a kilometre across the analysis window.
- The motion sensor did not show corresponding vessel movement.
That combination is stronger than either signal on its own.
Healthy-Looking GNSS Can Still Be Wrong
This case is also useful because the ordinary GNSS quality indicators did not look terrible.
The receiver was reporting a valid RMC status. Satellite count was high. HDOP was low. The feed did not disappear.
But valid-looking navigation fields are not the same thing as navigation integrity.
A receiver can continue producing coordinates, timestamps, satellite counts, and DOP values while the movement implied by those coordinates no longer matches the vessel.
That is the gap GeoWatch is designed to watch.
What This Could Mean
The right response is not to make an instant claim about cause.
A pattern like this could point to several possibilities:
- GNSS spoofing or induced drift
- local interference
- receiver or antenna fault
- correction-source issue
- navigation output smoothing, holdover, or stale-state behaviour
What matters operationally is that the position should not be treated as fully trusted without review.
Why Raw Evidence Matters
Final alert summaries are useful, but raw evidence is what explains the alert.
In this trial, the raw GNSS sentences showed the suspicious shape directly: frozen positions, zero-speed jumps, and repeated held coordinates.
The motion record added the other half of the picture: a healthy sensor reporting low physical motion while GNSS position continued to shift.
Together, those records create a much clearer incident story than a plotted track alone.
The Takeaway
Navigation integrity is not only about whether a receiver has a fix.
It is about whether the fix still agrees with physical reality.
When GNSS moves in steps, reports zero speed during displacement, and disagrees with independent vessel-motion data, the safest interpretation is not "normal slow movement."
It is: verify before trusting.