Tardisoft Systems Maritime IT management

Field notes

When Clean GPS Still Fails the Physics Check

May 30, 2026

During the current monitoring and tuning period, GeoWatch identified another GNSS integrity pattern worth reviewing.

This one was interesting because the GNSS feed did not look obviously broken.

It looked clean.

The receiver reported valid differential GNSS data. GGA and RMC agreed with each other. The speed over ground was steady. Satellite count was high. HDOP was excellent and unusually stable.

But the reported motion still did not fully agree with the vessel.

GNSS feed Coherent
GNSS SOG 13.5 kn
IMU motion Low
Integrity read Cross-checking

The Pattern

The GNSS stream showed a smooth northeast movement pattern at roughly 13.3-13.8 kn.

Across the reviewed GGA and RMC messages, the navigation solution was internally consistent:

  • RMC reported valid data.
  • GGA reported differential fix quality.
  • Satellite count stayed high, mostly 32-34.
  • HDOP stayed around 0.42-0.44.
  • Course over ground stayed mostly in the 73-80 deg range.
  • The calculated movement was consistent with the reported speed over ground.

In other words, the GNSS feed did not look like random corruption.

It looked like a plausible vessel transit.

Why It Was Suspicious

The issue appeared when the GNSS motion was compared with independent onboard motion data.

SIMATRA reported good IMU status throughout the reviewed period. The onboard motion data showed low vessel motion, with only small yaw-rate and dynamic acceleration changes.

At the same time, the GNSS feed was reporting steady underway movement at about 13.5 kn.

That created the important mismatch:

  • GNSS said the vessel was making a clean transit.
  • The onboard motion evidence did not fully support that movement.

The system also recorded repeated position_jump and gps_signal_degraded events, even while ordinary GNSS quality fields still looked strong.

That is the lesson from this case.

A GNSS feed can be coherent, valid, and checksum-correct while still being untrustworthy.

Healthy-Looking Is Not the Same as True

Operators often look for obvious signs of GNSS trouble:

  • loss of fix
  • low satellite count
  • poor DOP
  • invalid RMC status
  • large jumps on the chart

Those are useful signals, but they are not enough.

In this case, the ordinary GNSS fields were not the strongest evidence. The stronger evidence came from physics:

  • Does the reported GNSS movement match onboard motion?
  • Does the track match the vessel's yaw and acceleration behavior?
  • Do independent sensors tell the same story?

GeoWatch is designed to ask those questions continuously.

What This Could Mean

This kind of pattern can have several possible explanations:

  • local GNSS interference
  • spoofing or injected GNSS motion
  • receiver or antenna issues
  • correction-source or configuration problems
  • timing or data-integration problems

This evidence alone should not be treated as proof of malicious activity.

It should be treated as a credible navigation-integrity event that deserves cross-checking against ECDIS, radar, AIS, DP logs, receiver warnings, correction status, and onboard equipment records.

The Takeaway

Valid GPS is not the same as true GPS.

A navigation feed can have good checksums, good fix status, strong satellite counts, and excellent DOP while the reported movement no longer matches the physical vessel.

That is why motion integrity matters.

The safest question is not simply:

Is GNSS available?

It is:

Does the navigation answer still agree with the vessel?

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