GPS does not always fail loudly.
Sometimes it moves in small, believable steps while the vessel itself has not moved at all.
That is the dangerous case.
The Short Version
If your vessel is still but GPS says you moved, that is a lie.
Correction systems will not catch it. INS will not reliably catch it.
Only physics will.
The Quiet GPS Failure Nobody Notices
Sometimes GPS does not jump wildly across the chart.
It moves in small steps: 10 m, 20 m, 30 m.
The track can look smooth. It can look believable. It can look harmless.
But if the vessel is completely still, that movement is physically impossible.
Why This Is Dangerous
Quiet GPS lies can:
- confuse autopilots
- misalign INS filters
- break DP systems
- hide spoofing
- ruin track history
Because the motion looks plausible, operators may trust it.
That is exactly why it is dangerous.
Why Correction Systems Do Not Catch It
Fugro, SBAS, PPP, RTK, and LEO-based corrections are built to reduce navigation error.
They can help correct:
- drift
- bias
- atmospheric errors
They are not designed to prove that the reported motion was physically possible.
They do not reliably detect:
- step jumps
- spoofing
- impossible motion
In the worst case, a correction system simply corrects the lie.
Why INS Does Not Catch It
INS is good at dead reckoning.
But INS does not automatically detect deception.
If GPS lies quietly, an integrated navigation system may blend the bad position into the solution.
No alert. No warning. No simple statement that the motion is impossible.
Physics Is the Truth Source
If the vessel:
- did not accelerate
- did not rotate
- did not move
then any reported movement is false.
Physics does not lie.
GPS sometimes does.
The Fix
You need a system that asks simple physical questions:
- Did the vessel actually move?
- Did the IMU feel anything?
- Is the GPS motion physically possible?
This is motion integrity, not ordinary navigation.
The Takeaway
GPS can lie quietly.
Correction systems may not see it.
INS may not see it.
Physics will.
That is why motion-integrity monitoring exists.