A recent field report shared on LinkedIn described a less familiar but important GNSS failure mode:
almanac poisoning.
The worrying part was not only that the receiver could lose normal GNSS operation.
The worrying part was persistence.
According to the report, some commercial receivers could remain unable to function correctly for two to three days after leaving the affected area, unless a cold reset was performed.
That changes the operational question.
It is not only:
"Are we being jammed or spoofed right now?"
It is also:
"Did the receiver carry bad navigation assumptions out of the incident?"
Why Almanac Poisoning Is Different
Most GNSS interference discussions focus on the live event.
Jamming denies or degrades reception.
Spoofing can make a receiver compute a false but plausible position.
Almanac poisoning is different because it targets the receiver's stored knowledge about the satellite constellation.
GNSS receivers use almanac data to help find satellites and speed up acquisition. The almanac is not the high-precision navigation message used for an exact fix, but it helps the receiver know where to look.
If that stored data becomes wrong, the receiver can keep looking in the wrong places.
That means the failure can outlast the local interference environment.
The vessel, vehicle, or installation may have physically left the problem area, but the receiver may still be operating from poisoned assumptions.
The Recovery Problem
This is the part operators need to take seriously.
An obvious GNSS outage is disruptive, but crews know they have a problem.
A receiver that quietly remains impaired after the event is harder to reason about.
It may appear to be in a recovery phase.
It may intermittently see satellites.
It may produce partial or unstable navigation data.
It may require a cold reset or memory-clearing procedure before it behaves normally again.
That creates a practical risk: teams may treat the end of interference as the end of the incident.
But for the receiver, the incident may still be alive.
Why Stored State Matters
Navigation trust is usually discussed as if each fix stands alone.
It does not.
Receivers carry state.
They remember recent satellite information. They may keep backup data in memory. They use prior assumptions to reacquire signals quickly after power loss, antenna blockage, or movement between environments.
Most of the time, that is useful.
In a poisoned-data scenario, it becomes a liability.
The receiver's own memory can become part of the failure path.
That is why "we are no longer in the interference zone" is not enough.
The better question is:
"Has the receiver rebuilt a trustworthy view of the constellation?"
What This Means for Ships and Shore Systems
For maritime operations, persistent GNSS poisoning creates several uncomfortable possibilities.
A bridge team may leave a degraded area and expect GNSS confidence to return quickly.
A vessel may enter port with a receiver that is still recovering from a previous interference zone.
A shore dashboard may see fresh timestamps and assume the position stream has returned to normal.
An incident review may miss the beginning of the problem if it only looks for the live jamming or spoofing window.
That last point matters.
If the receiver remains impaired for hours or days, the evidence window is longer than the attack window.
The navigation record needs to show not only when the external interference began, but when the receiver actually became trustworthy again.
What GeoWatch Looks For
GeoWatch is built around that distinction.
It does not only ask whether a receiver is producing coordinates.
It asks whether the navigation data is behaving like trustworthy vessel motion.
For a persistent receiver-state problem, useful signals may include:
- slow or failed reacquisition after leaving a known interference area
- satellite lists that look unusual for the vessel's location and time
- repeated use of satellites with poor or inconsistent signal behavior
- fresh timestamps paired with missing or unstable fixes
- position, speed, and heading values that do not match onboard motion
- recovery patterns that require operator intervention, reset, or cold start
None of those signals alone proves almanac poisoning.
Together, they can show that the GNSS stream has not returned to ordinary trust.
That is the operational point.
The system needs to preserve the recovery evidence, not only the initial failure.
The Takeaway
Long-lasting GNSS poisoning is a reminder that navigation trust can fail in layers.
The radio environment may improve before the receiver does.
The vessel may leave the affected area before the stored receiver state is clean.
The dashboard may show activity before the navigation solution is trustworthy.
For operators, the lesson is simple:
do not treat the end of jamming or spoofing as automatic proof of recovery.
The better question is:
"Can we prove the receiver has returned to a trustworthy navigation state?"
That is exactly the kind of gap GeoWatch is designed to watch.