The first risk of almanac poisoning is operational.
The receiver may fail to recover cleanly after leaving the affected area.
The second risk is evidential.
If the receiver remains impaired for two or three days, then every system that records or relays that receiver's output may preserve a misleading navigation history.
That includes the VDR.
The uncomfortable point is simple:
a Voyage Data Recorder can faithfully record invalid navigation data.
The record may be complete.
The timestamps may be present.
The files may be intact.
But if the GNSS source feeding the record was still operating from poisoned or corrupted satellite assumptions, the position evidence may not describe where the vessel actually was.
The VDR Records What It Is Given
A VDR is not a truth machine.
It is an evidence recorder.
That distinction matters.
If the bridge receives good navigation data, the VDR can preserve valuable evidence.
If the bridge receives bad navigation data, the VDR may preserve that bad data with the same confidence, format, and timestamp discipline.
The recorder does not automatically know whether the GNSS receiver's stored almanac was poisoned.
It does not automatically know whether the receiver needed a cold reset.
It does not automatically know whether the position stream had returned to a trustworthy navigation state.
It records the data path.
So if the data path is contaminated, the evidence record can be contaminated too.
Two or Three Days Is a Serious Evidence Window
A short GNSS anomaly is already difficult to reconstruct.
A two or three day recovery problem is much larger.
During that time, the vessel may change operating area, enter port, depart port, anchor, transfer cargo, pass through traffic separation schemes, or interact with pilots, terminals, tugs, and other vessels.
If the primary GNSS receiver is still impaired, the recorded position history may be questionable across the whole period.
That does not mean the VDR is useless.
It means the GNSS portion of the VDR record needs to be treated as suspect until it is reconciled with other evidence.
Useful cross-checks may include:
- radar and ARPA context
- AIS received from other vessels and shore systems
- ECDIS history and alarms
- speed log and gyro heading
- engine and rudder commands
- bridge audio and operator actions
- port, pilot, tug, and terminal records
- independent onboard motion or integrity records
The key is not to throw away the VDR.
The key is to avoid treating a recorded GNSS track as automatically valid just because it came from the VDR.
The Dangerous Assumption
The dangerous assumption is:
"If it is in the VDR, it is reliable."
That is too broad.
The better assumption is:
"If it is in the VDR, it is evidence of what the vessel systems received or produced."
Those are not the same thing.
In a GNSS poisoning event, the VDR may prove that the receiver was outputting a position.
It may not prove that the position was true.
It may prove that timestamps advanced.
It may not prove that the receiver had recovered.
It may prove that the bridge display had data.
It may not prove that the data was safe for navigation.
That difference becomes critical during incident review, insurance investigation, regulatory reporting, and dispute resolution.
How the Record Can Become Misleading
Almanac poisoning can create a confusing recovery period.
The receiver may not simply go dark.
It may produce partial fixes.
It may show changing satellite lists.
It may report a position intermittently.
It may appear to recover, then fail again.
It may require a cold start before it rebuilds a clean view of the constellation.
If a downstream system records only the final position sentences, the failure mode can be hard to see later.
The VDR record may show navigation output without preserving enough receiver-state context to explain why that output was untrustworthy.
That is how two or three days of data can become evidentially weak.
The data may exist, but the chain of trust behind it has broken.
What GeoWatch Adds
GeoWatch is designed to preserve the context around the navigation stream.
That matters during persistent receiver failures because the incident is not only a bad coordinate.
It is a timeline:
- when the receiver first became suspicious
- whether the vessel was inside or outside a known interference area
- how satellite acquisition behaved during recovery
- whether position, speed, and heading matched physical motion
- whether the receiver produced stale, unstable, or contradictory data
- when the navigation stream became trustworthy again
That timeline can sit beside the VDR record.
It gives investigators and operators a way to separate two questions that are often blended together:
"What did the vessel systems record?"
and:
"Could that recorded navigation data be trusted?"
Why This Matters After the Event
The most damaging part of a persistent GNSS poisoning incident may happen after everyone thinks the event is over.
The vessel has left the affected area.
The bridge has resumed normal watch routines.
The dashboard is receiving fresh data.
The VDR is recording.
But the receiver may still be recovering from corrupted stored state.
If that continues for two or three days, the vessel may lose more than navigation confidence.
It may lose clean evidence.
The VDR may still contain those days, but the GNSS component of those days may no longer be defensible without independent verification.
The Takeaway
Almanac poisoning is not only a positioning problem.
It is an evidence problem.
If a poisoned receiver keeps feeding downstream systems for days, the VDR may preserve a detailed record of navigation data that should not be treated as valid.
That is why recovery monitoring matters.
Operators need to know not only when interference started, but when the receiver actually returned to a trustworthy state.
Until that point, the VDR record may be complete, but the GNSS evidence inside it may be compromised.