AIS Gap Analysis: Reading What Vessels Try to Hide

A common misconception in maritime OSINT is that AIS gaps are noise — receivers missing pings, weather, range limits. They are, sometimes. Most of the time they are not. Operators who want to disappear know exactly how AIS works, and what they are doing when they switch it off.

The job of an analyst is to separate the boring gaps (terrestrial coverage holes, common in the southern Indian Ocean or the deep Atlantic) from the meaningful ones — the ones that line up with port calls in sanctioned jurisdictions, ship-to-ship transfer corridors, or dark-zone rendezvous points.

What the silence usually means

Across roughly eighteen months of investigations into Iranian, Russian and Venezuelan oil flows, the same handful of patterns surfaced again and again:

• A vessel goes dark for 24–96 hours in a known STS (ship-to-ship transfer) corridor — the Strait of Hormuz approaches, the Lakonikos Gulf, the Sea of Japan east of South Korea — and reappears with a different draft.
• A vessel "loses AIS" three nautical miles outside a port whose call would be reported, then re-emerges twelve hours later on a course consistent with the same port.
• A vessel broadcasts a clean signal until it changes flag, then disappears for a week. The next AIS message often carries a new MMSI.

None of these on their own prove anything. Stacked together — and against a vessel's history — they are extremely difficult to dismiss as coincidence.

Reading a gap properly

When we open a vessel's pattern-of-life inside the Sentinel maritime view, we are not looking at a track. We are looking at three overlays at once:

• The terrestrial AIS feed (what near-shore receivers heard).
• The satellite AIS overlay (what S-AIS providers saw when the vessel was out of terrestrial range).
• The known coverage map — so we know whether the gap is forced or natural.

A gap inside a well-covered zone, especially a zone the vessel had been broadcasting through five minutes earlier, is almost always intentional. The platform flags it automatically. The analyst's job is to ask the next question: what was happening at the entry and exit of the silence? Speed, course, draft, weather. The four data points that turn a flag into a story.

Where it gets useful

Where this method earns its keep is in chains — when a vessel goes dark, surfaces, and another vessel went dark in the same zone twenty minutes later. The classic STS pattern. Pair that with a sanctions overlay on either vessel and you have a publishable lead in under an hour, not a week.

The reason platforms like Sentinel matter here is not that AIS data is hard to get — it is not. The reason is that nobody wants to be the analyst stitching together S-AIS providers, OFAC and EU sanctions lists, flag changes, and historical port calls by hand at three in the morning when an editor wants the brief by seven.

What is not a gap

A final note for analysts new to the field. The following are not, on their own, evidence of sanctions evasion:

• A gap in the southern Pacific east of Polynesia. Coverage is thin. It is geography, not intent.
• A two-hour gap during heavy weather. Class-A transponders can suffer in extreme conditions.
• A clean, short gap that coincides with a known terrestrial receiver outage. AIS receivers fail. So do their backups.

The skill — and it is a skill — is in the calibration. Stack enough patterns over enough vessels and the noise floor becomes obvious. The signal lights up. That is when the work begins.