The Automatic Identification System is a transponder that broadcasts a vessel’s identity, position, course and speed. It was designed for collision avoidance — so that ships can see each other — and only incidentally became the world’s primary record of where ships go.
That design origin matters. AIS is a self-reported broadcast. The vessel transmits what its equipment is set to transmit, and there is no verification of any kind built into the system.
How it works
There are two distinct behaviours, and conflating them causes most of the confusion in this area.
Disabling. The transponder is switched off and the vessel disappears from the record. This is permitted in defined circumstances. The 2020 advisory records that the Safety of Life at Sea convention requires certain classes of vessel on international voyages to operate AIS at all times with few exceptions1 , and acknowledges that safety issues may at times prompt legitimate disablement, and that poor transmission may otherwise occur2 . The exception is genuine, which is what makes gaps ambiguous.
Spoofing. The transponder transmits false data — a false position, a false identity, or both. Here there is no legitimate version. A vessel broadcasting a position it is not at, or an identity that belongs to another ship, is making a false statement.
The 2020 advisory describes manipulation of AIS data — spoofing — as broadcasting a different name, IMO number, MMSI or other identifying information3 . The 2023 Price Cap Coalition advisory supplies the monitoring formulation: industry should vigilantly monitor irregular AIS patterns or data that are inconsistent with actual ship locations4 .
Identity manipulation
The related behaviour is manipulation of the identity a vessel transmits or displays. A ship’s IMO number is assigned to the hull for life and does not change with name, owner or flag, which is exactly why it is the identifier that designation entries use.
Physical alteration of that marking has no innocent version, and the advisories name it directly. The 2018 North Korea vessel advisory lists physically altering vessel identification5 as a tactic, describing the painting over of vessel names and IMO numbers with alternate ones6 ; the 2020 global advisory records the same practice7 .
Why the absence of data is itself evidence
The intuitive assumption is that a vessel which stops transmitting becomes invisible. In practice a gap is highly informative, because it is bounded.
A vessel that stops transmitting at a known position and resumes at another known position has defined a region and an interval. Its speed capability limits where it can have been. Its draught before and after indicates whether cargo moved. Its bunkering and port records may place it somewhere its track does not. Independent observation — satellite imagery, coastal radar, other vessels’ reports — can fill the interval.
The result is that a well-documented gap frequently produces a stronger inference than the transmitted track would have, because it shifts the burden: the vessel has to account for an interval it chose to leave unrecorded.
How it is caught
Gap analysis. Recurrence is the signal. A single gap is weather, equipment or a safety judgement. A vessel with repeated gaps, all coinciding with voyages to the same area, is displaying a pattern.
Physical inconsistency. Transmitted data is checked against what the vessel physically is and does: speed against capability, draught against declared cargo, position against port and bunkering records.
Independent observation. Position data the vessel did not choose to broadcast can be obtained elsewhere. The 2023 advisory recommends that where there are AIS outages or suspected manipulation, stakeholders with access to long-range identification and tracking use it to establish a vessel’s true location8 , and treats combining the two as a best practice.
Reconstruction against the voyage. The UN Panel of Experts documented the pattern in detail: route detours almost always involved manipulating AIS transmissions9 while loading in one port before rejoining the original route and reactivating the system in time for delivery. The Panel lists AIS manipulation alongside loitering, voyage deviations and fraudulent documentation10 as a single combination of tactics.
Cross-vessel contradiction. Two vessels cannot occupy the same position, and a transmitted identity that belongs to a ship documented elsewhere is self-refuting.
Which vessels display these patterns, and their ownership and registry histories, is vessel- level reference work and is kept on the sister site.