AIS Gap (Going Dark): Maritime Glossary

An AIS gap is a period when a ship stops broadcasting its Automatic Identification System (AIS) signal, so its position, course and identity disappear from tracking services. People often call a deliberate gap "going dark". Some gaps are innocent, caused by equipment faults or poor coverage, but unexplained gaps are one of the most common red flags in sanctions screening.

Is a ship allowed to switch off AIS?

SOLAS regulation V/19 requires AIS on ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 gross tonnage and upwards on domestic voyages and all passenger ships, and requires it to be kept in operation at all times. IMO guidance allows the master to switch it off where continued broadcasting would put the ship at risk, for example in waters where pirates operate, and flag states expect the decision to be recorded in the ship's log.

Why AIS gaps matter in sanctions screening

The May 2020 sanctions advisory from OFAC, the US State Department and the US Coast Guard lists disabling or manipulating AIS as a deceptive shipping practice, alongside ship to ship transfers used to hide where cargo came from. A gap that lines up with a port call or a transfer in a sanctioned area is a reason to ask questions before you trade with the vessel.

What to check when you find a gap

How long the gap lasted and where the ship was before and after it
Whether the ship's draught changed across the gap, which can point to loading or discharging
Whether the gap matches a known high risk area or another vessel's gap
Whether the operator can show a log entry or a safety reason for it

AIS gaps are one signal among several. Combine them with ownership, flag history and sanctions list checks, as set out in how to screen a vessel for sanctions.

How AIS gaps are spotted

Tracking services collect AIS signals from shore stations and satellites. An AIS gap shows up as a stretch of time with no positions from a ship in an area where coverage is normally good. A related tactic is manipulation, where a ship broadcasts false positions or a false identity, which the May 2020 US advisory also lists as a deceptive practice. Analysts look at where a ship went dark, where it reappeared, and whether anything about it changed in between.

Tidal screens vessels and counterparties for sanctions, ownership and compliance history, and documents every decision. See maritime sanctions screening software or maritime due diligence.

Sources

Dark fleet Ship to ship transfer Port call ETA window All glossary terms →

Frequently asked questions

What is an AIS gap?

An AIS gap is a period when a ship stops broadcasting its Automatic Identification System signal, so its position and identity drop off tracking services. A deliberate gap is often called going dark.

Is it illegal to turn off AIS?

SOLAS regulation V/19 requires AIS to be kept in operation at all times on the ships it covers. IMO guidance lets the master switch it off when broadcasting would endanger the ship, such as in pirate risk areas, and the decision should be logged.

Why are AIS gaps a sanctions red flag?

US sanctions guidance lists disabling or manipulating AIS as a deceptive shipping practice used to hide port calls and cargo transfers. An unexplained gap near a sanctioned area is a reason for extra checks.

Does every AIS gap mean wrongdoing?

No. Gaps also come from equipment faults and poor receiver coverage. What matters is whether the gap is explained and whether it fits a pattern with other warning signs.

AIS gap