Glossary
Short definitions of the terms the entries use. The UN AIS Handbook and the IMO’s identification number page go deeper.
IMO number. A seven-digit identifier (the last digit is a check digit) that stays with a ship’s hull for life, through renames, reflaggings and sales. It is mandatory for most seagoing merchant ships under SOLAS. S&P Global assigns the numbers free of charge on behalf of the IMO. There is also an IMO company number for owners and managers. Start every vessel search from the IMO number if you have it.
MMSI. The nine-digit Maritime Mobile Service Identity that a ship’s radio and AIS transmit. The first three digits (the MID) identify the flag state. An MMSI changes when a ship reflags, so it is not a permanent identifier, and shadow-fleet ships sometimes transmit MMSIs that do not match their flag or that belong to another ship.
Call sign. The radio identifier assigned by the flag state. It also changes on reflagging. The ITU MARS database maps call signs and MMSIs to ships and administrations.
AIS. The Automatic Identification System, a VHF broadcast of position, course, speed and identity. Class A transponders are compulsory for ships of 300 gross tonnage and up on international voyages and for all passenger ships. Class B units are cheaper, transmit less often and at lower power, and are what most small craft carry. Terrestrial AIS is picked up by shore receivers (many of them volunteer-run) within a few tens of nautical miles. Satellite AIS fills the gaps at sea, at a cost.
AIS gap, or dark activity. A period with no AIS positions. Causes range from poor coverage and a switched-off transponder to deliberate concealment. A gap is a lead, not proof, and the free government and NGO sources on the tracking page are where to check coverage before drawing conclusions.
Spoofing. Transmitting false AIS data, most often a false position (a ship appears somewhere it is not) or a false identity. Radar imagery and port records are the usual ways to test a suspected spoof.
STS transfer. A ship-to-ship transfer of cargo at sea or at anchor, used legitimately for lightering and illegitimately to obscure the origin of sanctioned oil. Equasis began carrying STS data in 2026.
Port state control (PSC). Inspections of foreign ships in national ports. Results, including detentions and deficiencies, are published by regional agreements called MoUs (Paris, Tokyo, Indian Ocean, Black Sea and others) and by the US Coast Guard. A detention history is one of the strongest free risk signals.
Flag state and open registries. The country whose flag a ship flies sets its rules and issues its documents. Open registries (often called flags of convenience) register ships owned elsewhere. Several publish searchable registers; some do not.
Registered owner, beneficial owner and ISM manager. The registered owner is the company on the papers, often a one-ship shell. The beneficial owner is the person or group who controls and profits from the ship, and is usually what an investigation wants. The ISM manager (or DOC company) runs the ship day to day under the International Safety Management Code. Equasis and GISIS show registered owners and managers; beneficial ownership takes corporate records.
Classification society. An organisation that sets technical standards and surveys ships against them. Class status and class history are public in several registers. A class withdrawal is a warning sign.
P&I club. A mutual insurer covering a shipowner’s liabilities. Ships outside the main International Group clubs, or with no cover listed, are worth a second look.
Bill of lading. The shipping document that names shipper, consignee, cargo and route. Customs releases of bills of lading (in the United States, for example) are the backbone of trade-data tools.
EEZ. The exclusive economic zone, extending up to 200 nautical miles from a coast, in which the coastal state controls resources. EEZ boundaries matter for fishing and for where a flag state’s open AIS data stops.
Chokepoint. A narrow passage that a large share of trade must pass through, such as Hormuz, Bab el-Mandeb, Malacca, Suez, Panama or the Turkish Straits. Transit counts for the main chokepoints are published weekly by the IMF.
Shadow fleet. Ships used to move sanctioned oil and goods while hiding their ownership and movements: old tonnage, opaque owners, flags that change often, uncertain insurance, and AIS gaps and spoofing. The term is informal; the lists on the sanctions page use their own criteria.
IUU fishing. Illegal, unreported and unregulated fishing. Regional fisheries management organisations (RFMOs) publish lists of vessels caught doing it, and the Combined IUU Vessel List merges them.
SAR imagery. Synthetic aperture radar from satellites such as Sentinel-1, which sees ships through cloud and at night. It is the main free way to find a ship that is not transmitting AIS.