How AIS Ship Tracking Works: Classes, Messages and Gaps

The Automatic Identification System (AIS) is the reason anyone can watch ships move on a map. Each equipped vessel broadcasts its identity, position, course and speed over VHF radio, every few seconds when under way and every few minutes at anchor, and any receiver in range can pick those messages up. AIS was designed for collision avoidance, not surveillance, and many of its weaknesses as a tracking tool follow from that.

Who has to carry AIS

The carriage requirement is in SOLAS chapter V, regulation 19. According to the IMO, AIS must be fitted on all ships of 300 gross tonnage and above on international voyages, cargo ships of 500 gross tonnage and above not on international voyages, and all passenger ships regardless of size. Ships fitted with AIS must keep it in operation at all times, except where international rules provide for the protection of navigational information.

Warships, many fishing vessels and leisure craft are not covered by the requirement. Any map built from AIS therefore shows only part of the traffic.

Class A and Class B

AIS equipment comes in two main classes:

Class A transponders meet the IMO carriage requirement and are fitted on SOLAS ships, including virtually all ocean-going tankers.
Class B transponders are simpler, lower-powered units for vessels outside the SOLAS requirement. They report less often, and the system gives Class A units priority.

AIS uses two dedicated VHF channels, 161.975 MHz and 162.025 MHz. A ship's position report rate depends on its speed and whether it is turning. The figures below come from the IMO's revised guidelines for onboard use of AIS, resolution A.1106(29):

Class A ship status	Reporting interval
At anchor or moored, not moving faster than 3 knots	3 minutes
At anchor or moored, moving faster than 3 knots	10 seconds
0 to 14 knots	10 seconds
0 to 14 knots and changing course	3⅓ seconds
14 to 23 knots	6 seconds
Over 23 knots, or 14 to 23 knots and changing course	2 seconds

Class B units report less often. Carrier-sense (CS) units report every 30 seconds above 2 knots and every 3 minutes below; self-organizing (SO) units speed up with the vessel, to every 5 seconds above 23 knots.

Message types

AIS carries 27 message types. A handful matter for tracking ships, as listed by the US Coast Guard Navigation Center:

Message	Content
1, 2, 3	Class A position reports: position, course, speed, heading, navigational status
5	Class A static and voyage data: name, call sign, IMO number, ship type, dimensions, draught, destination, ETA
18, 19	Class B position reports
24	Class B static data, sent in two parts: the name, then details such as ship type
27	Long-range position report, for reception outside base station coverage

Static and voyage data are broadcast every six minutes. The distinction between the two matters. Identity, ship type and dimensions are set when the equipment is installed. Draught, destination, ETA and navigational status are entered by the crew, usually at the start of a voyage. A position comes from the ship's satellite navigation receiver and is generally reliable when the system is working honestly; a destination is only as accurate as the last person who typed it.

Tanker ship type codes

The ship type field is a two-digit code. The first digit 8 means tanker, so codes 80 to 89 cover tankers:

Code	Meaning
80	Tanker, all ships of this type
81 to 84	Tanker carrying dangerous goods, harmful substances or marine pollutants, IMO hazard or pollutant categories X, Y, Z and OS
85 to 88	Reserved for future use
89	Tanker, no additional information

Filtering on 80 to 89 is the standard way to pull tankers out of an AIS feed, and it is how the Baltic Sea tanker watch counts tankers reporting to Finland's open coastal receivers. The code is entered on board rather than checked by receivers, so a filter can miss tankers with a wrong or generic type and include ships that are not tankers.

Terrestrial and satellite reception

Terrestrial AIS relies on shore stations and other ships. The IMO guidelines give a typical range at sea of 20 to 30 nautical miles, depending on antenna height. Coverage is good near busy coasts and absent in the open ocean.

Satellite AIS uses receivers on low-orbit satellites to pick up transmissions far from land. The difficulty is that AIS was designed to share radio time slots within local cells. As the European Space Agency explains, a satellite sees many cells at once, and messages from ships that are not coordinating with each other collide. Detection is hardest exactly where traffic is densest. Message 27, a shorter position report for ships outside the coverage of shore stations, is intended for this kind of long-range reception. The Copernicus Sentinel-1C and Sentinel-1D radar satellites also carry AIS receivers, which lets radar images be compared with AIS collected at the same moment.

Gaps, switch-offs and spoofing

Several things create holes in the AIS picture:

Coverage gaps. No receiver in range means no data, even when the ship is transmitting.
Legitimate switch-offs. Under IMO resolution A.1106(29), the master may switch AIS off if continuous operation could compromise the ship's safety or security, or if security incidents are imminent. The action and the reason should be recorded in the logbook, and AIS restarted once the danger has passed.
Deliberate concealment. Ships involved in sanctions evasion switch off AIS to hide port calls and ship-to-ship transfers. The IMO's 2023 resolution on the "dark fleet" lists switching off AIS without a legitimate safety or security reason among the behaviors of such ships, while also noting that a ship's AIS going unreceived is not on its own proof of illegal activity. The guide to ship-to-ship transfers covers how these gaps are used.
Spoofing and manipulation. A transponder can broadcast a false position, a false identity or another ship's identifiers. Spoofed tracks often show impossible jumps in position or speed.
Interference with navigation signals. Because AIS positions come from satellite navigation, jamming or spoofing of those signals in a region corrupts the positions ships report, even from honest transponders.

These problems become severe during conflicts. The EIA noted that since the end of February 2026, AIS signal data for ships transiting the Strait of Hormuz have been especially unreliable, and its 2026 flow estimates for the strait rely on tanker-tracking data with additional analysis. The Strait of Hormuz guide covers that disruption.

The practical answer is to treat AIS as one source and check it against another. Satellite imagery shows ships whether or not they transmit, which is why the guide to Sentinel-2 ship detection is the natural companion to this one.

Key points

SOLAS requires AIS on ships of 300 GT and above on international voyages, cargo ships of 500 GT and above, and all passenger ships.
Class A units report as often as every 2 seconds under way and every 3 minutes at anchor; static and voyage data go out every 6 minutes.
Messages 1 to 3 carry Class A positions and message 5 carries identity, type, draught and destination; ship type codes 80 to 89 are tankers.
Terrestrial AIS reaches about 20 to 30 nautical miles; satellite AIS covers oceans but loses messages in dense traffic.
AIS can be switched off, spoofed or corrupted, so it should be checked against satellite imagery.