Which ships must have AIS?

Under SOLAS regulation V/19, ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 gross tonnage and upwards on other voyages, and all passenger ships — in force for all of them since 31 December 2004. The US adds commercial vessels of 65 feet or more.

How often does AIS update a ship's position?

A Class A ship under way reports every 2 to 10 seconds — faster when it is quicker or turning — and every 3 minutes at anchor. Its name, dimensions and voyage details go out every 6 minutes.

Can AIS track a ship in the middle of the ocean?

Not from the shore: VHF reaches about 20 to 30 nautical miles at sea. Satellites with AIS receivers pick up the signal instead, which is where mid-ocean positions on tracking maps come from.

Term

AIS (Automatic Identification System)

AIS, the Automatic Identification System, is the VHF transponder ships use to broadcast their identity, position, course, speed and voyage details to other ships and shore stations automatically. Under SOLAS, the IMO requires it on ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 tonnage and upwards on domestic ones, and all passenger ships.

Also called automatic identification system, AIS tracking, AIS data, satellite AIS, Class A AIS, AIS transponderUpdated 13 October 2026

What a ship sends

Type	Examples	Where it comes from
Static	MMSI, call sign and name, IMO number, length and beam, ship type	Set when the unit is installed
Dynamic	Position, course and speed over ground, heading, navigational status	The ship's sensors; navigational status is set by the officer of the watch
Voyage-related	Draught, hazardous cargo, destination and ETA	Typed in by hand at the start of the voyage

IMO Resolution A.1106(29), 2015

How often a ship reports

Ship's state	Position report every
At anchor or moored, moving no faster than 3 knots	3 minutes
0–14 knots	10 seconds
14–23 knots	6 seconds
14–23 knots and changing course	2 seconds
Static and voyage data	6 minutes

Recommendation ITU-R M.1371-6, February 2026 — a Class A unit

AIS runs on two VHF channels, so its reach is line of sight: the IMO's guidance puts a typical value at sea at 20 to 30 nautical miles, depending on antenna height. Further out, satellites with AIS receivers pick ships up; Class A units also send Message 27, built for long-range detection, nominally every 3 minutes. That is why a ship's position on a tracking map can be seconds old near the coast and minutes old mid-ocean.

Worked example: the gap between two positions

20 knots × 1,852 m per nautical mile ÷ 3,600 s = 10.3 m/s · a report every 6 s → about 62 m between positions · turning, a report every 2 s → about 21 m

Reporting intervals are ITU-R M.1371-6's for a Class A unit at 14–23 knots; the speed is illustrative.

In the US

US rules reach further than SOLAS. Under 33 CFR 164.46, as the US Coast Guard's navigation center sets it out, a type-approved Class A device is required on self-propelled commercial vessels of 65 feet or more and on commercial towing vessels of 26 feet or more with over 600 horsepower. AIS stays on at anchor, and a moored ship switches it on at least 15 minutes before getting underway.

The mistake people make

Reading the AIS destination and ETA as measured data. They are typed in by hand, and the IMO cautions that not all ships carry AIS, that a ship's master may switch it off, and that AIS may not give a complete or correct picture of traffic. Use AIS for where a ship is now; take its arrival time from the carrier's schedule or a model, and check one against the other.

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