What does AIS stand for?

Automatic Identification System. The word “automatic” matters: the ship’s transponder broadcasts without anyone on board doing anything, except for the fields the crew types in, such as destination and navigational status.

Is AIS the same as radar?

No. Radar detects any object that reflects radio waves, whether or not it wants to be seen. AIS only shows ships that transmit, but tells you who they are, where they are going and what they are doing. Bridge teams use both.

Is it legal to turn off AIS?

A ship required to carry AIS must keep it on at sea and at anchor. IMO guidelines let the master switch it off when transmitting would compromise the ship’s safety or security, and ask for the reason to be logged. Switching it off to avoid detection for illegal activity is not a legitimate reason.

How accurate is an AIS position?

The position comes from the ship’s GPS and is typically accurate to about 10 metres. What ages is the report: a position is only as current as the last message a receiver heard, which is why Voydar shows how old every position is.

What is AIS (Automatic Identification System)?

Updated Sep 28, 2026 · 5 min read

In short

AIS (Automatic Identification System) is a radio system that makes ships broadcast who they are and where they are going. A transponder on board sends the ship’s identity, position, course, speed and navigational status every few seconds on two VHF channels, and receives the same from ships around it. It was designed for collision avoidance and traffic services; it is also what makes public ship tracking possible. SOLAS requires it 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.

Who has to carry AIS

SOLAS regulation V/19 requires AIS on all ships of 300 gross tonnage and upwards engaged on international voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages, and all passenger ships irrespective of size. The requirement was phased in between 2002 and 31 December 2004. Warships and government ships are not required to carry it, and flag states can exempt some ships.

Many smaller boats carry AIS voluntarily, usually the simpler Class B equipment. Some national rules go further than SOLAS: in the United States, for example, the Coast Guard requires AIS on certain commercial vessels of 65 feet and more.

What a ship broadcasts

IMO resolution A.1106(29) divides the data into three kinds:

Static: MMSI, IMO number, call sign, name, length and beam, type of ship and where the GPS antenna sits. Set when the equipment is installed.
Dynamic: position (to about 10 metres), time, course and speed over ground, heading, rate of turn and navigational status. Updated automatically from the ship’s sensors, except the navigational status, which the crew sets by hand.
Voyage-related: draught, whether hazardous cargo is carried, destination and ETA, typed in by the crew at the start of each voyage.

The split explains most of what goes wrong with AIS data. Positions come from GPS and are usually right. Destination, ETA and navigational status depend on someone keeping them up to date, and are often stale.

How often a ship reports

A Class A transponder, the kind SOLAS requires, reports its position more often the faster it goes and when it turns. Static and voyage data go out every six minutes.

Class A reporting intervals (IMO resolution A.1106(29), table 2)
Ship’s state	Position every
At anchor or moored, not moving faster than 3 knots	3 minutes
At anchor or moored, moving faster than 3 knots	10 seconds
0–14 knots	10 seconds
0–14 knots and changing course	3⅓ seconds
14–23 knots	6 seconds
14–23 knots and changing course	2 seconds
Faster than 23 knots	2 seconds

Class B equipment, built for boats outside SOLAS, reports less often: the common “CS” type sends a position every 30 seconds above 2 knots and every 3 minutes when slower, and Class B position reports carry no navigational status.

How it works on the radio

AIS uses two dedicated VHF channels, AIS 1 (161.975 MHz) and AIS 2 (162.025 MHz), defined in Recommendation ITU-R M.1371. Transponders share the channels by dividing each minute into time slots and reserving slots ahead (self-organising TDMA), so hundreds of ships in range can transmit without talking over each other. Class A units transmit at 12.5 watts.

Because it is VHF, AIS travels about as far as the horizon from the antennas: IMO’s guidelines give a typical range at sea of 20 to 30 nautical miles, depending on antenna height. Shore receivers on high ground hear much further. Out of range of any receiver, only satellites can pick the signal up, which is the subject of how ship tracking works.

What Voydar is hearing right now

Live from Voydar

39,021 ships reported a position to Voydar in the last hour.
330,191 vessels and 13,219 ports have a Voydar page.

Open the live map See receiver coverage

Ships heard: distinct MMSIs received by Voydar’s realtime service in the last hour. Catalog counts: Voydar’s own records. As of Oct 2, 2026, 17:02 UTC

What AIS does not tell you

Not every ship transmits. Small craft, many fishing boats and warships may carry no AIS, and IMO’s guidelines allow a master to switch it off if continuing to transmit “might compromise the safety or security” of the ship, for example where pirates operate. The switch-off should be logged and AIS restarted when the danger has passed.
Not every transmission is true. Crew-entered fields can be wrong or out of date, and transponders can be misprogrammed. Deliberately switching AIS off or disguising a ship’s identity to avoid scrutiny is one of the marks of the shadow fleet.
Not every transmission is heard. A gap in a track often means no receiver was in range, not that the ship was hiding.

In 2004 the IMO’s Maritime Safety Committee voiced concern that publishing AIS data on the web could harm the safety and security of ships and ports. Voydar publishes positions only from feeds whose licences allow it and from its own receivers, and protects sensitive locations such as war graves.

Questions and answers

What does AIS stand for?: Automatic Identification System. The word “automatic” matters: the ship’s transponder broadcasts without anyone on board doing anything, except for the fields the crew types in, such as destination and navigational status.
Is AIS the same as radar?: No. Radar detects any object that reflects radio waves, whether or not it wants to be seen. AIS only shows ships that transmit, but tells you who they are, where they are going and what they are doing. Bridge teams use both.
Is it legal to turn off AIS?: A ship required to carry AIS must keep it on at sea and at anchor. IMO guidelines let the master switch it off when transmitting would compromise the ship’s safety or security, and ask for the reason to be logged. Switching it off to avoid detection for illegal activity is not a legitimate reason.
How accurate is an AIS position?: The position comes from the ship’s GPS and is typically accurate to about 10 metres. What ages is the report: a position is only as current as the last message a receiver heard, which is why Voydar shows how old every position is.

Look up a ship

Search any vessel by name, IMO number or MMSI for its free Voydar profile: live position, identity history, ownership, inspections and sanctions.

Sources

Primary sources for the facts in this guide. Figures labelled as Voydar data come from Voydar’s own records, with their method stated next to them.

Published Sep 28, 2026 · For information only. Not for navigation, and not a substitute for the official sources cited.