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What is AIS? How vessel tracking works
What AIS is, which ships must carry it, what data it broadcasts, how terrestrial and satellite AIS differ, and where the data reaches its limits.
- AIS
- Vessel tracking
- MMSI
In short
AIS (Automatic Identification System) lets ships broadcast their identity, position, course, speed and navigational status automatically over VHF radio. SOLAS makes it mandatory on ships of 300 GT and above on international voyages and on all passenger ships. Because the crew types in fields such as destination and ETA, they can be wrong or outdated.
AIS (Automatic Identification System) is the system ships use to broadcast their identity, position, course, speed and navigational status automatically over VHF radio. Other ships, shore stations and satellites receive these broadcasts, and they are the source of the positions you see on vessel tracking screens. Some fields are entered by hand, though, so AIS data needs to be read with care.
What is AIS and which ships must carry it?
AIS helps ships identify each other and helps shore-based traffic services monitor shipping. According to IMO, SOLAS regulation V/19 makes AIS mandatory on:
- all ships of 300 GT and above engaged on international voyages,
- cargo ships of 500 GT and above not engaged on international voyages,
- all passenger ships, regardless of size.
Ships fitted with AIS must keep it running at all times, except where international rules provide for the protection of navigational information.
The AIS unit on board is connected to the ship’s position, course and heading sensors and broadcasts its data on its own, with no action needed from the officer of the watch. Nearby ships and shore stations receive the broadcasts and see the ship on their displays with its name, course and speed. With a suitable display, AIS can also calculate the closest point of approach (CPA) between two ships and the time left until that point (TCPA).
Class A and Class B AIS
Class A units meet IMO’s mandatory carriage requirements and are used on SOLAS ships. Class B units are designed for craft outside the SOLAS requirements, such as leisure boats. The system gives priority to Class A; Class B units report less often or when free time slots are available. Both use the two VHF channels dedicated to AIS.
What information does AIS broadcast?
IMO’s guidelines for the onboard use of AIS (resolution A.1106(29)) group the data ships send into three types:
| Type | Content | How it is entered | How often it is sent |
|---|---|---|---|
| Static | MMSI, call sign and name, IMO number, length and beam, type of ship, location of the position-fixing antenna | Set when the unit is installed | Every 6 minutes or on request |
| Dynamic | Position, UTC time stamp, course over ground (COG), speed over ground (SOG), heading, rate of turn, navigational status | Automatic from ship sensors; navigational status by hand | Every 2 seconds to 3 minutes, depending on speed and course changes |
| Voyage-related | Draught, hazardous cargo type, destination and ETA, route plan | Entered by hand at the start of the voyage | Every 6 minutes or on request |
A Class A ship at anchor or moored and moving no faster than 3 knots reports its position every 3 minutes. Underway, the interval drops to seconds as speed rises and the ship changes course.
The MMSI (Maritime Mobile Service Identity) is the ship’s nine-digit identity in AIS. Its first three digits identify the flag administration, for example 271 for Türkiye. Our IMO number guide explains how the MMSI differs from the IMO number.
Why can navigational status and destination be wrong?
Navigational status is a field the officer of the watch selects by hand: underway using engine, at anchor, moored, restricted in ability to maneuver, constrained by draught, aground, engaged in fishing and so on. Destination, ETA and draught are also typed in at the start of the voyage and should be updated when they change. IMO recommends entering the destination as a UN/LOCODE, but the field is just a 20-character text box.
In practice, these fields are sometimes left unchanged, abbreviated or mistyped. You may see a destination left over from the previous voyage or an ETA that has already passed. That is why an AIS destination alone does not mean “this ship is coming here.”
What is the difference between terrestrial and satellite AIS?
AIS signals are collected in two ways:
| Terrestrial AIS | Satellite AIS | |
|---|---|---|
| Receiver | Shore-based VHF stations and other ships | Satellites in low Earth orbit |
| Coverage | VHF range; per IMO, typically 20-30 nautical miles at sea, depending on antenna height | Wide area, including the open ocean |
| Updates | Frequent and continuous within receiver range | Depend on satellite passes, so gaps can be longer |
| Weak point | Coverage ends as ships move away from the coast | Messages can collide in busy waters |
EMSA notes that receiving AIS messages by satellite extends the geographical range over which ships can be tracked. At the same time, as ESA explains, ships close to each other coordinate their transmission slots, but a satellite in orbit hears many ships that are not coordinated with one another, so messages can collide. Near the coast, terrestrial receivers usually deliver more frequent positions, while satellite data fills the gaps at sea.
What are the limits of AIS data?
IMO’s guidelines explicitly warn that AIS may not give a complete or correct picture of the traffic around a ship. The main limits are:
- Not every vessel has AIS. Leisure craft, fishing boats and warships may not carry it.
- AIS can be switched off. The master may turn it off if keeping it on could compromise the ship’s safety or security.
- Manually entered fields can be wrong. Navigational status, destination, ETA and draught are only as accurate as what the crew types in.
- Data is only as good as its source. Position, speed and heading come from the ship’s sensors, and a poorly calibrated sensor broadcasts wrong information.
- Coverage gaps and delays happen. Outside receiver range or between satellite passes, a position may not update for a while.
- False data is possible. The U.S. Coast Guard Navigation Center notes that AIS can be spoofed and advises users to verify.
How does VesselCall use AIS data?
VesselCall combines AIS positions with a range circle drawn around each port:
- You choose the Turkish ports you serve and set a range, in nautical miles, for each one.
- VesselCall watches AIS positions. When a commercial vessel (cargo, tanker, passenger) enters the circle, it is detected as an approaching vessel. Tugs, pilot boats, fishing boats, yachts and similar craft are filtered out by default.
- Detection is based on the ship’s actual position, not on the destination field typed in by the crew. A vessel moving back and forth across the edge of the circle does not trigger detection again and again.
- The detected vessel’s IMO number is used to find its managing company, and your introduction e-mail is sent from your own domain. The same vessel never receives a second e-mail within 7 days, even if it appears at another port.
You can find port-level details on the ports page and every step on the How it works page. The Data package uses the same position data to record port calls at Turkish ports and anchorages (arrival, departure, length of stay, at anchor or at berth). The records build up from the day monitoring starts.
