What Is AIS and How Does It Work? A Complete Guide

The Automatic Identification System (AIS) is one of the most important pieces of navigation equipment aboard any commercial vessel. AIS broadcasts a ship's identity, position, course, and speed over VHF radio, giving bridge teams and shore stations a real-time picture of surrounding traffic.

For any mariner preparing for port state control inspections or keeping a safe watch, understanding how AIS works is essential knowledge.

What Is AIS (Automatic Identification System)?

AIS is an automated tracking system that uses VHF transponders to exchange navigation data between vessels and shore stations. A vessel equipped with AIS continuously broadcasts its identification, position, heading, speed, and voyage details to other AIS-equipped ships and coastal Vessel Traffic Services (VTS).

The International Maritime Organization (IMO) developed AIS in the late 1990s as a collision avoidance tool, then adopted mandatory carriage requirements under SOLAS Chapter V in 2000 with phased implementation between 2002 and 2004. Since then, AIS has expanded into traffic management, search and rescue coordination , and fleet monitoring.

AIS supplements marine radar rather than replacing it. Radar detects targets independently, while AIS relies on vessels actively broadcasting their information. A sound bridge team uses both systems together as part of standard navigation equipment practice.

How Does AIS Work?

Each AIS unit collects positioning data from GPS, combines that with pre-programmed vessel information, and transmits the package at regular intervals over VHF radio. A standard AIS station consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and connections to shipboard sensors. Heading, speed, and rate of turn are pulled from the gyrocompass, speed log, and other bridge instruments through NMEA data links.

VHF Channels and Transmission

AIS transmits on two dedicated VHF frequencies: Channel 87B (161.975 MHz) and Channel 88B (162.025 MHz). The system uses Self-Organizing Time Division Multiple Access (SOTDMA) technology, dividing each 60-second period into 2,250 time slots. Every AIS station autonomously selects its own transmission slot based on traffic history and the schedules of nearby stations. According to the U.S. Coast Guard Navigation Center, the system can handle over 4,500 reports per minute , making AIS effective even in congested waterways .

AIS Data on the Bridge

AIS data can appear on a standalone display, a radar overlay, or directly on an ECDIS. When integrated with an ECDIS like the SEALL system , AIS targets appear as labeled symbols on the electronic chart, giving the officer on watch immediate access to each vessel's name, course, speed, and closest point of approach (CPA).

What Data Does AIS Transmit?

AIS data falls into three categories. Static data includes the vessel's MMSI number, IMO number, call sign, name, ship type, and dimensions, all entered during initial setup and broadcast every six minutes. Dynamic data changes continuously: position from GPS, course over ground (COG), speed over ground (SOG), heading, rate of turn, and navigational status. Class A transponders broadcast dynamic data every two to 10 seconds while underway and every three minutes at anchor. Voyage-related data covers the current trip, including destination, ETA, draught, and cargo type. The officer on watch updates this information before departure.

Class A vs. Class B AIS Transponders

Class A transponders are mandatory for SOLAS vessels. Operating at 12.5 watts with updates every two to 10 seconds underway, Class A systems use SOTDMA technology to guarantee message delivery. A DSC receiver, external GPS, and heading sensor are all required. Class A units can also transmit and receive short safety-related text messages.

Class B transponders serve non-SOLAS vessels, including recreational craft and smaller commercial boats. Operating at two watts, Class B systems transmit less frequently (every 30 to 180 seconds) and use CSTDMA technology, which only transmits when open slots are available. Because of the lower power, Class B signals typically reach five to six nautical miles.

For watchkeeping officers, this distinction matters. Smaller Class B-equipped vessels may not appear on the AIS display as consistently as commercial traffic, and some small craft carry no AIS at all.

SOLAS Carriage Requirements for AIS

SOLAS Chapter V, Regulation 19.2.4 requires AIS on all ships of 300 gross tonnage and above engaged on international voyages, cargo ships of 500 gross tonnage and above regardless of voyage type, and all passenger ships irrespective of size. Some flag state administrations impose additional domestic requirements on non-SOLAS vessels.

AIS must remain switched on at all times while the vessel is operational, unless the master determines a safety or security risk. Port state control officers routinely check AIS functionality and data accuracy during inspections. Incorrect static data or a non-functional transponder can result in deficiencies. Vessels should keep current SOLAS publications aboard to verify applicable regulations and amendment timelines.

Limitations of AIS Every Mariner Should Know

AIS is a cooperative, self-reporting system with inherent limitations. A vessel without AIS, or one that has switched its transponder off, will not appear on the display. Terrestrial AIS range is limited to approximately 20 nautical miles, though satellite AIS (S-AIS) extends tracking to open ocean areas. Data accuracy depends on proper GPS calibration and correct manual input by the crew, and errors in MMSI, vessel dimensions, or navigational status are common.

AIS should always be used alongside radar, visual observation, and sound signals. Setting proper ECDIS alarm thresholds for CPA and TCPA adds another safety layer. The history of maritime navigation has consistently proven that no single technology replaces sound seamanship.

Stay Compliant, Stay Informed

Every AIS transponder aboard a commercial vessel represents a direct link to safer waterways. Whether verifying CPA in a busy shipping lane or confirming compliance before an inspection, understanding AIS is part of the job.

For the latest IMO publications , ECDIS solutions, and navigation equipment, contact American Nautical Services at +1 (954) 522-3321 or sales@amnautical.com.

Frequently Asked Questions

Q. What does AIS stand for in maritime?

AIS stands for Automatic Identification System, a VHF-based transponder system that automatically broadcasts and receives vessel identification, position, course, speed, and voyage information between ships and shore stations.

Q. Is AIS required on all vessels?

AIS is required under SOLAS Chapter V, Regulation 19.2.4 on ships of 300 GT and above on international voyages, cargo ships of 500 GT and above, and all passenger ships regardless of size. Some flag states mandate AIS on smaller vessels through domestic regulations.

Q. What is the difference between AIS Class A and Class B?

Class A transponders are mandatory for SOLAS vessels, transmit at 12.5 watts with updates every two to 10 seconds, and use SOTDMA technology. Class B transponders serve non-SOLAS vessels, transmit at two watts with updates every 30 to 180 seconds, and have a shorter range.

Q. Can AIS replace radar for collision avoidance?

AIS cannot replace radar. AIS depends on other vessels having functioning transponders, while radar detects targets independently. Professional mariners use both systems together as part of layered watchkeeping.

Q. What information does AIS transmit?

AIS transmits static data (vessel name, MMSI, IMO number, dimensions), dynamic data (position, COG, SOG, heading, rate of turn), and voyage-related data (destination, ETA, draught, cargo type).

Q. How far can AIS signals reach?

Terrestrial AIS has a typical range of approximately 20 nautical miles, depending on antenna height. Satellite AIS extends coverage globally by receiving signals from low Earth orbit, enabling tracking beyond coastal VHF range.