AIS (Automatic Identification System) is a globally standardized maritime radio system designed primarily to enhance safety at sea and prevent collisions between vessels. The system allows an automatic, real-time exchange of critical navigation data for a comprehensive digital awareness of their surroundings, even in poor visibility or high-traffic areas.

The system relies on the Very High Frequency (VHF) maritime radio band at 161,975 MHz (Kanal 87B) and 162,025 MHz (Kanal 88B). The visibility range of surrounding ships depends on the height of the receiving and transmitting antennas. This is because waves propagate in a straight line, making them dependent on the line of sight, which is ultimately limited by the curvature of the earth. A typical reception range onshore is around 20 nautical miles (around 37 km), depending on the height above sea level and obstructions nearby.

The installation of an Class A AIS transmitter is mandatory for commercial vessels over 300 BRT, and it is possible to equip private ships with AIS Class B. Every vessel that is equipped with an AIS transmitter is obligated to turn it on. Only authorities or navy ships are allowed to turn it off. Class A vessels are given priority and transmit AIS data at a significantly higher power level of approximately 12.5 W, compared to Class B vessels, which transmit at 2 to 5 W.

To manage the vast amount of data being transmitted simultaneously by multiple vessels, the system utilizes Self-Organizing Time Division Multiple Access (SOTDMA) technology. This protocol allows transceivers to autonomously organize available time slots, effectively preventing signal interference and ensuring that every ship in the vicinity can communicate its position without overlapping with others.

The data transmitted through AIS messages actually includes 28 different message types. Two types of messages are important to provide a full profile of a ship's status. Every AIS message includes the vessel's MMSI to assign the related messages for the ship.

Static data (message type 5)

If possible, transmission occurs every 6 minutes. Covers permanent details such as:

Dynamic data (message types 1, 2, 3, and 27)

If possible, transmission occurs every few seconds, related to the speed of the vessel. Provides the real-time movement of the ship, including:

Terrestrial receiving stations are located on land to receive AIS data. Reception depends on the altitude above sea level and an unobstructed line of sight to the sea. A standard station equipped with a dipole antenna at approximately 30 meters above sea level can receive signals from ships at a distance of 20 nautical miles. With a permanent internet connection, the received data can be transmitted and processed with a very low latency of just a few milliseconds. These stations generally receive all messages from all vessels, regardless of whether they are Class A or Class B.

Satellites in Low Earth Orbit (LEO) receive AIS data while traveling at speeds of approximately 28.000 km/h, allowing them to cover the open sea. Due to their altitude above the Earth, they monitor very large areas. This results in overlapping SOTDMA slots, which makes it difficult to differentiate between individual AIS messages. Especially in ports and busy maritime areas, such as the North Sea, this leads to "dark spots" in coverage.

The transmission path from the satellite to the ground station can result in significant delays, ranging from minutes to several hours. Satellites primarily receive Class A transmissions, though not every message is captured. Due to their lower transmission power, Class B transmitters are rarely received.

Ships receive AIS data from surrounding ships for their own systems, effectively making them mobile terrestrial stations. With the availability of permanent internet even on the high seas, it is now possible to transmit this AIS data. With a stable Internet connection, the data can be transmitted within milliseconds and without significant delays. Because ships are not stationary, gaps in coverage are possible, and networks with a high number of ships are required to ensure comprehensive coverage.

https://www.itu.int/rec/R-REC-M.1371