What’s Inside an AIS Message? The Ultimate Beginner’s Guide

Whether navigating geopolitical tensions in the Red Sea, rerouting around the Cape of Good Hope, or braving extreme weather events - today’s global shipping industry is facing unprecedented risks. In heavily congested chokepoints and volatile crisis zones, precise ship tracking is no longer just a nice-to-have; it is vital for safeguarding crews, cargo, and global supply chains. To mitigate navigational hazards, prevent collisions, and maintain full situational awareness even in times of crisis, the maritime world relies on one indispensable foundation:
AIS (Automatic Identification System).

But how does maritime data transmission actually work, and what information is a ship broadcasting into the world?

In this first edition of our AIS Deep Dive Series, we break down step-by-step—and without requiring any prior technical background—what information is packed into an AIS message, why these details are split into distinct categories, and what makes this system so remarkably efficient.

What is AIS and How Does It Work?

AIS stands for Automatic Identification System. It is an automated radio tracking system operating in the VHF maritime band that transmits vessel positions and navigation parameters between nearby ships, shore stations, and satellites.

Originally developed to enhance maritime safety and collision avoidance, AIS has also transformed into the foundational dataset for global commerce, supply chain visibility, and ocean management.

An onboard AIS transceiver does not simply broadcast one single, massive data packet. To conserve bandwidth and prevent network congestion, the information is split into three core categories broadcast at different time intervals.

The 3 Data Categories of an AIS Message

To ensure smooth and reliable communication across crowded waterways, AIS data is divided into dynamic, static, and voyage-related data.

Dynamic Data (Real-Time Information)

Dynamic data constantly changes while a vessel is underway. Because this information is critical for immediate navigation and collision risk assessment, it is broadcast every few seconds (the exact frequency depends on the ship's speed and navigational status).

Key dynamic data points include:

• MMSI Number (Maritime Mobile Service Identity): A unique 9-digit identification number—essentially an international phone number for the ship. It is assigned by the flag state and may change if the vessel changes flag or ownership.
• Position: Highly precise GPS coordinates for Latitude and Longitude.
• Navigation Status: Indicates what the vessel is currently doing (e.g., underway using engine, at anchor, moored).
• SOG (Speed Over Ground) & COG (Course Over Ground): The vessel's actual speed and direction of movement.
• True Heading & ROT (Rate of Turn): The exact direction the ship’s bow is pointing and how fast it is turning.

Static Data (Identity Information)

Static data describes the permanent physical characteristics of the ship. Since these details rarely change, they are typically broadcast only every 6 minutes to save radio bandwidth.

Static data includes:

• IMO Number: A permanent, globally unique identifier linked directly to the ship's hull. Unlike the MMSI, the IMO number never changes throughout the life of the vessel.
• Vessel Name & Call Sign: The official name of the ship (e.g., Ever Given) and its radio call sign.
• Ship Type & Dimensions: Details on the vessel category (container ship, tanker, passenger vessel) along with its length and beam dimensions.

Voyage-Related Data (Destination Information)

Voyage-related data changes with every trip. It is manually configured by the ship’s crew prior to departure and is also transmitted roughly every 6 minutes. Static data and voyage-related data are broadcasted within the same data packet.

Key voyage details include:

• Draft: How deep the ship sits in the water. This is essential for safe navigation in shallow waters and harbors.
• Destination: The planned destination port.
• ETA (Estimated Time of Arrival): Expected arrival time in UTC.

Why This System Design is Genius

The structured scheduling of AIS messages is a masterclass in efficient communication protocols. If every ship broadcasted its full static and voyage details every few seconds, VHF radio frequencies in high-density regions - like the English Channel or the Strait of Malacca - would instantly become overloaded.

By utilizing clever timing - frequent updates for fast-changing data, infrequent updates for static info - AIS keeps data traffic light while maintaining maximum situational awareness.

Conclusion & What’s Next

AIS turns complex ship movements into clean, actionable data streams. By pairing real-time dynamic updates with static and voyage metadata, maritime logistics professionals, captains, and port authorities can track global fleet movements with remarkable precision.

In the second article of our AIS Deep Dive Series, we’ll take a closer look under the technical hood: Did you know there are 28 different AIS message types? Tune in again to find out how they work!