The Automatic Identification System (AIS) powers most of what we know about vessel traffic today. From its roots in collision avoidance to its role in modern surveillance and risk detection, AIS is foundational—but far from flawless. This article walks through how AIS works, what it transmits, how often, and why some vessels still remain unseen.
AIS at a Glance
AIS is a self-reporting system. Vessels equipped with transceivers continuously broadcast data via VHF radio—everything from identity and location to cargo status and destination. These signals can be received by other vessels, shore stations, and increasingly, satellites.
AIS was mandated under the IMO SOLAS Convention in 2002 (IMO MSC.74(69)) and rolled out globally by 2008. Today, it is a primary input to most Maritime Domain Awareness (MDA) platforms — including Starboard.
Types of AIS Transceivers
The kind of transceiver a vessel uses affects how often it transmits, what it transmits, and who can receive it.
Class A: Commercial Standard
Who uses it: Cargo ships ≥300 GT, all passenger vessels on international voyages
Power: 12.5 watts
Range: 30–40 NM (shore), 400+ NM (satellite)
Update rate: Every 2–10 seconds underway, every 3–6 minutes at anchor
Message set: Full (static, dynamic, voyage, safety, binary, long-range)
Class B: Lighter Footprint
Who uses it: Yachts, fishing boats, small commercial vessels
Power: 2 watts
Range: 5–15 NM
Update rate: Every 30 seconds to 3 minutes
Message set: Limited (no voyage data; simplified safety messages)
Specialized Transmitters
AIS-SART, EPIRB-AIS, MOB-AIS: Transmit distress messages with position (IMO SN.1/Circ.227)
AIS AtoN: Aids to navigation (buoys, lighthouses, virtual markers)
Base Stations: Shore infrastructure managing slot assignments and time synchronisation
What Data Does AIS Transmit?
AIS broadcasts several categories of information. These messages follow strict formats defined in ITU-R M.1371 and are transmitted on VHF channels 87B (161.975 MHz) and 88B (162.025 MHz). We can group AIS data into four main types:
1. Static Vessel Information
Data entered manually at installation or during registration.
Field | Description |
MMSI | Maritime Mobile Service Identity (9-digit unique ID) |
IMO Number | International Maritime Organization vessel ID |
Call Sign | International radio call sign |
Vessel Name | Plain text ship name |
Ship Type Code | Numeric classification (IMO SN.1/Circ.227 and IMO SN.1/Circ.227/Corr.1) |
Length and Beam | Overall vessel dimensions |
Position Reference Point | GPS antenna location relative to hull |
⚠️ Static values can become outdated or misconfigured if not regularly maintained.
2. Dynamic Positioning Data
Generated automatically by onboard GNSS and heading sensors.
Field | Description |
Latitude / Longitude | WGS-84 geodetic coordinates |
Speed Over Ground | In knots, accurate to 0.1 kt |
Course Over Ground | In degrees, relative to true north |
Heading | Vessel’s bow direction (gyro/compass) |
Rate of Turn | Degrees per minute |
Navigation Status | Underway, moored, restricted, aground |
Timestamp | Seconds past the minute of last report |
Position Accuracy | Binary flag: high/low GPS accuracy |
See also: IALA Guideline 1082 – An Overview of AIS (June 2016).
3. Voyage-Related Data (Class A only)
Entered manually by crew and intended to provide voyage context.
Field | Description |
Destination Port | Plain text (often abbreviated) |
Estimated Time of Arrival (ETA) | UTC date and time format |
Draught | Reported to 0.1 m accuracy |
Cargo Hazard Indicator | Encoded per IMO pollutant categories (X, Y, Z, OS) |
AIS Version | Indicates message format compatibility |
🛑 This is often the least reliable category due to human error and irregular updates.
Learn more:
4. Safety and System Messages
Includes text-based alerts, binary payloads, and long-range transmissions.
Message Type | Use Case |
Safety Broadcast (Msg 14) | Maritime safety alerts (weather, hazards) |
Addressed Text Msg (Msg 12) | Direct ship-to-ship communication |
Interrogation (Msg 15) | Base station queries |
Long Range Broadcast (Msg 27) | Optimized for satellite detection of Class A signals |
Technical message format reference: ITU-R M.1371-5.
How Often Do AIS Messages Transmit?
Transmission rates vary by ship behavior and equipment class:
Condition | Class A Rate | Class B Rate |
>23 knots or turning | Every 2–3 seconds | Every 30 seconds |
14–23 knots | Every 6 seconds | Every 30 seconds |
<14 knots | Every 10 seconds | Every 30 seconds |
Anchored or moored | Every 3 minutes | Every 3 minutes |
Static/Voyage data | Every 6 minutes | Every 6 minutes |
Satellite-compatible long-range messages (Message 27) are typically sent every 3 minutes when beyond shore VHF range (Wahl et al., 2005).
Who Can Receive It?
Receiver Type | Typical Range | Notes |
Ship-to-Ship | 10–20 NM | Direct line-of-sight |
Terrestrial Stations | 20–40 NM | Dependent on antenna height and terrain |
Satellites (S-AIS) | 400+ NM | Susceptible to signal congestion and collision (ITU-R M.2123) |
The Tradeoff: Accessibility vs. Trust
AIS is powerful because it’s open and interoperable—but that also makes it vulnerable:
AIS messages can be spoofed, jammed, or turned off
Manual entries (like voyage destination or draught) may be incomplete or misleading
In congested areas, satellites may miss broadcasts due to time slot collisions
AIS is essential—but incomplete. At Starboard, we fuse AIS with satellite imagery, historical patterns, and ownership records to build a verified, risk-graded maritime picture.
Learn more:
Summary
AIS provides a continuous stream of ship identity, position, and intent—globally. It’s regulated, machine-readable, and critical for enforcement and transparency. But it’s also only one piece of the puzzle.
Understanding what each message really means—how often it’s sent, what field it came from, and whether it can be trusted—is key to using AIS data effectively in any operational setting.