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What Is AIS? A Practical Guide to the Vessel Tracking System

Mark Douglas avatar
Written by Mark Douglas
Updated over a week ago

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


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

AIS Version

Indicates message format compatibility

🛑 This is often the least reliable category due to human error and irregular updates.


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.


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.

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