What is AIS tracking? A SeaLight primer

This post explains how the AIS vessel tracking system works and how bad actors abuse the system.
Miao Shou, Gaute Friis | AUGUST 21, 2023
What is AIS tracking? A SeaLight primer

Miao Shou


Gaute Friis




Tracking vessels' Automatic Information System (AIS) signals is a core means SeaLight and other open-source intelligence collectors use to monitor maritime activity. AIS is a broadcast system that maritime authorities use to identify a vessel's unique identification number, type, position, course, speed and navigation status. 

The International Maritime Organization (IMO) mandates the use of AIS to ensure maritime safety and direct marine traffic more efficiently. According to the International Maritime Organization's International Convention for the Safety of Life at Sea (SOLAS) Regulation V/19.2.4, all vessels over 300 gross tons (or 500 tons for those not on international voyages) and passenger ships of any size are required to "carry automatic identification systems capable of providing information about the ship to other ships and to coastal authorities." 

SOLAS came into effect in 1974—before the development of AIS—and 167 nations (including the People's Republic of China) have agreed to abide by its provisions. Its updated regulations now include the requirement for specified ships to have AIS "in operation at all times except where international agreements, rules or standards provide for the protection of navigational information". 

The limited exceptions SOLAS lays out cover two main categories:

  1. Military ships are exempt from the regulations, though many navies choose to broadcast AIS during routine operations as a safety precaution. Coast guard ships are not exempted.
  2. A ship can suspend broadcast of its AIS signal for valid security reasons, such as when it is in imminent danger due to sailing in pirate-infested waters. In such circumstances, the AIS signal should be switched back on as soon as the threat is no longer imminent, and a report detailing the actions and reasons should be saved in the ship's log and submitted to the relevant authority.

Class A AIS vs Class B AIS
There are two types of AIS transponders: Class A and Class B. The distinction is that Class B transponders are low-powered devices with limited functionality, and are intended for voluntary use by vessels not covered under SOLAS AIS regulations, such as small fishing boats and pleasure craft. 

Only Class A signals are detectable by satellite receivers, with Class B signals having a limited range of about 8-10 nautical miles. Therefore, Class B-equipped ships are generally only visible when they navigate very close to a land-based receiver or another passing ship. 

Many larger ships include information about AIS signals they receive in their own satellite uplinks. This means that some ships' Class B data only becomes visible to the ship-tracking community when a properly equipped larger ship passes by its location.

The following scenarios illustrate how ships may or may not be visible on AIS depending on circumstance:  

SeaBoats_Boat 1.png
Scenario 1: The Class B vessel is in range of a land-based antenna while the Class A ship is in range of a satellite. In this scenario, both A and B are visible on AIS.


SeaBoats_Boat 2.png
Scenario 2: Both the Class A and Class B vessels are in range of a satellite, but only the Class A ship will be visible on AIS as Class B signals are not collected by satellites and the Class B vessel is too far from a land-based antenna. 


SeaBoats_Boat 3.png
Scenario 3: Both the Class A and Class B vessels are in range of a satellite, but only the Class A ship is connected to the satellite. However, the Class B vessel will still be visible on AIS because its signals are received by the Class A vessel via a ship-to-ship connection and transmitted to the satellite. 

AIS Abuse and Countermeasures
AIS was designed in the early 1990s and lacks basic cybersecurity features such as independent verification and encryption. It is therefore easy for the gray-zone operator to manipulate the system to obscure its activities. The most common methods these actors use include:

  1. Switching from Class A to Class B or turning off AIS transmissions completely. We call this tactic "going dark".
  2. Deliberately broadcasting false information (such as a ship's flag state, type or location), which we call "spoofing". 

A second-generation evolution of AIS called VHF Data Exchange System (VDES) is currently in the early stages of implementation. VDES will be more robust to malicious manipulation, but it is still years away from widespread adoption. 

In the meantime, AIS abuse can be counteracted by methods such as close analysis of AIS signals; AI-enabled behavioral analysis; and tracking vessels using other sensor types, such as satellite imagery or radio frequency (RF) detection. 

The continued evolution of these sensors will make it harder for bad actors to hide in the maritime gray zone.

Miao Shou

Miao Shou is a student at Stanford University, and a Defense Innovation Scholar at Stanford's Gordian Knot Center for National Security Innovation.

Gaute Friis

Gaute is a Defense Innovation Scholar at Stanford's Gordian Knot Center for National Security Innovation.

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