What is AIS? A guide to the maritime safety system

AIS is a maritime safety system that’s compulsory for commercial vessels and can be used by recreational boaters too. In this guide, we discuss the different types of AIS, how it works, and some of its limitations.

What is AIS?

AIS stands for Automatic Identification System and is a safety and navigation system used on boats and ships to broadcast and receive information about nearby vessels. It uses VHF radio signals to automatically share details about a vessel – including its identity, position, speed, course, heading, type, size, and navigation status – with other AIS-equipped boats, shore stations, and traffic control centres.

Types of AIS

There are three main types of Automatic Identification Systems. 

Class A

• Required on commercial vessels
• Higher transmit power
• Sends more frequent updates
• Large range

Class B

• Common on recreational boats
• Lower cost and smaller units
• Transmits less frequently while remaining effective

AIS receiver only

• Receives data from other vessels but does not broadcast its own position

History of AIS

The history of AIS begins with early experiments in vessel tracking and collision-avoidance technologies in the 1970s and 80s. It was during this time that maritime authorities started to look for a system that could automatically identify vessels and help with collision prevention. This initial research included looking into the use of VHF radio and digital data exchange.

In the 1990s, the IMO (International Maritime Organisation) and the ITU (International Telecommunication Union) came together to design a global standard for ship-to-ship and ship-to-shore communication. This is when AIS protocols were drafted and adopted by the IMO under SOLAS (Safety of Life at Sea) regulations.

Compulsory implementation of AIS was established in the early 2000s, and all SOLAS vessels needed to have this system from 2002. In 2004, AIS also became mandatory on commercial ships of 300 gross tonnage and above on international voyages, passenger ships, and large domestic commercial vessels.

As AIS hardware became smaller and cheaper, Class B AIS was introduced for smaller, non-commercial vessels. Boaters implemented it for collision avoidance, racing, and offshore cruising.

Benefits of AIS

AIS is useful for several reasons. Some of its main benefits include:

• Collision avoidance: AIS enables you to locate nearby boats, often before they appear on radar, helping you avoid potential collisions
• Improved situational awareness: Using AIS, you can see which vessels are around you, where they’re heading, and their speeds
• Search and rescue: Rescue teams can use AIS to track your last known position if your Automatic Identification System is active
• Traffic monitoring: It’s easier to navigate busy waterways when using AIS data

How AIS works

AIS is essentially a digital messaging system for boats and uses VHF radio to send and receive information. It has a built-in GPS receiver and constantly updates your location, speed, and direction. This data is then packaged into messages and automatically sent over VHF radio to other AIS units nearby. Anyone in range with AIS will see your boat appear on their chartplotter, radar overlay, AIS receiver, or via an AIS app. You also receive their AIS signals and will instantly see their positions too.

A smart timing system called TDMA (Time Division Multiple Access) prevents signals from clashing by assigning a time slot to each boat, so they take it in turns to send their broadcasts. This prevents multiple vessels from sending information at the same time and allows hundreds of boats to share the same channels without interference.

Satellites and shore stations can pick up AIS too. For example, coastal stations listen for AIS signals and forward them to maritime authorities. And satellites orbiting above capture AIS signals far offshore, giving global tracking. This is how ship tracking websites show vessel locations.

AIS limitations

AIS is a fantastic safety tool, but it has several limitations that boaters, especially recreational users, should be aware of. Understanding these helps avoid over-reliance and improves safety.

• Range limitations – AIS operates over VHF radio, which is line-of-sight and has a typical range of 10 to 20 nautical miles. This means that vessels over the horizon may not appear without satellite AIS.
• Only tracks vessels with AIS – Small boats, dinghies, kayaks, or older vessels may not carry AIS, so you cannot rely solely on this system to see all nearby traffic. This means that radar or visual lookout is still essential.
• Potential for signal interference – Busy waterways may have many AIS messages, causing displays to become cluttered, and signal overlap or weak reception can cause delayed or missing position updates.
• Data accuracy depends on the vessel – AIS information is self-reported by the vessel, and some may input incorrect data, which can mislead navigation decisions if relied on too heavily.
• Cannot detect non-AIS hazards – Floating debris, logs, buoys, rocks, and swimmers are not visible on AIS, so radar, sonar, or visual lookout are still vital for detecting hazards.
• Vulnerable to technical failures – Power loss, antenna damage, and electronics failure can stop your AIS from working, and software glitches or misconfigured units can prevent transmissions from being sent or being received.
• Privacy concerns – AIS broadcasts your position publicly so recreational boaters may expose their location to the public, apps, or potential trespassers. Be mindful that some areas require transmissions to be limited for privacy or security.
• Limited update frequency (Class B) – Class B AIS updates less frequently than commercial Class A units and can be less precise in congested areas.

When should recreational boaters use AIS?

Recreational boaters don’t always need AIS but using it can improve safety and awareness in certain situations, including:

• when using busy or confined waterways
• at night or in poor visibility
• when far from shore or in open water
• when sharing navigation information
• in emergency situations

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