What is AIS and how does it work?

AIS is a practical system that shares information about the vessel, its position and direction of travel.

Throughout history, sailors and adventurers have utilized various methods to determine their location and direction at sea, as well as identify potential hazards like shoals and rocks, which they marked on nautical charts. To communicate from afar, they developed a specific signaling system. Nowadays, sailors have access to a wide range of sophisticated and dependable tools and systems for navigation, communication, and information gathering. These tools include radar, radios, analog and digital navigation instruments, and the advanced AIS system. With these tools at their disposal, modern-day sailors and captains no longer have to rely solely on the stars, compass, and lighthouse signals. How exactly does the AIS system work to prevent collisions, and what benefits does it offer?

AIS helps to automatically identify a vessel and its route

In maritime navigation, AIS stands for Automatic Identification System and is a tool for locating, identifying, and aiding navigation. Although it has some features in common with radar and is often linked to it, it differs in many ways and the two should not be confused with radar. Radar is more expensive, more complex and works on the principle of sending out waves that bounce off nearby ships, obstacles and other objects. The reflected waves are then displayed on a radar monitor.

In basic terms, AIS is a system that regularly transmits and receives information about a vessel's position, speed, heading, and identification data. Its main purpose is to establish an identification network between vessels, between vessels and land, and between land stations to prevent collisions at sea. The information is shared through satellite or internet links and can be received by all ships equipped with AIS. While larger ships are required to have AIS under the Law of the Sea, it is not mandatory for smaller vessels, such as charter boats. Nonetheless, using AIS can significantly enhance navigation safety for all vessels.


The Airborne AIS system, © Agung Wahyudiono

By utilizing a VHF radio, which is typically incorporated into most charter boats, the Automatic Identification System broadcasts pre-configured information including the ship's name, type, size, location, velocity, and heading. VHF, an acronym for Very High Frequency, is employed by AIS to transmit signals. The frequency band for VHF radio transmission ranges from 130 MHz to 174 MHz, while the range reserved for shipping is designated as MARINE and operates at around 156 MHz.

Navigation technology and on-board instruments

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How does AIS work?

The AIS system uses a VHF radio to transmit in the MARINE band, which is reserved for shipping. The data is updated between 2 and 10 seconds if the ship is moving, every 3 minutes when at anchor and once every six minutes other secondary data is updated.

There are two classes within ASI. AIS Class A is used by large transport and commercial ships (specifically ships of more than 300 GT, commercial ferries and fishing boats over 15 metres). These are expensive and sophisticated instruments capable of processing large amounts of data that cannot even be specified for smaller ships. AIS Class B are cheaper, simpler and suitable for private yachts and charter boats. The second class transmits less data, which is updated every half minute, and the signal range is about 10 nautical miles. The only exception is if the boat exceeds 14 knots, at which point the vessel's position information is updated more quickly.

Thanks to the AIS system, we can transmit and receive a wide range of data. For better understanding, these can be divided into several groups:

1. Static information (transmitted every 6 minutes or on demand):

  • MMSI number (a unique nine-digit code for communication)
  • IMO number (fixed hull number)
  • Ship's name and call sign
  • Length and bream of the ship
  • Type of ship
  • Positioning antenna location

2. Dynamic information (depends on speed and course alteration)

  • Ship's position with indication of the accuracy of the measurement
  • Position time stamp (in UTC local time)
  • Course over ground (COG)
Yacht capsized in strong winds

YACHTING.COM TIP: For recreational boats, there are also AIS receivers available that only receive data but do not transmit it, and they are well priced. For navigation, take a look at our top 10 mobile apps for sailors.

3. Cruise-related information (broadcast every 6 minutes, when data changes or on request)

  • Ship's draught
  • Type of cargo
  • Destination and estimated arrival
  • Itinerary (waypoints)

4. Safety related short messages — a text message in any format addressed to one or more receivers within range (e.g. information about a missing buoy, sightings of icebergs and other unexpected obstacles, etc.).

How to avoid a collision with AIS

All positioning and navigation systems are a very good way to prevent a collision with another vessel or other collision. However, in practice, it is always important to rely on your own common sense, experience and observation of the actual situation. Particularly when offshore and in busy waters, you cannot assume that all boats are equipped with AIS, as it is not mandatory for smaller and recreational vessels. It may also be the case that large ships equipped with Class A AIS filter out the transmissions of Class B AIS instruments. If the situation is in any way unclear or you are unsure, you are always obligated to try to avoid a collision. In addition to on-board instruments and positioning or navigation systems, continuous monitoring of your surroundings will serve you well. We have prepared a handy guide for you on how to judge distance at sea.

Flooded ship in the harbour

AIS systems are equipped with an alarm that alerts you to the danger of a possible collision. You can see from the data at what distance the vessels will pass and when this situation will occur. However, this is only indicative data. Especially for manually steered boats, where their speed and direction fluctuate depending on waves, currents and wind gusts. Also, in busy waters, the alarm may be triggered when no other vessel is visible from the boat.

You can have the data from the AIS system displayed on a chart plotter along with other data such as your position, shore, shoals, buoys, lighthouses and other nearby vessels. Nowadays it is no longer a problem to have it sent to your smartphone, tablet or computer. If you are sailing on a private yacht or charter boat, you can always turn off the ASI transmission (so-called AIS Silent mode). This is practical in situations where you are sailing or anchored in dangerous waters. While you are invisible to other ships in the system, you can see the ships that are transmitting the ASI signal.

Would you like to be on board a boat as a captain? Take a look at the possible dates of the skipper courses!

On which boat will you be trying out the navigation and positioning systems?

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FAQs: AIS Identification and Location System