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The Essential Guide To The Automatic Identification System (AIS)

How AIS works, the challenges it presents, and how we collect and use it in our advanced vessel data.


Safety at sea

The automatic identification system, or AIS, is a vessel tracking system that allows shipping to operate more safely and efficiently. Vessels over a particular size, and all passenger ships, are required to carry equipment that broadcasts information about their position, course and speed, as well as other important data.

AIS was initially purely a short-range system developed to give vessels and shore stations a picture of the traffic around them and avoid problems with traffic flow.

Now, AIS signals, collected by various methods such as terrestrial receivers on the ground, at sea, and by satellites in orbit, have become an important source of data that can be used to judge maritime risk and compliance and monitor trade all over the world in an increasingly sophisticated and complex industry.

How does AIS work?

AIS equipment uses Very High Frequency (VHF) transceivers to broadcast and receive information about the positioning and identification of vessels. These radios communicate on two VHF frequencies, 161.975 MHz and 162.025 MHz (known as maritime channels 87B and 88B), and they have an effective range of about 40 nautical miles (74 km).

The International Maritime Organization (IMO) requires any ship weighing 300 gross tonnes or more to be fitted with AIS equipment when on an international voyage. The regulation also applies to cargo ships of 500 gross tonnes or more involved in a local voyage and to all passenger ships, regardless of size.

The transmitters automatically broadcast information at regular intervals. Depending on its activity and status, a ship’s AIS system transmits navigational data every two to 180 seconds and broadcasts voyage data every six minutes.

Why and how do vessels use AIS?


AIS was originally developed by the IMO to allow vessels and land-based systems to build up a real-time picture of shipping traffic in their vicinity, and to receive position reports.

The systems are generally used in conjunction with radar for navigation. A range of data sources can be used to generate a visual chart or display for safer movement.

In addition, AIS is a useful tool for conducting search and rescue missions or investigating accidents when collisions do occur. The system can provide rescue teams with information about other ships in the vicinity, for instance.

Portable AIS devices, called AIS-SART (Search and Rescue Transmitters), can be used to provide positional information from survival craft. 

Maritime security

In an uncertain world, it is often important to decide whether a vessel is acting unusually or departing from its typical patterns of activity. AIS means it’s possible to identify ships accurately and build up a precise picture of their movements, making this task easier.

If a ship deviates from expected patterns or ‘goes dark’ (loss of AIS signal), this can be considered alongside other data to consider the possibility of a compliance risk, breach of sanctions, or generally illicit trading activity. Alerts for this type of activity can highlight potential violations and assist the authorities tasked with maintaining maritime security. It can also be used by compliance teams in finance and shipping when considering the operations risk factors of certain vessels, owners or managers.  

Logistics, statistics and efficiency

For managers of busy fleets, AIS data provides a valuable source of information on their vessels’ location and activities. This may be useful for forward planning; to anticipate arrival times at port or organise the ship’s departure, for example. Equally, it can be an important tool for identifying potential problems and ensuring compliance with regulations.

During March 2020, the United Nations fed AIS data into its UN Global Platform, a community of statisticians and analysts working to develop big data cloud solutions. It examined how this information could be used to make port traffic more efficient, map fishery activities and reduce shipping’s carbon footprint, among other use cases.  

What information is transmitted via AIS?

IMO regulations require AIS to provide a ship’s “identity, type, position, course, speed, navigational status and other safety-related information” automatically to other ships and appropriately equipped shore stations.

The identity data includes the vessel’s unique nine-digit maritime mobile security identity (MMSI) number. The navigational status will indicate whether the ship is “at anchor”, for example, or whether it is “underway using engines”. Longitude, latitude, “course over ground”, “true bearing” and “true heading” will be provided.

Every six minutes typically, a ship with AIS can also broadcast information about its cargo, dimensions, estimated time of arrival and other data about its voyage and systems. 

How is AIS data collected?

different AIS


While AIS data is transmitted vessel to vessel, for many purposes its collection by land-based or terrestrial receivers is critical.

The signals are processed by algorithms that turn them into useful real-time information on all relevant ships within a particular vicinity. Combined with an internet connection and data-sharing between systems, a more detailed picture of coastal activity can be assembled.

We noted earlier that transceivers on ships and land broadcast over a maximum range of around 46 horizontal miles (74 km). For that reason, terrestrial AIS signals (T-AIS) are lost beyond coastal waters. This has necessitated a longer-range collection system.


In the early 2000s, organisations including the Applied Physics Lab at Johns Hopkins University discovered that AIS could be read by satellites orbiting the Earth. VHF signals travelled further and more easily through the thinner atmosphere at high altitude and into space.

This offered, in theory at least, the chance of global coverage of AIS signals. Ships could be monitored wherever they were in the world, providing there was an appropriately placed satellite.

Combining data from earth and space

There are potential weaknesses with both data collected on land and by satellite. While terrestrial signals have only a limited range and do not reveal the picture away from shore, satellites too have their limitations.

AIS crowding

In the vastness of our oceans there can be significant gaps in coverage. Ships may sail for several hours without having their signals received. In addition, crowded waters can cause signal interference. Satellites deal with data from a larger area than terrestrial receivers, which means vessels may be misidentified in busy areas where a lot of signals are being transmitted at the same time.

This is why it is sensible to draw data both from terrestrial and satellite sources. 

The challenge of AIS gaps

Inevitably, there will still be gaps in AIS data due to human error, problems with coverage or deliberate avoidance of the system. As regulation of the high seas increases, so too do the efforts of those who want to skirt sanctions or skip detection.

Sometimes a vessel turns off its AIS equipment, or ‘goes dark’, in order to call at ports where international sanctions apply. In addition, sometimes ships ‘go dark’ to enable illicit transfers of goods between them.

How advanced analytics and AI can help

Malign activities at sea can often be difficult to distinguish from perfectly innocent behaviour.

Many events can be explained by gaps in AIS coverage or equipment failure. This is where artificial intelligence and in-depth data provide extra layers of insight, revealing where genuinely suspicious conduct has taken place.

For example, the known direction of a vessel and its speed can be calculated automatically by the software and compared to AIS data. In conjunction with information about its closeness to any sanctioned port, events like probable dark port calls, ship-to-ship transfers and other suspicious behaviour will be flagged automatically.

Advanced data analysis and machine learning power our Seasearcher Advanced Risk and Compliance platform. With access to more than 3,000 sources of information and receiving over 420 million AIS messages daily, generating 274 million vessel positions each month, the database finds signal gaps and calculates whether or not these are linked to risk, flagging voyage risks in near-real time for customers. We help thousands of professionals in finance, shipping and insurance, among others, to spot and assess incidences of risk quickly, with reports to evidence the findings to support their decisions.

The pairing of VHF signals from ships around the globe with the latest in advanced analytics and machine learning now illuminates the darkest corners of our oceans, providing a previously unattainable level of insight into vessel behaviour and possible illicit activity, giving organisations a clearer picture of risk and opportunity.

Key takeaways

  • AIS is a vessel-tracking system that automatically broadcasts information about a ship’s position, course, speed, planned voyage and characteristics.
  • The system can help identify security and compliance risks when suspicious behaviour at sea is highlighted by advanced analytics..
  • AIS signals are collected by equipment on vessels, land stations and satellites.
  • There can be gaps in the receipt of AIS data by any method of collection. These can be plugged by combining terrestrial and satellite data and using the latest data analysis and AI tools.
  • AI can use AIS data points and additional machine learning information about vessel activities to identify probable suspicious behaviour like dark ship-to-ship transfers and dark port calls.


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