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Recent news coverage has stoked fears of vessels "going dark" as cover for criminal and terrorist activity, but the headlines betray a lack of understanding of AIS and its limitations.


Using AIS and a range of other sources, the Lloyd’s List Intelligence database shows that 3,869 vessels made 24,868 callings in ports/countries that have been sanctioned in the last 12 months. Does this make all of those vessels law breakers? No.


Achieving comprehensive vessel tracking and deriving actionable intelligence from this insight is highly complex and relies on detailed industry and technical knowledge. The activities of these 3,869 vessels may or may not have been illicit, but jumping to the conclusion that they were based solely on AIS movements is a dangerous path to go down without context and an understanding of the intricacies of AIS technology.


Here are some of the common myths around AIS, and the realities behind them:


1. AIS was designed for vessel tracking

AIS was not originally developed with the purpose of providing commercial vessel position data. AIS was introduced to improve maritime safety by allowing vessels to see the position and direction of other ships in their close vicinity, not to give a global picture of vessel movements. The commercial adoption and commoditisation of this technology only occurred less than a decade ago.


2. AIS is the same as GPS

Global Positioning System is different to AIS. While ships do have GPS receivers on board to determine the latitude/longitude and time stamp data for a vessel, once this information is collected it is transmitted by AIS on Very High Frequency radio waves along with many other pieces of data such as speed over ground, course over ground, draught and destination. GPS is tracked by specific GPS satellite constellations that can work in any weather condition and in any position in the world, AIS is subject to variable performance levels.


3. AIS can pick up any vessel, anywhere

AIS operates over VHF radio waves, which travel in straight lines. The distance of transmission for VFH radio waves approximates to line of sight limits, roughly 20-30 miles from land receivers situated on high locations and is also limited by the curvature of the earth. VHF radio waves can also be effected by atmospheric conditions and landmass, resulting in variances in the range of signal and reach of land based receivers.


4. The addition of satellite AIS must mean I can see all vessels, all of the time

Satellite AIS is helpful, but is not an answer to the limitations of terrestrial AIS. Due to the nature of satellite orbits and their trajectories, satellite AIS receivers cannot pick up AIS messages as frequently as terrestrial receivers which are in a fixed position. Satellite detected AIS data provides valuable but far less granular vessel position records than land based receivers.


5. AIS accurately shows one vessel, with one signal

Vessel AIS signals are identified by their MMSI number.  The MMSI number is normally issued as part of a ship station’s radio license, typically in conjunction with the international radio call sign (IRCS). A new MMSI number is issued when a vessel changes flag and unlike unique IMO vessel numbers, some flags reuse and redistribute MMSI numbers. This results in a changing population of a few hundred vessel pairs with the same MMSI number. This means a vessel may appear to be in two locations at the same time, commonly known as “spoofing”. The highest quality AIS vessel tracking providers have dedicated teams who cleanse this data to minimise the display of spoofed data. 


6. An AIS network should have 100% coverage of the global fleet at all times

AIS receivers and transceivers are technical pieces of equipment that make up a network. By their very nature they are subject to technical failure and manipulation. No maritime data provider can offer a completely “always on” AIS network. Also, placing terrestrial receivers in certain regions is impossible due to geopolitical sensitivities. Vessel tracking providers with coverage in these areas cannot guarantee that the receiver has been placed their legally, or will remain online indefinitely. It has also been reported that some countries block satellite AIS transmissions.


7. “Going Dark” means the vessel is acting illicitly

Going Dark” does not necessarily mean the vessel has manually and deliberately switched off the on-vessel AIS transceiver. It can mean the vessel has moved beyond AIS detection range or has entered an area where the signal is difficult to detect. A vessel’s activity must be reviewed within the context of the commercial activity that they are engaged in, for example, has the vessel changed it normal voyage pattern? Is it acting differently to other vessels of a similar type? Are there indications that the vessel’s owning or operating companies have a history of prior illicit activity?


8. AIS is a true indication of a vessel’s position

AIS is generally a good indicator. However, when AIS was first introduced calibration errors were often observed when the vessel’s actual position was slightly offset, leading to plots of vessels on the quayside rather than alongside. This idiosyncrasy is becoming rarer, but there will always be a small subset of vessels which through deliberate action, spoofing, or through error, appear to show their positions are hundreds of miles from their actual location. 


9. AIS provides constant information on cargo and vessel draught 

It is true that additional information is available within the AIS message stream. Not all AIS records contain such information and the population of cargo data is sparse. This additional information is entered by the crew, often through a keypad similar to texting on an old mobile phone. The information is useful as a guide but its source and the method of input should always be considered while using it.


10. AIS is the only way to view global fleet movements

To avoid relying on AIS data alone, fleet port callings and departures information can be accessed through port agents. Vessels also advise port agents on their next destination when leaving a port. As they are reporting to a person, the location given is often more accurate than the one keyed into the AIS system. Often the AIS destinations provided by the crew can be miss-typed or obscured to mask the true destination. To address this issue, Lloyd’s List Intelligence also collects information from ports about the vessels they expect to be arriving at their location to provide you with as much information and accuracy as possible. This human intelligence is the only way to verify AIS data on port activity and bound for destination.



At Lloyd’s List Intelligence, we have been working with international commercial organisations and governments for many years to provide vessel tracking insight and assist with their market context and technical understanding. Our customers use Lloyd’s List Intelligence because they understand the limitations of relying on AIS information alone and need a more robust solution.  


For more information on AIS technology, you can view our full ‘Understanding AIS’ article which explains the technology, its limitations and how to overcome them.


For more information, please email If you would like to speak in more detail on the topic, our AIS and security expert, Daryl Williamson is available.

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