Automatic identification system (AIS) devices are a type of GPS tracking device that vessels use while at sea.
AIS messages contain two types of information: positional messages which contain the vessel’s GPS location, course, heading and speed; and static messages which contain the vessel’s identity information including vessel name, IRCS, IMO, shiptype, dimensions and destination.
VMS messages include positional messages, and similar identity information as well as VMS identification information used by governments.
Global Fishing Watch produces its vessel activity data based on positional messages transmitted by vessels through their automatic identification system (AIS), transmitted alongside self-reported identity data.
Limitations
The AIS information contained in the static AIS message is manually entered in the vessel’s AIS transmitter device.
This manually entered information is referred to as being ‘self-reported’ and as such, is susceptible to containing errors and can be changed by the vessel operator.
“Identity spoofing” – inaccurate information on AIS
Lastly, a fraction of vessels engage in behaviors that make their AIS data unreliable, such as by simultaneously broadcasting the same maritime mobile service identity (MMSI) as another vessel or generic MMSIs, commonly referred to as “identity spoofing”.
A small group of actors may transmit inaccurate information on AIS to hide their identity and make their monitoring control and surveillance more challenging. Examples of this include vessels transmitting information appearing to be another ‘known’ vessel that might be authorised in the area observed or transmitting misleading vessel type information e.g. a vessel type ‘pleasure’ for a fishing vessel to make them receive less attention.
Our map and downloadable datasets omit these vessels in order to remove misleading information, as we continue to pursue ways to identify and correct the dataset for these behaviors.