What is AIS (Automatic Identification System)?

Shipfix works with both terrestrial and satellite AIS data to track live and historical ship positions

AIS explained:

The Automatic Identification System (AIS) is an automatic tracking system that uses transponders on ships and is used by vessel traffic services (VTS). When satellites are used to detect AIS signatures, the term Satellite-AIS (S-AIS) is used. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport.

Information provided by AIS equipment, such as unique identification, position, course, and speed, can be displayed on a screen or an ECDIS. AIS is intended to assist a vessel's watchstanding officers and allow maritime authorities to track and monitor vessel movements. AIS integrates a standardized VHF transceiver with a positioning system such as a GPS receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator. Vessels fitted with AIS transceivers can be tracked by AIS base stations located along coast lines or, when out of range of terrestrial networks, through a growing number of satellites that are fitted with special AIS receivers which are capable of deconflicting a large number of signatures.

The International Maritime Organization's International Convention for the Safety of Life at Sea requires AIS to be fitted aboard international voyaging ships with 300 or more gross tonnage (GT), and all passenger ships regardless of size. For a variety of reasons, ships can turn off their AIS transponders.

The AIS signals emitted by ships can be received by either land-based receivers or Satellite. There are many established professional and amateur communities running land-based receivers. Land based receivers are more accurate than satellite at this point but their range is limited to coastal areas. Therefore, deep sea/ocean tracking of ships is achieved almost exclusively via satellite.

Source: Wikipedia