Terrestrial vs satellite
AIS was developed as a short range, high intensity system with a line-of-sight range of 10-20 miles between the transponders and terrestrial (shore and vessel based) receivers. Its primary purpose was to allow vessels to see who else is operating in their immediate vicinity so as to prevent collisions.
What is less well known is that, thanks to improvements in technology, AIS signals can also be received by suitably equipped, low orbit, satellites. This has transformed the range of AIS, giving it a truly global reach and allowing users of AIS services to locate and track vessels anywhere on the face of the earth from their desk or bridge.
With Big Ocean Data the data from terrestrial and satellite feeds merge with each other to give a seamless and uninterrupted service.
Who has AIS?
International maritime law requires AIS transponders to be fitted aboard international voyaging ships with a gross tonnage of 300 tonnes or more, and on all passenger ships regardless of size. Given its visibility and safety advantages many smaller vessels voluntarily install AIS units. In many countries no license is required to purchase and operate either transponders or receivers.
The result is that AIS is used almost universally in the worldwide commercial maritime industry and increasingly so in the leisure marine sector. Not all vessels can be tracked by AIS. Naval and security ships generally prefer not to be tracked when on active duty, and cases are regularly reported of commercial vessels underway with their transponders turned off for unspecified reasons.