What is AIS?
The Automatic Identification System (AIS) is a VHF radio transponder system that ships continuously broadcast at regular intervals, transmitting: Maritime Mobile Service Identity (MMSI number), vessel name and type, GPS position, course over ground, speed over ground, heading, and navigation status. AIS was developed by the IMO and became mandatory for vessels over 300 gross tonnes on international voyages under SOLAS regulations from 2002.
Terrestrial AIS vs. Satellite AIS
Terrestrial AIS (T-AIS) uses coastal receivers with a range of approximately 74 km (40 nautical miles). This provides excellent density in ports and coastal waters but leaves vast ocean areas dark. Satellite AIS (S-AIS), introduced commercially from 2008, captures AIS transmissions from orbit. A LEO satellite at 600 km altitude can receive AIS signals from ships across a footprint of approximately 6,000 km diameter. The main challenge is signal collision: in dense shipping lanes, hundreds of vessels transmit simultaneously, causing messages to overlap and corrupt. Advanced receivers and signal processing algorithms extract individual ship signals from this collision environment.
Commercial satellite AIS providers
Spire Global (which acquired exactEarth in 2021) is the leading commercial S-AIS provider, operating 40+ CubeSats with AIS receivers. Orbcomm also operates S-AIS payloads on its OG2 constellation. These providers sell near-real-time global vessel tracking data to maritime authorities, port operators, commodity traders, insurance companies, and fishing authorities monitoring illegal fishing activity.
AIS limitations
AIS is self-reported — a vessel can transmit false position data or switch off its transponder to avoid detection (known as 'going dark'). Maritime intelligence firms correlate AIS data with SAR satellite imagery (which cannot be spoofed) and optical imagery to detect dark vessels and suspicious behaviour, used extensively in sanctions enforcement and counter-piracy operations.