What is AIS?
AIS is the radio system ships use to broadcast their identity and position. Public ship tracking statistics are built from it.
Explainer, updated 2026-09-19 (19 September 2026).
Explainer
AIS stands for Automatic Identification System. Most large ships carry a transponder that broadcasts the vessel’s identity, position, course and speed, so that other ships and coastal receivers can see it.
Why it matters here
Because AIS signals can be received by shore stations and satellites, they can be used to count how many ships are in a place at a given time. IMF PortWatch builds daily port call and chokepoint transit statistics from AIS data supplied through the United Nations Global Platform. Those statistics are the public data behind the measured effects on this site.
Limits
AIS coverage is not perfect. Signals can be missing in some areas or for some days, ships can switch transponders off, and small vessels may not carry one. A sudden odd value in a daily count can be a gap in coverage rather than a change in traffic, which is why we are careful about reading single days.
Commercial vessel tracking services exist, but this site does not use them. See sources and licences and the methodology.