The Automatic Identification System (AIS) is a VHF-band radio broadcast carried by every SOLAS-class commercial vessel and most large fishing and pleasure craft. AIS transponders broadcast a structured set of static and dynamic information at intervals defined by ITU-R Recommendation M.1371; the broadcasts are received by other vessels in line of sight, by terrestrial coastal stations, and by AIS-equipped satellites in low-earth orbit. The aggregate of all these reports is the dataset that powers PortWatch and every comparable maritime tracker.
What an AIS message contains
Each AIS message is a short binary payload carried in an over-the-air slot allocated by a self-organising time-division multiple-access scheme (SOTDMA). The headline static fields are the vessel’s name, IMO number, MMSI, vessel type code, dimensions (length, beam, draft), and call sign. The dynamic fields are latitude, longitude, course over ground, speed over ground, true heading, rate of turn, navigational status (underway, anchored, moored, restricted manoeuvrability, etc.) and a UTC timestamp. There is also a destination and ETA field, free-text and operator-entered, which is notoriously inconsistent in practice.
How often AIS reports
The reporting interval is defined by speed and manoeuvring state: a vessel underway above 23 knots reports its position every two seconds, a vessel underway at moderate speed reports every six to ten seconds, and a vessel anchored or moored reports every three minutes. Static information (name, dimensions) is broadcast every six minutes regardless of state. This means that under normal conditions a coastal AIS receiver sees a constant stream of position updates from every vessel within VHF range, while satellite receivers see a much sparser sample because of the limited time any single satellite spends overhead.
Coverage gaps
Terrestrial AIS coverage is excellent in densely-trafficked European, North American and East Asian coastal waters, where dense networks of coastal receivers operated by national authorities, port operators, and volunteer enthusiasts capture nearly every AIS broadcast within VHF line of sight. Coverage thins rapidly with distance from the coast: at 50–80 nautical miles offshore a vessel may only be in occasional satellite contact. The South Atlantic, parts of the Indian Ocean, and high-latitude polar waters have especially patchy coverage. PortWatch indexes positions as reported and does not interpolate across coverage gaps.
Spoofing and silence
AIS is an unauthenticated broadcast protocol. There is nothing in the message format that proves a transmitter is who it says it is, and there is no enforcement against transmitting false MMSI or position data. Spoofing — broadcasting deliberately false positions — is a documented practice in sanctions-evasion shipping and in some military operations. “AIS silence”, where a vessel turns off its transponder entirely, is also documented in fishing, smuggling, and dark-fleet contexts. Treat individual position reports as suggestive rather than authoritative when the consequences of being wrong are material.
From AIS to a port profile
Every position-and-status update arriving from the AIS feed is joined to the nearest port in the PortWatch port reference table by a coordinate-radius rule. A vessel reporting a position within the catchment of a port, with a navigational status of moored or anchored, is shown on that port’s profile under “moored at port”. A vessel underway with a declared destination matching the port and an ETA in the active reporting window is shown under “currently arriving”. A vessel that recently transitioned from moored to underway and is heading away from the port appears under “recently departed”. The catchment radius is generous — large enough to capture vessels at outer anchorages — which means PortWatch occasionally lists a vessel as being at a port when it is in fact still in transit.
Reading the directory critically
The most useful skill in reading any AIS-derived directory is healthy scepticism about specific data points combined with confidence in aggregate patterns. Any single vessel’s reported position might be an hour or a day old, might be misclassified, or might be deliberately falsified. But the aggregate count of, say, container vessels arriving at the Port of Rotterdam over the past month is a robust signal even when individual reports are noisy. Use PortWatch as a directory and a research starting point, not as a source of operational truth.