Why AIS alone cannot describe maritime activity

The Automatic Identification System is the backbone of most maritime activity pictures, and it is very good at the job it was designed for. That job was collision avoidance.

Understanding that origin explains almost every gap in the data.

What AIS was built to do

AIS is a VHF broadcast system. A transponder announces a vessel’s identity, position, course and speed, and nearby vessels and shore stations receive it. Under SOLAS Chapter V, carriage is required for ships of 300 gross tonnage and above on international voyages, cargo ships of 500 gross tonnage and above not on international voyages, and all passenger ships regardless of size.

Class A transponders, the SOLAS-mandated type, transmit at 12.5 W and report position every two to ten seconds while under way. Class B units, intended for smaller and non-mandated vessels, transmit at 2 W and report far less often.

Read that list again as a description of coverage rather than of compliance, and the shape of the blind spot appears.

Where the picture ends

Below the threshold. The tonnage thresholds exclude most of the vessels operating in nearshore African waters. Artisanal and small-scale fishing craft are not required to carry AIS and largely do not. A monitoring approach built on AIS is, by construction, blind to the fleet that operates closest to shore.

Out of range. Terrestrial receivers are limited by VHF line of sight, typically a few tens of nautical miles. Satellite AIS extends coverage offshore, but in areas of dense traffic many transponders transmit within the same receiver footprint and messages collide, so detection probability falls exactly where activity is highest.

Switched off. A transponder is a device on the bridge. It can be turned off. A gap in a track is not evidence of wrongdoing, and a vessel may have entirely legitimate reasons to be dark, but the gap is also not an observation of absence.

Self-declared. Everything AIS reports about identity is asserted by the transmitting vessel. Position can be falsified and identifiers can be reused. The system has no built-in verification, because collision avoidance never needed one.

The inference that does not hold

The failure mode is a single quiet assumption: that no AIS report means no vessel.

It does not. It means no report. Those are different statements, and the difference is the entire space in which unreported activity happens.

What independent observation adds

This is the specific reason Earth Observation belongs in a maritime picture. A satellite radar sensor does not ask a vessel to identify itself. It records what is physically present in the scene, whether or not anything on board chose to broadcast.

The useful signal is not either source on its own. It is the comparison:

• A detection with a matching AIS report is a corroborated vessel.
• An AIS report with no detection may be an artefact of sensing conditions, or a timing mismatch.
• A detection with no corresponding report is an observation that something is present which is not reporting.

That third case is not a finding of illegality. Small vessels are not required to carry AIS, equipment fails, and the association itself can be wrong when a satellite pass and a position report are separated in time. It is a prompt for a human to look, and prioritising those prompts well is a more useful contribution than any automated verdict would be.

Why this matters for design

A system that treats AIS as ground truth will report high confidence and a clean picture, because it can only see what agreed to be seen. Adding an independent sensing modality makes the picture messier and more honest at the same time.

For anyone building maritime intelligence, the design question is not which source to trust. It is what each source is physically capable of observing, and what follows when two of them disagree.