A point on a maritime chart may seem to prove that a ship is at a particular location. In reality, it may come from an AIS radio message broadcast by the ship itself. The automatic identification system exchanges identity, position, heading, and speed with other ships and coastal stations. Satellite receivers extend the range of collection. The Italian IRIDE/Eaglet II constellation combines, according to ESA, multispectral observation and AIS capability. This combination is powerful, provided that a declared signal is not confused with an independent observation of the hull.
What the AIS says
The International Maritime Organization describes AIS as a safety and automatic exchange tool. The obligations to carry it depend on the type and size of the ship and its voyage; not all small vessels are therefore equipped. The information includes static elements entered or configured, as well as dynamic data from onboard sensors. If the equipment is misadjusted or the entered information is incorrect, the map may display a false identity or position. The IMO guidelines explicitly warn that the accuracy of the received data depends on the accuracy of the transmitted data.
A satellite does not magically "follow" a ship when it receives its AIS. It captures radio emissions during a visibility window, and then those emissions are processed and made available. Messages may be missing, delayed, or overlapping in a dense area. The coverage of the Gulf of Guinea, the revisit rate, and the latency of a given service must be measured, not assumed based on the presence of a constellation alone.
What does a silent ship mean?
The absence of a message does not prove illegal activity. A vessel may not be subject to a reporting obligation, may be experiencing a malfunction, may be leaving a receiving area, or may have turned off the device under permitted circumstances. The IMO also recognizes security concerns related to the public dissemination of AIS data and discourages certain irresponsible uses of this data. Publishing in real time the detailed trajectory of a vessel can have consequences that a visually appealing infographic does not show.
Conversely, an AIS message does not guarantee that the identified vessel is the only object visible in an image. To reconcile the two, the time difference between the optical acquisition and the radio message must be corrected, taking into account speed and position uncertainty, and then analyzing the image resolution. A feature corresponding to a vessel may be difficult to distinguish from a fixed structure or a wave. An automatic match provides a hypothesis, not a verdict.
The sensors complement each other, without replacing each other.
A multispectral image can show shapes at sea in fair weather, but clouds and night limit some uses. A spaceborne radar, if available, can observe under other conditions, with its own geometry and false echoes. AIS adds a context of identity and declared route. Fusion requires reliable time-stamping, traceability of sources, and rules for conflicting cases. When two data sets do not agree, the disagreement can be more informative than a map filled with supposedly accurate points.
Reconstitute a data match
Suppose we have a satellite image acquired at 10:05 and an AIS message at 9:55. A ship traveling at several knots may have moved significantly during those ten minutes. The expected position calculation uses the reported heading and speed, but the ship may have turned or slowed down. The analyst must therefore construct an uncertainty zone, not superimpose two points as if they were measured simultaneously. The longer the interval, the less convincing a simple match is.
A series of pre- and post-image messages help to test a trajectory. An identification may also be compared to the visible characteristics of the ship, if the resolution permits. If several nearby ships meet the criteria, multiple candidates must be presented or the match declared indeterminate. This principle is particularly important for an accusatory publication: a mis-match could attribute the acts of one crew to another.
Manage security data
IMO draws attention to the risks of the irresponsible publication of AIS data accessible on the web. An organization conducting a historical study of transport flows does not necessarily need to display a real-time position or personal identity. A public map may aggregate trajectories, delay their display, or remove certain details, depending on the purpose and applicable rules. These decisions should be made with maritime actors and the relevant authorities, not left to the sole discretion of a graphic service provider.
The same principle applies to spatial imagery. Detecting a shape at sea may be a scientific finding; publishing an interpretation of the ship's course requires more evidence and context. The report should present a clearly marked fictional example rather than an unverified real-world case. The educational goal is to make the limitations of each source visible, not to produce improvised surveillance.
How to honestly present an AIS chart
A map intended for the public must show the period it covers and the source of the reception. A point at 10 o'clock does not mean that the ship was still at that location at 12 o'clock. Interruptions in transmission can be represented by a dashed line rather than an imaginary continuous line. If the trajectory is interpolated, the legend must state this. The displayed identity comes from a declared message, not from independent visual recognition. These details can be included in a small graphic note, but they profoundly change the interpretation.
An educational example may use fictitious data to illustrate three cases: strong correspondence between image and AIS, ambiguous correspondence between several ships, and no message with no conclusion about the cause. Students or readers must decide what additional information would be required for an investigation. A real capture of Gibraltar acquired by IRIDE may illustrate the presence of ships visible in an optical image, but it does not contain their AIS identities. The caption should explicitly state this.
The risk of a hasty accusation
In an area where security and fishing issues can be sensitive, the term "dark ship" carries heavy connotations. A lack of AIS may have banal or regulatory explanations; an incorrect AIS position may be unintentional and not intended to mislead. To attribute an activity, multiple sources would need to be gathered, timestamps preserved, and responses sought from the relevant actors. A detection tool provides an indication for a competent authority, not an automatic journalistic proof.
This principle also protects the credibility of a future monitoring service. Repeated false positives consume operational time and can harm innocent people. A thorough evaluation therefore counts missed cases and unjustified alarms, and sets thresholds for intervention in advance. The expanded orbit increases coverage; it does not eliminate the need for human judgment and procedure.
Validation on documented routes, where identity and position have been independently verified, would allow for the measurement of image-AIS matching errors. Without this control set, the advertised accuracy of an algorithm would remain a vendor claim rather than an observable result in the Gulf of Guinea.
What this changes – or does not change – for Central Africa
For the Gulf of Guinea, the potential interest ranges from navigation safety to the analysis of certain maritime activities. None of these applications dispenses with a legal framework, cooperation with the relevant authorities, and the protection of sensitive data. A RECA educational demonstration could show an official IRIDE image and a hypothetical AIS pairing scheme, with a clear warning that the positions in the scheme are fictitious. It would not establish that a real ship committed an offence. The quality of the investigation depends less on the number of points scored than on the ability to explain the shortcomings and errors.
