Article Series 02 • AIS • Dark Vessel Detection • SAR Satellite
AIS (Automatic Identification System) is a radio transponder installed on vessels that automatically broadcasts ship information via VHF radio. Under the IMO's SOLAS convention, all ocean-going vessels over 300 gross tons and all passenger ships are required to have AIS installed and activated.
AIS broadcasts every few seconds, allowing nearby vessels, port authorities, coast guards, and global satellite networks to track the ship's real-time position.
| Data Field | Description | Example |
|---|---|---|
| MMSI | Maritime Mobile Service Identity (9 digits, like a national ID) | 416123456 |
| Vessel Name | Registered ship name | XING SHUN 39 |
| Call Sign | Radio communication identifier | VRCS7 |
| IMO Number | Permanent IMO identifier (theoretically unchangeable) | 9876543 |
| Vessel Type | Cargo, tanker, fishing, passenger, etc. | Cargo |
| Flag State | Encoded in first 3 digits of MMSI (MID code) | Tanzania (677) |
| Lat / Lon | GPS position, ~10m accuracy | 25.12°N 121.8°E |
| SOG | Speed over ground (knots) | 4.2 kn |
| COG | Course over ground (degrees) | 185° |
| Destination | Manually entered by crew — may be inaccurate | KAOHSIUNG |
This information is itself valuable intelligence. Vessel type, flag state, speed, and heading can all be used to identify anomalous behavior. This site collects exactly this data.
The biggest problem with AIS is that it's entirely voluntary broadcasting — nobody can force a ship to keep its signal on. Any crew member can make a vessel disappear from all AIS tracking platforms in a few button presses.
This is called "going dark" in the industry. Going dark isn't necessarily illegal — there are legitimate reasons for a captain to disable AIS, such as for safety in piracy-prone areas. But in Taiwan's surrounding waters, a vessel suddenly disabling AIS while approaching a submarine cable route is a very serious warning signal.
Beyond switching off, AIS can also be spoofed — vessels can broadcast false position coordinates, fake speeds, or even another vessel's identity. This "AIS Spoofing" is an item our scoring system specifically monitors.
Going dark on AIS doesn't mean a vessel has disappeared — only that it's vanished from conventional tracking systems. Synthetic Aperture Radar (SAR) satellites can detect any metallic object on the water surface via radar scanning, completely unaffected by weather, cloud cover, or time of day.
The SAR data used by this site comes from Global Fishing Watch, which scans Taiwan's surrounding waters via SAR satellites and cross-references detected vessel positions against AIS data:
These "dark vessels" become priority monitoring targets. A vessel detected by SAR near a submarine cable route with no AIS signal immediately triggers high-level alertness.
Some vessels go further — not just disabling AIS, but frequently changing their name, call sign, or even falsely reporting their IMO number to create identity confusion. This behavior itself is a major red flag.
Now we can see the clear logical chain:
This logic is the core of our monitoring system. For the full scoring methodology, see Article 06: Our Methodology.
An AIS signal has to be received before it can be "seen." Understanding the two ways it is received is what separates real "going dark" from a mere coverage gap.
This matters for monitoring: when a ship vanishes beyond terrestrial range, between satellite passes, it may have genuinely switched off AIS (gone dark) — or it may just be in a coverage hole. Telling the two apart requires independent SAR satellite detection, which is exactly why this site cross-references AIS "going dark" with SAR dark-vessel hits.
The concepts on this page can be verified against these public, authoritative sources:
This page is for research and education; detection signals are leads to investigate, not legal or factual findings. See the methodology.