How to detect a vessel that has turned off its AIS

A vessel that has switched off its transponder is not invisible — it is only invisible to the one sensor that depends on its cooperation. Detecting it is a fusion problem, and the method is now well established. Here is how it works.

Start with the gap

AIS (the Automatic Identification System) is a cooperative sensor: the ship broadcasts its own position over VHF. That cooperation can stop for legitimate reasons — patchy satellite coverage far from shore, or a deliberate safety decision — or because someone wants to hide loading, a transfer, or a destination.

An AIS gap — a vessel going silent then reappearing — is the first signal. But a raw gap is noisy. To be useful it must be normalised against expected satellite AIS coverage: in a region where the constellation rarely passes, silence means little; in a well-covered corridor, the same silence is loud. Getting this normalisation right is what separates a real dark-vessel pipeline from a false-positive generator.

See the hull with radar

The gap tells you when to look. Non-cooperative sensors tell you what is there.

Synthetic aperture radar (SAR) is the workhorse. It is an active sensor — it provides its own illumination — so it works in any weather, day or night, and penetrates cloud. The open Copernicus Sentinel-1 constellation images in C-band; its Interferometric Wide swath (roughly 5×20 m) is well suited to maritime detection. Crucially, Sentinel-1C (launched December 2024) and 1D (November 2025) carry an onboard AIS receiver, so the radar detection and the transponder return can be correlated natively.

Optical imagery (Sentinel-2 at 10 m, or commercial Planet, ICEYE and Umbra down to ~25 cm) confirms and classifies detections in clear conditions.

Fuse detections to AIS

The intelligence is in the association:

1. Detect candidate vessels in the imagery — classical CFAR detectors or CNN ship detectors on the SAR scene.
2. Associate each detection with any AIS track present at that place and time.
3. Reason over what is left. A detection that matches an AIS return is a cooperative vessel. A detection with no matching AIS is a candidate dark ship.

Layer behavioural context on top — loitering, two-vessel rendezvous consistent with an STS transfer, and pattern-of-life scoring — and you convert a scene full of blips into a short, ranked list of leads.

Why this is not optional

The scale of the blind ocean was quantified in a 2024 Nature study that processed two petabytes of Sentinel-1 and Sentinel-2 imagery from 2017–2021 with machine learning. It found that roughly 75% of industrial fishing vessels, and more than a quarter of transport and energy vessel activity, were absent from public AIS tracking. For sanctions enforcement, IUU-fishing patrols and maritime domain awareness, the dark vessels are not the edge case — they are the majority.

Watch the spoofing, not just the silence

Turning AIS off is one tactic; broadcasting a lie is another. Location spoofing transmits a false GNSS position — circular tracks, impossible jumps, land-locked coordinates. Identity spoofing broadcasts another vessel’s MMSI, a scrapped vessel’s MMSI, or a duplicated “pirate” MMSI. Detection leans on kinematic plausibility checks, the frequency-of-arrival of AIS messages at satellites, and cross-validation against the same SAR and optical imagery.

This detection-to-AIS fusion is the engine behind our maritime domain awareness and sanctions screening work — wrapped in an analyst-in-the-loop copilot so every lead arrives with the evidence a human can defend.