Oil and shipping
Satellite Ship Detection with Sentinel-2: What It Can See
How satellite ship detection with Sentinel-2 works: 10 m bands, revisit times, cloud and size limits, Sentinel-1 radar, unidentified vessels and data licensing.
Satellite imagery shows ships whether or not they choose to be seen. That makes it the natural cross-check on AIS, which depends on ships broadcasting honestly and on a receiver being in range. The European Union's Copernicus program makes imagery from its Sentinel satellites free to use, and Sentinel-2's optical images are sharp enough to pick out large ships at anchor. They also have hard limits, and knowing them is the difference between a useful observation and a false conclusion.
What Sentinel-2 is
Sentinel-2 is designed as a pair of identical optical satellites in the same sun-synchronous orbit at 786 km, placed on opposite sides of the planet. Each carries the MultiSpectral Instrument (MSI), which images a 290 km wide swath in 13 spectral bands. According to the Copernicus mission documentation:
| Resolution | Bands | Use for ship detection |
|---|---|---|
| 10 m | Blue (B2), green (B3), red (B4), near infrared (B8) | Main bands for spotting hulls and wakes |
| 20 m | Six red-edge and shortwave infrared bands | Supporting information |
| 60 m | Three atmospheric correction bands | Cloud and haze correction |
Near infrared is particularly useful at sea, because water absorbs it strongly and ships stand out as bright objects against a dark background.
Sentinel-2A launched in June 2015 and Sentinel-2B in March 2017. Sentinel-2C, launched in September 2024, replaced 2A in the operational pair in January 2025, and 2A has continued in an extension campaign since March 2025. One satellite revisits a location every 10 days; the two-satellite constellation gives a revisit of five days at the equator, more often at higher latitudes where swaths overlap. Images are taken in the morning, with the orbit crossing the equator at 10:30 local solar time.
Where it looks
Sentinel-2 does not image the whole ocean. Its systematic acquisitions cover land between 56° south and 82.8° north, coastal waters up to 20 km from shore, islands larger than 100 km², all EU islands, the Mediterranean Sea and closed seas such as the Caspian. Ports, anchorages and narrow straits fall inside that coverage. A transfer 100 km offshore in the open Atlantic does not.
How a ship appears at 10 meters
At 10 m resolution, each pixel covers a 10 by 10 m patch of sea. Typical tanker dimensions translate roughly as follows:
| Vessel | Approximate size | Approximate size in 10 m pixels |
|---|---|---|
| Aframax tanker | 247 m × 44 m | 25 × 4 |
| Product tanker | 183 m × 32 m | 18 × 3 |
| 30 m tug or fishing vessel | 30 m × 8 m | 3 × 1 |
Tanker dimensions are typical figures from shipbuilder specifications; the small vessel is illustrative. A large tanker is a clear, elongated object. A tug is a few bright pixels that are easy to confuse with noise, small platforms or rocks. Moving ships often leave a wake that is more visible than the ship, which helps with detection and gives a rough heading.
Optical limits
Optical imaging is passive: it records reflected sunlight. That brings several constraints:
- Cloud. Cloud and thick haze hide the sea entirely. If a location is clear on only half its passes, the useful revisit drops from five days to roughly ten, and in a persistently cloudy season it can be weeks.
- Daylight only. No images at night, and every image is taken at roughly the same morning hour.
- Sun glint. Sunlight reflecting directly off the water can wash out parts of a scene.
- Ships alongside each other. Two tankers moored together for a ship-to-ship transfer can merge into one large object, or be split only by a thin line of darker pixels.
- One moment in time. An image shows where ships were for a fraction of a second, every few days. It says nothing about what happened between passes.
Sentinel-1 radar as the complement
Sentinel-1 carries a C-band synthetic aperture radar (SAR). Radar supplies its own illumination and sees through cloud, day and night. Steel hulls reflect radar strongly, so ships show up as bright points against the sea. In its main Interferometric Wide swath mode it covers 250 km at 5 by 20 m resolution, according to ESA, and the two-satellite constellation revisits every six days at the equator.
The Sentinel-1 constellation has changed recently. Sentinel-1B's mission ended in 2022 after a technical anomaly. Sentinel-1C launched in December 2024 and Sentinel-1D in November 2025, and Sentinel-1A ended its mission in June 2026 after more than 12 years. Sentinel-1C and 1D carry their own AIS receivers, which allows radar detections to be matched against AIS messages received at the same moment.
Radar has its own weaknesses. Rough seas create clutter, and radar images are harder to interpret visually than optical ones. Used together, the two sensors cover more of the gaps than either does alone.
Why detections without AIS are unidentified
A satellite detection gives a position, an approximate length and width, sometimes a heading, and the time of the image. It does not give a name, flag, IMO number or cargo. Identity comes only from matching the detection with an AIS report from the same place at the same time.
That match can fail for ordinary reasons: no AIS receiver in range at the moment of the pass, a ship type the AIS filter excluded, or a small timing offset that moves a ship out of the matching window. It can also fail because the ship is not transmitting. The imagery alone cannot tell these cases apart. A detection without a matching AIS report is therefore unidentified, not "dark", and the IMO itself notes that unreceived AIS is not on its own proof of illegal activity. The guide to AIS ship tracking covers why AIS coverage has gaps.
FXPM Tanker Watch works within these limits. Its counts are automatic proposals from 10 m Sentinel-2 imagery over fixed watch areas at anchorages and chokepoints, and the detector is not a validated classifier. Cloud-covered water is not searched, so a low count under cloud means missing data rather than an empty anchorage, and satellite detections carry no names unless an AIS feed covers the same water. The tanker watch map shows the vessels alongside the image crops and capture times they came from.
Licensing
Copernicus Sentinel data are available on a free, full and open basis under EU law. Anyone can reproduce, adapt, combine and redistribute them. The Copernicus Data Space FAQ and the Sentinel data legal notice require attribution: "Copernicus Sentinel data [year]" for unmodified data, or "Contains modified Copernicus Sentinel data [year]" when the data have been processed, which covers detections and crops.
Key points
- Sentinel-2's four 10 m bands can detect large ships clearly; small vessels become a few ambiguous pixels.
- Two satellites give a five-day revisit at the equator, but cloud and daylight limits reduce usable images.
- Systematic coverage includes coastal waters within 20 km of shore, not the open ocean.
- Sentinel-1 radar sees through cloud and at night, and its newest satellites carry AIS receivers.
- A detection without an AIS match is unidentified, not proof of a dark ship.