Maritime Tool Guide

Dark Vessel Detection From Satellites: Free Tools Compared

Finding a dark vessel takes two steps: spot the hull in satellite imagery, then check it against AIS. The free tools split along those two steps. Compared by job.

Watch a port or strait Open the Sentinel-1 SAR viewer

Quick answer — what Off-Nadir Delta is

Four free tools cover most dark-vessel work. Global Fishing Watch publishes Sentinel-1 radar detections already matched against AIS on a public map. Skylight, part of the Allen Institute for AI family, combines radar, optical and night-light detections for maritime agencies, with a public map delayed by 48 hours. Copernicus Browser shows the radar scene for you to inspect by eye. Off-Nadir Delta runs ship detection on an area and scene you choose, or on every new Sentinel-1 pass over a watched port, but does not match to AIS. Pick by whether you need the published picture or a count at your own place.

Dark vessel detection tools compared at a glance

Facts last reviewed 2026-10-17.

Tool	What it detects	Matched to AIS	Satellites	Your own area or scene	Access
Global Fishing Watch	Industrial vessels in Sentinel-1 SAR	yes — matched / unmatched	Sentinel-1	Browse any area of the global dataset	Public map; API
Skylight	Vessels in radar, optical and night-light imagery	Shows vessels sharing their location and those that do not	Sentinel-1, Sentinel-2, Landsat 8-9, Night Lights, commercial	Browse the public map; full platform for authorized users	Public map without account; platform login; API
Copernicus Browser	Nothing automatically — you look	No	Sentinel-1 (and other Sentinels)	Any area	Free account
Off-Nadir Delta	Vessel-like radar targets, with positions	no — pair with your own AIS source	Sentinel-1; NISAR L-band on plans that include it	yes — any area you draw, on any scene or every new pass	Free account; detection metered

Third-party cells are from each provider’s own pages: the Skylight home page, Global Fishing Watch’s May 2024 dataset release, the Skylight platform page and public map article, and the Copernicus Browser documentation. Off-Nadir Delta is an independent project and is not affiliated with any of them.

Detection and matching are different steps

A satellite finds a hull. Whether that hull is dark is a second question, answered by comparing the detection with AIS positions at the time of the image. Knowing which step you need tells you which tool to open.

Detect a vessel at my place

You have a port, anchorage or strait and want to know how many vessel-like targets were there on a pass, or on every pass. Run a detector on the scene.

See where dark vessels have been

You want the pattern of detections with no AIS across a region. Use a published dataset that has already done the matching.

The free dark vessel detection tools, by job

Global Fishing Watch

Best for seeing where unbroadcast vessels have been, worldwide

A public map that shows vessel detections from Sentinel-1 radar, each matched against vessels transmitting AIS so you can see which detections had no public position report. Its May 2024 release describes the dataset as industrial vessels detected with satellite imagery between 2017 and five days before the present, and says the same data is available through its APIs.

Best for:: Looking at the pattern of unmatched radar detections in a region — fishing grounds, coastal waters, around a port — over months or years.
Limitation:: It publishes its own processed detections; you do not run a detector on a scene of your choosing. Its 2024 release notes that the map download omits fields such as vessel length, which are in its separate data portal.

Skylight

Best for maritime agencies combining radar, optical and night-light detections

A maritime monitoring platform that describes itself as part of the Allen Institute for AI (AI2) family. It says it shows vessels voluntarily sharing their location and those that do not. Its platform page lists Sentinel-1, Sentinel-2, Landsat 8-9 and Night Lights among its free data sources, with commercial partners, and an API. Its public map can be explored without an account and covers 18 months of history, with data delayed by 48 hours.

Best for:: Organizations protecting their waters that want vessel detections from radar, optical and night-light sources in one place.
Limitation:: The public map is, in Skylight’s own words, not for real-time monitoring or enforcement, and detailed vessel information is available only to authorized users.

Copernicus Browser

Best for looking at the radar scene yourself

The official viewer for the Copernicus Sentinel missions, including Sentinel-1 in SM, IW and EW modes, with compare and timelapse tools. Ships show as bright points on dark water in the radar image.

Best for:: Checking one scene by eye — whether there was a vessel at a place on a date — straight from the official source.
Limitation:: It is a viewer: it does not detect or count vessels and has no AIS. Its documentation says a free account is currently needed to use it.

Off-Nadir Delta(that’s us)

Best for counting vessels at your own port or strait on every new pass

Runs CFAR ship detection over an area you draw on a Sentinel-1 GRD scene (or a NISAR GCOV scene on plans that include it) and returns each detection’s position, not just a picture. Watch a port or chokepoint and every new Sentinel-1 pass is counted into a time series, with a count flagged when it breaks from the place’s own history. Maritime events reported near the site arrive on the same watch.

Best for:: A specific port, anchorage or strait you want to watch over time, with counts that carry their caveats (land mask, coastal exclusion, partial coverage).
Limitation:: It detects radar returns, not identities: no AIS, no name, flag or MMSI, and offshore structures or sea clutter can appear as targets. To find which detections are dark, pair them with an AIS source. Detection and monitoring are metered with tokens.

Frequently Asked Questions

What is a dark vessel?▼

A vessel that is not broadcasting AIS at the time it is seen — the transponder is off, spoofed, or out of range of a receiver. Being dark is not by itself evidence of wrongdoing; many vessels are not required to carry AIS.

How do satellites find dark vessels?▼

In two steps. First, a detector finds hulls in satellite imagery; synthetic aperture radar is used because it images day and night and through cloud, and a hull returns a strong signal against calm water. Second, each detection is compared with AIS positions at the time of the image. A detection with no matching AIS position is a candidate dark vessel.

Which free tool should I use?▼

To see where unbroadcast vessels have been across a region, use the Global Fishing Watch map, which publishes Sentinel-1 detections already matched against AIS. If you are an agency protecting your waters, Skylight combines radar, optical and night-light detections. To look at one radar scene yourself, use Copernicus Browser. To count vessels at your own port or strait on a scene you choose, or on every new Sentinel-1 pass, use Off-Nadir Delta and pair the detections with an AIS source.

Does Off-Nadir Delta match detections to AIS?▼

No. Delta does not ingest AIS and does not identify vessels. A detection is a bright target on the water with a position, and the count travels with its caveats. Global Fishing Watch and Skylight are the free options that do the AIS matching for you.

How small a vessel can Sentinel-1 detect?▼

It depends on the scene, the sea state and the detector, so no single figure applies. Global Fishing Watch describes its Sentinel-1 dataset as industrial vessels. For a vessel that is too small for 10 m radar, the next step is higher-resolution commercial imagery; satellite pass prediction shows when a commercial SAR satellite can next see the place.