Every day, millions of vessels traverse the world's oceans—container ships, fishing boats, ferries, tankers, warships, and everything in between.
To prevent collisions, ships are legally required to transmit signals via an Automatic Identification System (AIS). Authorities also use AIS signals to monitor fishing activity, passenger ferries, sovereignty violations, and rescue operations.
Nevertheless, many ships deactivate their AIS signals. They are then referred to as dark ships. American research published in 2024 in the scientific journal Nature found that up to 30 per cent of the world's vessels emit no AIS signal, prompting researchers at DTU to develop methods for identifying these ghost ships.
"Not all dark ships are engaged in illegal activity, but nearly all ships involved in unlawful acts are dark ships," says Kristian Aalling Sørensen, postdoc at DTU Space, specializing in maritime surveillance and providing research-based support to authorities.
Such activities range from illegal fishing to drug and arms smuggling, espionage, and attacks on critical infrastructure. Hence, authorities need to be able to detect these ships swiftly and accurately.
"We naturally wish to prevent such actions, but the military does not have sufficient vessels to be everywhere. Surveillance of dark ships is one of our most effective tools, as it serves as a deterrent—the better our surveillance, the stronger the deterrence," Kristian Aalling Sørensen explains.
Disguised as accidents
Alexander With, a military analyst at the Royal Danish Defence College who researches maritime hybrid warfare, notes a significant rise in incidents involving sabotage of maritime infrastructure since the war in Ukraine.
"There were also cases of damaged subsea cables and pipelines under suspicious circumstances prior to the war, but the scale has increased dramatically," he says.
While Alexander With sees Russia as the primary threat in Danish waters, there have also been incidents involving Chinese vessels exhibiting suspicious behaviour near cables shortly before the cables were damaged. Typically, commercial ships drag their anchors across the seabed to damage cables and pipelines.
"That way it can be claimed to be an accident," Alexander With says.
