It has to be more difficult for ships to hide their routes through Danish waters.
Henning Heiselberg, head of Center for Security DTU, has embarked on this mission together with colleagues from DTU Space.
Every day, there are ships in Danish waters that switch off the legally required VHF-based navigation and anti-collision tool AIS (automatic identification system).
It could be because they are fishing illegally, dumping oil or chemicals, violating territorial boundaries, or simply forgetting to turn on their AIS transponder.
In the Arctic, some cruise ships switch off the AIS signal because they are not allowed to sail so close to calving glaciers.
They must not be able to get away with that in the future, which is why DTU is working on a rogues’ gallery, i.e. a fingerprint database of ships that are known to switch off their AIS transponders.
The primary tool for identifying dark ships is satellite imagery from two European satellites, Sentinel-1 and Sentinel-2.
They regularly fly over Danish waters and take pictures from between 690 and 780 kilometres above the Earth.
“We get both optical images and radar images, which enable us to identify ships—even if they have turned off AIS—and assign them a primitive fingerprint in the form of size, length, and width. If the weather is clear, we can also see colour signatures on the ship,” Henning Heiselberg explains.
The automatic identification system (AIS) is a VHF-based navigation and anti-collision tool which makes it possible to exchange information between ships. All ships over 300 gross tonnes, as well as passenger ships and fishing vessels over 15 metres are legally required to use AIS. AIS data: Identification of, for example, the ship’s name, call sign, or IMO number, as well as the MMSI number. Navigation data, for example, the ship’s position, course, and speed. Information about the ship’s journey—for example, the ship’s destination, arrival time, and current draught. Source: The Danish Maritime Authority
The automatic identification system (AIS) is a VHF-based navigation and anti-collision tool which makes it possible to exchange information between ships. All ships over 300 gross tonnes, as well as passenger ships and fishing vessels over 15 metres are legally required to use AIS.
Identification of, for example, the ship’s name, call sign, or IMO number, as well as the MMSI number.
Navigation data, for example, the ship’s position, course, and speed.
Information about the ship’s journey—for example, the ship’s destination, arrival time, and current draught.
Source: The Danish Maritime Authority
The information is used to identify and catalogue ships which have previously been known to turn off their AIS signals in Danish waters.
“We are compiling a rogues’ gallery of ships in Danish waters. We use an image recognition algorithm, and we are currently investigating how well we can recognize ships. So far, the results are promising,” Henning Heiselberg says.
In recent years, the tools for spotting dark ships have improved greatly thanks to the new European Sentinel satellites used for Earth observation.
Radar images from the Sentinel-1 Synthetic Aperture Radar (SAR) are especially useful because they can see through clouds even at night.
The SAR emits and receives radio waves with a wavelength of about six cm, with which it scans the Earth’s surface perpendicular to the direction of the satellite’s orbit around the Earth.
The new database works like a fingerprint database, except for ships instead of people.
“We can designate ships with a certain probability, depending on how many parameters we have available. For example, whether the ship has an unusual sailing route, whether the ship has been in the area before, and whether there are other similar ships in the area,” Henning Heiselberg says.
Hunt for the jammer ship
The method is not only used for pure research.
Right now, Center for Security DTU is working to identify which ships were in a given area off Sjællands Odde on 3 October between 14:47 and 14:57.
During this time, a lot of radio noise was recorded on the two frequency bands, L1 and L2, over which the navigation equipment receives signals from GPS satellites.
GPS expert and PhD student at DTU Space Søren Skaarup Larsen investigated a number of private and public GNSS stations that register GPS signals, and he found a lot of radio noise at the Samsø, Røsnæs, Halsnæs, and Sjællands Odde stations.
“It’s most likely a powerful jammer that was in the area either in the air or on the water. It’s possible to jam large areas over water because there are no buildings or terrain that reflect and disturb the signal—especially on a clear day like 3 October,” Søren Skaarup Larsen said to Ingeniøren in October.
The jamming incident was so severe that, among other things, the navigation equipment on Molslinjen’s ferries between Aarhus and Sjællands Odde went down.
It is not yet known who is behind the powerful jamming attack.
But with the help of three types of information, Henning Heiselberg and his colleagues at DTU Space hope to get closer to an answer.
Initially, GPS data from four government stations has been triangulated to pinpoint the most likely location from which the jamming attack was carried out. It points to an area off the coast north of Sjællands Odde.
Everything points to the jamming attack having been carried out from a ship.
Henning Heiselberg subsequently received AIS data from Gatehouse Maritime, a company which tracks AIS signals in Danish waters, and then the AIS data was compared with a satellite image from the area two hours before the actual attack was carried out.
Based on that data, it is evident that there were one or more dark ships in the area during the time of the jamming attack.
The dark ships are currently being identified.
“We have now narrowed it down to one or more suspect ships, which we are investigating further,” Henning Heiselberg says.
Large ships near Nord Stream
DTU Space was not quite so lucky when it comes to the Nord Stream explosions.
It was cloudy at the time the explosives were allegedly found, and it is not yet known when the explosive charges were placed at the Nord Stream pipelines.
“This means that we only have radar images, which provide a bit less information and details, and the case is further complicated by the fact that we do not have an exact time for when the explosive charges were placed at Nord Stream 1 and 2. But we are working on the case and have found dark ships in the area in the days leading up to the sabotage,” Henning Heiselberg says.
A similar analysis by the American satellite monitoring company SpaceKnow has shown that in the days before the suspected sabotage on Nord Stream 1 and 2, two dark ships sailed past one of the leaks.
The peculiarity of the radar images is that the ships are much larger than is normally recorded.
One million icebergs examined
In the Arctic, the Danish authorities spend a lot of resources on territorial surveillance.
Center for Security DTU now has a database of 10,000 ships that they have identified in Arctic waters and the Atlantic.
The database also contains about one million icebergs.
Right now, the biggest challenge in the Arctic is to distinguish between calved icebergs and ice floes and ships that have switched off their AIS signal.
“The glaciers in north-western Greenland calve thousands of icebergs and ice floes, and some of them look like ships in the satellite images recorded at a distance of 800 km. We have succeeded in differentiating between the two using deep learning and thus reducing false alarms significantly,” Henning Heiselberg says.
The method can be used for more than just dark ships. The DTU researchers have discovered that they can use satellite images to identify ground-based radars.
Radar signals on Earth produce noise in the images taken from space, and this results in clear interference stripes on the satellite images.
“It happened somewhat by accident, but we can see that we can pinpoint positions from which radars have been set up on land or on ships,” Henning Heiselberg says.
A group of students has also used the same type of satellite data to pinpoint the locations of forest fires.
“We have a few students who are also volunteers in the emergency services, so they were interested in learning more about fires. Identifying wildfires with the Sentinel satellites has proven to be surprisingly easy. When the war broke out in Ukraine in February, we started following forest fires in the country. We could actually follow the course of the war via fires which started near Belarus and slowly moved over Chernobyl towards Irpin near Kyiv and back again,” Henning Heiselberg says.