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The bottom of the sea is the Danish Defence’s blind spot: 4 possible surveillance technologies

Denmark has not had submarines since 2004, but they may come back in the future, says Henning Heiselberg, head of Center for Security DTU. Illustration: Nanna Skytte

The surface of the sea is meticulously mapped. Anyone with a smartphone can download an app and see each and every ship that sails by.

The Royal Danish Navy and the Danish Home Guard patrol belts and straits, and they keep an eye on radar signals and radio waves from both sea and land.

But the bottom of the sea is dark.

After the ruptures in four places on the Nord Stream gas pipelines, Danish authorities have determined that they were caused by deliberate explosions, but who or what is behind them has not yet been clarified.

The incident has made many aware of how exposed Denmark is from the sea, especially below sea level. A large amount of critical infrastructure necessary for society to function is located on the seabed, of which no Danish authorities have a particularly good overview.

“We don’t constantly have eyes on everything underwater. Therefore, we have no way of identifying a submerged submarine,” says Johannes Kidmose, navy commander and head of the Centre for Maritime Operations at the Royal Danish Defence College.

Johannes Kidmose does not consider it improbable that an enemy vessel would be able to sabotage a submarine cable, internet cable, or gas pipeline on the bottom of the sea in Danish waters without anyone discovering it before the damage has been done.

But there are technologies that can monitor the area below the surface of the sea and detect hostile vessels, says Henning Heiselberg, head of Center for Security DTU.

“Just the fact that one can monitor them, find them, and see them means that they won’t dare to come close. This requires systems that constantly monitor the huge area. It’s not easy, and it requires large investments.”

Henning Heiselberg mentions four technologies in particular that can make a crucial difference:

Danish frigates have no sonars on board and are thus blind to what is happening down in the depths of the sea. Illustration: NOAA's National Ocean Service via Wikimedia Commons

1. A sonar network

From ships and shore stations all around the coast, the Danish Defence constantly scans the sea surface using coastal radars and other technologies. In addition, they use AIS signals to identify ships: According to the law, ships of a certain size must have a transponder on board, which transmits information about the ship’s name, position, etc.

However, radar signals and radio waves do not work under the sea. But sonar does. The sonar sends out a sound, and with the help of the echo that returns, it can paint a picture of what is happening in the area.

Henning Heiselberg sees sonar technology as crucial to being able to monitor the sea for submarines, drones, frogmen, or other things.

“One can have a network of buoys that are spread over a large area and that listen for and measure different things,” Henning Heiselberg says.

Sonars can also be mounted on ships and submarines or dipped from a helicopter.

“Sonars are most accurate on a submarine if you need to examine the seabed. Those on-board ships are also more accurate than those dipped from a helicopter. On the other hand, the helicopter can get to get to the area where submarines are suspected the fastest, so that they don’t disappear in the meantime.”

Denmark previously had a sonar research unit that looked for mines on the seabed with so-called side-scan sonars. However, it has been disbanded, says Henning Heiselberg.

Sonobuoys for environmental monitoring. Sonobuoys can also be used to search for submarines. Illustration: tempestz / Bigstock

The Danish Defence uses sonars to some extent, for example on minesweepers and inspection ships in the Arctic, according to DR. But according to Johannes Kidmose, the former equipment has been removed decades ago from both helicopters and frigates to save money, and it is also not found on the six patrol boats that patrol the waters around Denmark.

In 2018, the Danish Parliament decided to invest in new sonars for both helicopters and frigates. In addition, a number of sonobuoys were ordered, which can be dipped from a helicopter and monitor a larger sea area as part of an anti-submarine mission.

But that order has been delayed. It is not expected to arrive until the end of next year, and then the Danish Defence will have to spend at least a year learning how to use the equipment, the Danish Ministry of Defence Acquisition and Logistics Organisation informs Ingeniøren.

“We have a huge area, comparable to few other countries. So we’re in hot water, so to speak. It’s resource-intensive to have such a large area to cover as a small country,” Henning Heiselberg says.

Although the Danish Defence does not have a large stock of sonar equipment, there are actually several large manufacturers of it in Denmark, such as Zealand’s Teledyne Reson, for example. This is what Frederik Bergenfelt Friis, political consultant at the Danish Chamber of Commerce, tells us. He suggests that Danish expertise be brought into play if the monitoring of critical infrastructure is strengthened in the future.

Fibre-optic cables are mostly used for internet cables but can also function as a sensitive sensor. Illustration: Lasse Gorm Jensen

2. Multifunctional internet cables

The Danish seabed is not just home to gas pipelines but also to thousands of kilometres of fibre-optic cables used for data transmission. The cables consist of optical fibres which, in addition to sending data at the speed of light, can also function as a sensitive and accurate sensor covering its surroundings.

Fibre-optic cables on the seabed can function as a supplement to sonar technology to keep a constant eye on what is moving around selected areas in the sea, Henning Heiselberg believes.

Lars Dittmann, researcher at DTU Fotonik, agrees with him. He explains that the fibres in the cable consist of glass through which light is sent; an optical signal. If the signal is subjected to tension, pressure, twisting, or other forms of physical impact, it is possible to measure it.

“Everything that can affect an optical signal can be recorded and converted into an understanding of a given parameter. Fibre-optic cables are used, for example, to monitor fishing nets in the Norwegian fjords and prevent salmon from fish farms from escaping.”

Fibre-optic cables are primarily used to send data, but according to Lars Dittmann, many fibre-optic operators also use the optical fibres as sensors that provide detailed information about the condition of the submarine cables. Therefore, it would be natural to spread the technology to other forms of critical underwater infrastructure.

“You can place a fibre-optic cable on top of a gas pipeline or cast it into the concrete casing that encloses a gas pipeline,” Lars Dittmann says.

The California company SoCalGas uses fibre-optic cables to monitor its gas pipelines. Illustration: SoCalGas

The advantage of using fibre-optic cables is that they are relatively cheap and do not require physical modifications to be used as sensors. Today, signals can be sent over a distance of up to 100 kilometres before a repeater is needed to amplify the signal.

However, submarine fibre-optic cables are rarely laid together with gas pipelines or power cables to avoid faults on one type of cable affecting the other cables. At the same time, the fibre-optic cables can only send a warning once the damage has occurred, and they cannot provide an early warning like sonar.

A magnetometer that the U.S. Navy lowers from a ship to detect submarines. Illustration: U.S. Navy / John F. Williams via Wikimedia commons

3. An underwater metal detector

The third technology that can be used to spot submarines or underwater drones is a so-called magnetometer.

“We need magnetometers that cover the seabed and can pick up iron objects moving, such as submarines or ships,” Henning Heiselberg says.

Magnetometers are able to measure tiny changes in the magnetic field. As well as lying on the seabed, they can be towed through the water by a ship or mounted on a maritime patrol aircraft built to spot submarines. When used from the air, the technology is called MAD (magnetic anomaly detector), and it is used by e.g. the United Kingdom and Norway.

However, there is one problem with the magnetometer technology that we have to be aware of, Henning Heiselberg says.

“Great efforts are made to demagnetize submarines so that they cannot be found and measured.”

The underwater drone SAAB Double Eagle, which belongs to the Royal Danish Navy and is currently near Bornholm investigating the Nord Stream crime scene. Illustration: SAAB

4. Submarines and underwater drones

The first three technologies are useful for finding out if there may be a submarine or underwater drone present in Danish waters. But if we want to be absolutely sure whether it is a friend or an enemy, it is best to move under the surface ourselves, Henning Heiselberg believes.

The Royal Danish Navy has underwater drones, for example as fixed equipment on the minesweepers Hirsholm and Saltholm, and uses them, among other things, to destroy old mines. Saltholm is currently near Bornholm, investigating the Nord Stream leaks.

Underwater drones are also used to inspect internet cables for damage, says Lars Valdemar Mogensen, head of department at a Danish company that produces the small submarines.

And DTU is working on developing a system of independent underwater drones that can scout for abnormal things around offshore oil platforms, such as leaks or foreign vessels, Henning Heiselberg says.

“Seen in the light of recent observations of flying drones at the Danish and Norwegian platforms in the North Sea, that system is very relevant for underwater monitoring.”

Underwater drones are therefore no strangers to Danish waters.

On the other hand, Denmark does not have a single manned submarine. The last Danish submarines were retired in 2004, and one is currently on display at Holmen in Copenhagen.

Henning Heiselberg believes that both submarines and underwater drones may have a renaissance in the coming years, as we look towards a time of increasing military tensions and the risk of terrorism and sabotage.

“Surveillance in itself can ensure that enemy ships do not come close. And if they were to do it anyway, then we can respond to it with warships and perhaps in the future also submarines.”

Norway, Germany, the United Kingdom, the Netherlands, and Sweden all have submarines, and Sweden in particular is an obvious place to look to for inspiration, Henning Heiselberg believes.

“Sweden has for many years been a leader in finding and chasing Russian submarines, and they have even captured one.”

From the Baltic Sea, when gas was still bubbling up from the Nord Stream gas pipeline. Illustration: Forsvaret

What will the defence of the future look like?

The Nord Stream explosions have put renewed focus on Denmark’s lack of an overview over its underwater area, Henning Heiselberg believes.

“We are quite vulnerable because we have been living in peacetime for decades. The Nord Stream sabotage is a wake-up call.”

Denmark differs from its neighbouring countries because it is responsible for a sea area that is enormous in relation to both the country’s size and population. Thousands of merchant ships sail to and from Denmark and through the massive waters around Denmark, Greenland, and the Faroe Islands, and this will only increase with the melting of the Arctic.

It is therefore extremely important to use resources wisely in the Danish Defence in the coming time, says navy commander Johannes Kidmose.

“Based on everything that is happening right now, we really have to think about how to develop our fleet. Because it has to be developed. It’s not just about getting new and more ships, but also getting people to sail them and training the crew for the types of tasks that are needed, such as seabed warfare.”

On 6 March, a broad majority in the Danish Parliament agreed to gradually increase the Danish Defence’s budget so that it will [from 2030]( -of-defence/) have DKK 18 billion more to spend per year than today. On 18 August, Minister of Defence Morten Bødskov announced that DKK 40 billion will be invested in new ships for the Royal Danish Navy in the coming years.

In the aforementioned DR article, the Chief of the Royal Danish Navy, Carsten Fjord-Larsen, states that the politicians should buy warships that are equipped with sonar and, unlike the current frigates, can keep sailing in the Arctic in winter.

Kristian Søby Kristensen from the Centre for Military Studies at the University of Copenhagen also stated to DR that it is important to get control over our own waters, because if we leave it to the USA, it could provoke Russia.

“Russia will fly and sail more aggressively, and this could start an arms race spiral in the Arctic, which Denmark is not at all interested in.”

Navy commander Johannes Kidmose agrees with that statement.

“One thing that is clear in the NATO treaties is that, above all, we must have control over our own countries and our own waters.

Right now, I believe that we have a fleet that is not at the level it should be considering the fact that Denmark is one of the world’s most significant maritime nations.”