Underwater networking: Captain Nemo goes online

Networking: Emerging undersea data networks are connecting submarines, aquatic drones and other denizens of the deep

DURING the Cuban missile crisis of October 1962 the Soviet Union stationed submarines in the region in order to be able to sink American ships in the event of war. But communication between the submarines and Soviet high command was hard. Electrically conductive salt water absorbs radio waves, so exchanging information with Moscow required the submarines to ascend to periscope depth in waters patrolled by American planes and ships. Between scheduled transmissions Soviet crews also poked antennae out of the water and listened to commercial radio to see if hostilities had broken out. A blunder could have been catastrophic, says Alexandre Sheldon-Duplaix, a naval-warfare historian at France’s defence ministry, because the Soviet submarines were armed with nuclear torpedoes that the West did not even know existed.

In the decades that followed navies continued to be dogged by the difficulty of establishing underwater data links. In the 1980s America decided not to integrate its new “Los Angeles” class of attack submarines with aircraft-carrier groups, says Norman Friedman, a former consultant to the secretary of the US Navy, because it was concerned that flaky communications links between ships and submarines might lead to collisions or friendly fire. A former commander of an American carrier in the 1990s says he was unable to establish submarine contact “more times than I can even remember”. But now better underwater-communications technologies are changing the rules.

Although traditionally more a “sneaky sniper” than a team player, as Eric Wertheim, an American naval analyst, puts it, submarines are increasingly expected to co-ordinate with surface and land forces. Subsea networks being developed by America and its allies will shuffle data between submarines, aquatic drones and sensors via devices that sway beneath the water or bob on its surface. They will make it possible to detect enemy vessels and mines and allow submarines to link up with surface ships, aircraft and distant command centres, says Vernon Clark, America’s former chief of naval operations. Undersea networks could also monitor waterways and gather scientific data.

Making waves

In the 1970s and 1980s both America and the Soviet Union built underwater-signalling systems based on “extremely low frequency” (ELF) radio waves. Both systems needed large electrodes, buried in the ground 50-60km apart, and could then send signals to submarines thousands of kilometres away—but only one way, and at a rate of around ten characters per minute. To overcome these limitations, more recent research has focused on signalling using sound, over shorter ranges.

The big technical challenges have been mostly overcome, says John Potter of NATO’s Centre for Maritime Research and Experimentation (CMRE) in La Spezia, Italy, who heads one of several groups building underwater networks based on small devices called acoustic nodes. These could be “shovelled out of a plane”, says Mr Potter, to provide the military alliance with an unprecedented ability to gather intelligence, communicate and co-ordinate its forces underwater. Akin to mobile-phone towers but communicating using pulses of sound rather than radio waves, the nodes are placed a kilometre or so apart. A few extra nodes are needed because acoustic signals can be scrambled or lost near a choppy surface, in fast currents or in a region with a sharp temperature change.

Each node, which costs $5,000-10,000, is around the size of a canister of tennis balls. It contains a computer to process signals, a microphone to pick up sound waves, and a piezoelectric transducer to emit them. This transducer, similar in size to a small plate, is made of a special material that expands and contracts suddenly when a voltage is applied, creating an acoustic wave in the water. This signal is picked up by other nearby nodes, which retransmit it as appropriate to more distant ones, like an underwater internet.

Seaweb, an underwater network being designed at the Naval Postgraduate School in Monterey, California, and the Space and Naval Warfare Systems Command in San Diego, takes a similar approach. Its nodes have been tested under the Arctic ice sheet and in water 300 metres deep. Data packets hop from one node to another until they reach their destination—a submarine, a subsea drone, a warship or a “gateway” node at the surface. Gateway nodes use a satellite-radio link to connect the underwater network to the rest of the world.

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