The fine art of surfacing

ON JULY 20th 1960, a missile popped out of an apparently empty Atlantic ocean. Its solid-fuel rocket fired just as it cleared the surface and it tore off into the sky. Hours later, a second missile followed. An officer on the ballistic-missile submarine USS George Washington sent a message to President Dwight Eisenhower: “POLARIS—FROM OUT OF THE DEEP TO TARGET. PERFECT.” America had just completed its first successful missile launch of an intercontinental ballistic missile (ICBM) from beneath the ocean. Less than two months later, Russia conducted a similar test in the White Sea, north of Archangel.

Those tests began a new phase in the cold war. Having ICBMs on effectively invisible launchers meant that neither side could destroy the other’s nuclear arsenal in a single attack. So by keeping safe the capacity for retaliatory second strikes, the introduction of ballistic-missile submarines helped develop the concept of “mutually assured destruction” (MAD), thereby deterring any form of nuclear first strike. America, Britain, China, France and Russia all have nuclear-powered submarines on permanent or near permanent patrol, capable of launching nuclear missiles; India has one such submarine, too, and Israel is believed to have nuclear missiles on conventionally powered submarines.

As well as menacing the world at large, submarines pose a much more specific threat to other countries’ navies; most military subs are attack boats rather than missile platforms. This makes anti-submarine warfare (ASW) a high priority for anyone who wants to keep their surface ships on the surface. Because such warfare depends on interpreting lots of data from different sources—sonar arrays on ships, sonar buoys dropped from aircraft, passive listening systems on the sea-floor—technology which allows new types of sensor and new ways of communicating could greatly increase its possibilities. “There’s an unmanned-systems explosion,” says Jim Galambos of DARPA, the Pentagon’s future-technology arm. Up until now, he says, submariners could be fairly sure of their hiding place, operating “alone and unafraid”. That is changing.

Hello, buoys

Aircraft play a big role in today’s ASW, flying from ships or shore to drop “sonobuoys” in patterns calculated to have the best chance of spotting something. This is expensive. An aeroplane with 8-10 people in it throws buoys out and waits around to listen to them and process their data on board. “In future you can envision a pair of AUVs [autonomous underwater vehicles], one deploying and one loitering and listening,” says Fred Cotaras of Ultra Electronics, a sonobuoy maker. Cheaper deployment means more buoys.

But more data is not that helpful if you do not have ways of moving it around, or of knowing where exactly it comes from. That is why DARPA is working on a Positioning System for Deep Ocean Navigation (POSYDON) which aims to provide “omnipresent, robust positioning across ocean basins” just as GPS satellites do above water, says Lisa Zurk, who heads up the programme. The system will use a natural feature of the ocean known as the “deep sound channel”. The speed of sound in water depends on temperature, pressure and, to some extent, salinity. The deep sound channel is found at the depth where these factors provide the lowest speed of sound. Below it, higher pressure makes the sound faster; above it, warmer water has the same effect.

Changes in the speed of sound (or for that matter light) cause sound (or light) waves to bend, a phenomenon known as refraction. The higher speed of sound above and below the deep sound channel thus bend sound back into it, allowing it to propagate for thousands of kilometres, especially if the sound’s wavelengths are long. It is a natural analogue to the process that keeps light in an optical fibre. Some zoologists believe whales use it as an ocean-wide telephone system.

In the POSYDON system, buoys on the surface would receive a GPS fix from satellites, then retransmit that data into the deep sound channel in acoustic form to submerged submarines and AUVs. Dr Zurk’s team is now determining the optimum frequencies for propagation, and modelling ways to correct for variable conditions. The simplicity of POSYDON would allow AUVs to offload a lot of the expensive equipment that they currently use to decipher positioning, says Dr Galambos. That means the possibility of more room on the drones for other useful stuff, or more money for more drones.

Even in heavily surveilled seas, spotting submarines will remain tricky. They are already quiet, and getting quieter; new “air-independent propulsion” systems mean that conventionally powered submarines can now turn off their diesel engines and run as quietly as nuclear ones, perhaps even more so, for extended periods of time. Greater autonomy, and thus fewer humans—or none at all—could make submarines quieter still. “As we pivot from manned to unmanned, no air cavity, maybe no propulsion motor, that’s a really challenging platform to find,” says Dr Zurk.

A case in point is a Russian weapon called Status-6, also known as Kanyon, about which Vladimir Putin boasted in a speech on March 1st. America’s recent nuclear-posture review describes it as “a new intercontinental, nuclear-armed, nuclear-powered, undersea autonomous torpedo”. A Russian state television broadcast in 2015 appeared to show it as a long, thin AUV that can be launched from a modified submarine and travel thousands of kilometres to explode off the shore of a major city with a great deal more energy than the largest warheads on ICBMs, thus generating a radioactive tsunami. Such a system might be seen as preserving a second-strike capability even if the target had a missile-defence system capable of shooting ICBMs out of the sky.

Despite such disturbing possibilities, many experts think that the balance of advantage is currently with seekers, not hiders. Sebastian Brixey-Williams of the British American Security Information Council thinks that “tracking and trailing” submarines will, within a decade, become significantly easier. Passive systems which simply listen will be a key part of this. Mr Brixey-Williams predicts that a few important choke points, such as the gap between Scotland and Iceland could now be completely surveilled by an array of just 15 acoustic sensors, far more sophisticated than the chain of hydrophones which did that job in the cold war. If a submarine is detected by such a system, it can then be trailed by another submarine, or some new form of drone.

New cold war

One part of the ocean that has become particularly interesting in this regard is the Arctic. Tracking submarines under or near ice is difficult, because ice constantly shifts, crackles and groans loudly enough to mask the subtle sounds of a submarine. With ever less ice in the Arctic this is becoming less of a problem, meaning America should be better able to track Russian submarines. Its Assured Arctic Awareness programme, also run by Dr Zurk, aims to develop new sensing techniques that can provide year-round monitoring without requiring a human presence. It is working on probes that can be deposited on the ice by drone, then melt their way down to the ocean beneath. Dr Zurk also talks of tagging icebergs with sensors, thus getting free rides across the ocean.

Greater numbers of better sensors, better networked, will not soon make submarines useless; but even without breakthroughs, they could erode the strategic norm that has guided nuclear thinking for over half a century—that of an unstoppable second strike. If a country even suspects that the location of its second-strike submarines might be known, their value for nuclear deterrence decreases. As Mr Brixey-Williams wrote in 2016: “The political tensions and threat to strategic stability that [tracking and trailing] would create should not be underestimated...and may be more dangerous than the technology itself.”