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Ever since the development of submarines and other sub-surface threats, such as naval mines, the ability to detect objects lurking below the world’s oceans has been a top priority for navies all over the world. This remains the case today, especially as sub-surface threats increase with the proliferation of mini-submarines and robotic systems such as unmanned underwater vessels (UUVs), which can penetrate traditional defences.

In March 2018, Russian President Vladimir Putin revealed the development of six new strategic weapons, one of them a long-range nuclear-tipped torpedo called Poseidon. This nuclear torpedo, if detonated near the coast of the US, could engulf vast areas with a radioactive tsunami that would cause devastation for years. While the US and allies have a network of man-made underwater sensors to detect sub-surface threats - both manned and unmanned - these alone can never completely cover the vast distances and depths of the world’s oceans.

A nuclear torpedo, for instance, could easily slip through the cracks.

This is just one of the reasons why the Pentagon is looking at strengthening its underwater detection capabilities, firstly by increasing the effectiveness of existing technologies, and also by further exploring innovative concepts that are still in their research stages. Unsurprisingly, the Department of Defense has turned to DARPA - which is known for its radical ideas that often resemble science fiction - for help on how it could improve underwater detection.

And DARPA hasn’t disappointed. In February the agency announced that it was funding a programme known as Persistent Aquatic Living Sensors (PALS) through its Biological Technologies Office, which aims to use marine organisms and tap into their highly tuned sensing to detect underwater threats.

“The US Navy’s current approach to detecting and monitoring underwater vehicles is hardware-centric and resource intensive,” noted PALS programme manager Lori Adornato.

“If we can tap into the innate sensing capabilities of living organisms that are ubiquitous in the oceans, we can extend our ability to track adversary activity and do so discreetly, on a persistent basis, and with enough precision to characterise the size and type of adversary vehicles,” she added.

Earlier this year Northrop Grumman was selected by DARPA to begin developing biological sensing hardware, along with several other teams from academia, industry and the US Navy. All the teams will develop or apply technologies “to record stimulus responses from observed organisms, and develop combined hardware and software systems that interpret those responses, screen out false positives, and transmit analysed results to remote end users,” according to DARPA.

An array of technologies will be used including hydrophones, sonar, cameras, and magnetic, acoustic, and kinetic sensors.

“We've been building and delivering undersea technologies for many years, developing different kinds of sonar and related processing systems, so this research is really an extension of that,” says Vern Boyle, vice-president of advanced technology at Northrop Grumman Mission Systems. “And really what we're trying to look at with PALS is, how do you move beyond some of the basic research and move more in the direction of applied capability?”

“We're working with snapping shrimp and some different bioluminescence organisms to see what kind of range and what kinds of sensitivity we need in the sensors.”

Northrop Grumman will record and analyse acoustics from snapping shrimp and optical activity by bioluminescent organisms, which are “pervasive in the aquatic environment”, says Boyle. Snapping shrimp will react to their environment by emitting signals with different characteristics, which can be analysed to determine what it is reacting to. Like most sensing systems, such as sonar or radar, it is about measuring and processing the data collected to determine what the object is.

“We're working with snapping shrimp and some different bioluminescence organisms to see what kind of range [and] what kinds of sensitivity we need in the sensors,” says Boyle. “We're looking at more advanced machine learning-based processing of the signals themselves [as well as] feature extraction and classification of the signals to see if the organisms can reveal something about the object in the environment: Is it manmade or natural? If it is manmade, what kind of object is it? Can you ascertain something about its size or shape or speed?”

A team from Florida Atlantic University will also analyse vocalisation cues from the goliath grouper saltwater fish in tropical and subtropical waters, while the University of Maryland Center for Environmental Science will tag black sea bass with sensors to track their behaviour around underwater vehicles. The Naval Research Laboratory and Naval Undersea Warfare Center will also take part in PALS.

The use of marine life as one large network of sensors would effectively cost the US Navy nothing. There would be no cost associated with deployment or sustainment as nature would effectively take care of that.

Boyle also highlights that unlike active sonar pulses, there is no danger to aquatic organisms because the entire network would use passive ‘listening’ sensors, rather than emitting sound waves and listening for the echo coming back. This persistent passive approach - similar to how vessels use passive sonar to listen for submarines - would also ensure that threats were unaware of being monitored.

“The teams involved will have to push the boundaries of what is technically feasible with current sensing and processing technologies.”

The Northrop Grumman team has now taken baseline measurements in a lab environment to characterise the hardware and processing, and has already begun collecting signals from controlled and live environments. DARPA envisions PALS as a four-year research programme - likely spread across several phases - with teams able to publish results for review by the scientific community unless it is identified as controlled unclassified information.

The programme is still in its initial phases and whether PALS will result in a fielded solution is too early to tell. The teams involved will have to push the boundaries of what is technically feasible with current sensing and processing technologies, and some will inevitably not achieve the results expected, while others could achieve a breakthrough that will ultimately give the US a powerful capability for the future.