DARPA is embarking on an undersea submarine detection system so weird it may just work. The Pentagon’s research and development arm wants to learn the behaviors of undersea animals including fish, shrimp, and microscopic phytoplankton so it can use them to detect manned and unmanned submarines passing by. Such a network would enhance the U.S. military’s ability to detect even the quietest submarines.
Traditionally there are two methods to detect submarines at sea: active and passive sonar. Active sonar broadcasts pulses of sound into the water. When the pulses bounce back to the sender, they can be studied to locate possible enemy submarines. While effective, broadcasting sound underwater reveals the location of the friendly ship or sub to those around it. Passive sonar involves detecting sonar broadcasts or other noises made by enemy ships. While much safer for the searching ship, if a submarine is quiet enough it can’t easily be detected.
DARPA is now looking at a process that, although similar to passive sonar, could be a third way of detecting underwater vessels. The Persistent Aquatic Living Sensors (PALS) program seeks to use sea life, including black bass, goliath groupers, and snapping shrimp, as a living underwater sensor network. Goliath groupers, for example, make booming barking sounds that can be felt as well as heard. If a passing submarine disturbs a grouper, causing it to bark, that vocalization could be picked up by an underwater listening post no matter how quiet the submarine is.
DARPA describes PALS as a “a four-year fundamental research program requiring contributions in the areas of biology, chemistry, physics, machine learning, analytics, oceanography, mechanical and electrical engineering, and weak signals detection.” DARPA is currently currently funding five teams to study the problem.
One team, led by Raytheon, is studying the use of snapping shrimp as a possible underwater sensor. Snapping shrimp snap their claws at super-fast speeds, creating a high-pressure cavitation bubble. The collapse of this bubble creates a loud snapping noise powerful enough to stun prey. Snapping shrimp also use the snapping noise to communicate with other shrimp, and large colonies of shrimp can create a cacaphony of snapping noises. During World War II, U.S. Navy submarines used the din of snapping shrimp colonies to avoid detection entering Japanese harbors.
Raytheon’s team, led by research scientist Alison Laferriere, told Popular Mechanics “[Raytheon] is developing a novel system to detect manned or unmanned underwater vehicles in coastal waters that will leverage the sounds made by organisms found naturally in the environment. The system will use the loud, impulsive sounds produced by snapping shrimp as sources of opportunity in a multi-static sonar system—detecting reflections of those sounds off of the underwater vehicle. To enhance performance and versatility, the system will also listen to the underwater soundscape (i.e., the sounds produced by all animals in the environment), utilizing machine-learning algorithms to detect changes in these sounds caused by the intrusion of an underwater vehicle.”
Using sea life to detect submarines is a novel idea. While passive sonar relies on sound emitted from submarines, sea life such as goliath groupers and snapping shrimp might be disturbed by the underwater pressure wave from a passing submarine or from a large shadow passing over them. As a sensor network sea life is naturally self-sustaining. On an individual level snapping shrimp are self-repairing, regenerating lost claws to become part of the network once again. Unlike dolphins and seals that have military jobs, fish and shrimp wouldn’t need to be trained—they’re just naturally reacting to stimuli that just happen to be potentially hostile undersea vehicles. And a network of sea life would cost taxpayers nothing.
Of course, all of this relies on viable numbers of sea life to keep them useful. The goliath grouper, for example, might be a great submarine detector but it is also critically endangered in the Atlantic waters it calls home. (Snapping shrimp are a lot more ubiquitous.) In the near future, overfishing and climate change could affect the Pentagon’s ability to detect hostile submarines.