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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.

In attempt to enhance the U.S. military’s ability to detect submarines, DARPA 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.  

DARPA is looking at a process that, although similar to passive sonar, could be a new way of detecting underwater vessels. The Persistent Aquatic Living Sensors (PALS) program seeks to use sea life, 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.

The research program requires 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. The team told “[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.”

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.