Sound Surveillance System (SOSUS): The “Fish Hook” That Catches Chinese Submarines

Today, China is building an underwater Great Wall that reaches out to the first island chain that stretches from Japan to Taiwan to the Philippines to Indonesia, composed of its own sound surveillance system (SOSUS) nets and anti-submarine warfare (ASW) forces designed to deny the area above all to the US 7th Fleet, but also to other allied navies such as the Japanese Maritime Self Defense Force, Indian Navy or the Australian navy.

The US Navy’s pioneering Sound Surveillance System (SOSUS) became a key, long-range early-warning asset for protecting the United States against the threat of Soviet ballistic missile submarines and in providing vital cueing information for tactical, deep-ocean, anti-submarine warfare.

Since the early 2000s, when PLAN submarine patrols are supposed to have turned aggressive, the U.S. Navy and the Japan Maritime Self Defense Force (JMSDF) began setting up a chain of fixed arrays to monitor the movement of Chinese submarines in the East China Sea and South China Sea. This resulted in the establishment of the “Fish Hook Undersea Defense Line” in early 2005, stretching from Japan to Southeast Asia with key nodes at Okinawa, Guam, and Taiwan. The system reportedly consists of two separate networks of hydrophones, one stretching from Okinawa to southern Kyushu, and the other from Okinawa to Taiwan.

The JMSDF and U.S. Navy personnel jointly manage the JMSDF Oceanographic Observation Centre in Okinawa, all the information is available to the U.S. Pacific Command,as the facility is under the operational control of the U.S. Navy.

The US ‘Fish Hook’ Undersea Defense Line Source Australian National University

This includes placing acoustic sensors in the deep ocean near US bases in the western Pacific and hydrophone networks along its own coast. Its growing ability to deny uninhibited American access to surface and aircraft military forces inside this island chain has shifted competition underwater where the US remains dominant. The resulting force structure that combines US and allied underwater, surface and aerial forces amounts to a giant fish hook that is intended to bait and capture Chinese naval and submarine forces – or at least make it impossible for Chinese forces to be sure of victory in a war in the western Pacific with the US.

The combination oceanography and science-based engineering development can lead to extraordinary operational effectiveness of the US Navy.

The modern submarines have a unique advantage over other types of military vessels because they are able to stay hidden below the sea surface. One way of detecting and locating submarines is by using passive acoustics or active acoustics.

The objective of passive acoustics is to detect the sounds produced by a submarine, such as propeller, engine, and pump noise. These sounds can be identified by experienced sonar operators. Each type of submarine has a unique sound profile that makes up the acoustic “signature” of the vessel. Submarines themselves are equipped with passive sonar systems, such as towed arrays of hydrophones that are used to detect and determine the relative position of underwater acoustic sources.

Active Sonar Suite

The Navy can also use active acoustics to find submarines much the same way people use active acoustics to find fish (See “How is sound used to locate fish?“). By transmitting a sound pulse and receiving the echo on an array, the sonars can determine the direction of the echoes that return from objects hit by the sound. They can also measure the time it takes for echoes to return and calculate the distance to the object causing the echo. A skilled sonar operator or a computer program can distinguish submarine echoes from those of ocean bottom features, whales, schools of fish, etc. Much research continues to be done on classifying the kinds of echoes that different objects make.

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