Thanks KQN for locating Owen R. Cote Jr.’s excellent article Invisible nuclear-armed submarines, or transparent oceans? Are ballistic missile submarines still the best deterrent for the United States? Published online on January 7, 2019 at https://www.tandfonline.com/doi/full/10.1080/00963402.2019.1555998.
I've placed very small parts of it in Section A. (below) in quotation marks. I have bolded some words for emphasis and added links and comments in brackets [...] for extra information. Sections B. and C. further support A.
In addition to the well known Sound Surveillance System (SOSUS) there is an additional term/concept Reliable Acoustic Path (RAP).
“Like SOSUS, RAP arrays are bottom-mounted [or vertically mounted from the bottom to a buoy], deep water arrays, but unlike SOSUS, they are upward-looking, and there are thousands of nodes in a single RAP array. [RAP arrays can be weaponized.] Each individual, upward-looking array node only receives signals from a tea cup-shaped zone of coverage several miles deep and 20 miles wide at the surface."
"Consequently, an individual RAP array node has two huge advantages over the nodes in a SOSUS array: It is no more than a few miles away from its potential targets (which is point blank range for a sophisticated, passive acoustic sensor) and very little of the broad ocean’s noise is competing with the target’s signal. The flip side is that even a RAP array with thousands of nodes can only cover a small fraction of the ocean area that SOSUS covered during its heyday."
"This means that RAP arrays do not provide anything close to ocean-wide surveillance. But they do provide reliable if fleeting, preliminary indications (“cues” in submariner-speak) of even the quietist submarines at natural chokepoints in the ocean, such as the one that exists between Greenland, Iceland, and the United Kingdom [GIUK] – or, more to the point, the Luzon Strait or the Ryukyus (i.e., the main exits from China’s Inner Seas to the Philippine Sea).”
[see map at B. below]
[see map at B. below]
"Some Reliable Acoustic Path arrays are called Fixed, Distributed System [FDS] arrays capable of detecting the more quiet Russian nuclear submarines that pass over them (eg. Akula multipurpose SSNs). “A modernized version of the original Fixed, Distributed System likely is being deployed in the Western Pacific, if it has not already been done.” [perhaps covering] “...of chokepoints like the Ryukyus and the Luzon Strait...”
B.
Already in the public realm and on Submarine Matters at How to Trap the Chinese Dragon - SeaWeb's Fixed Undersea Array, since September 4, 2015 is the map below of a current or past hook shaped SOSUS and RAP/FDS array line in the Western Pacific hooking around to the Indian Ocean. The "point" of the hook probably terminates at Port Blair in India's Andaman Island territory. In 2018 I suspected and published that the array has been extended from Port Blair, west across the Bay of Bengal, to Chennai, India.
The map is from page 54 “Map 4. The US ‘Fish Hook’ Undersea Defense Line” by (the late) Desmond Ball and Richard Tanter, The Tools of Owatatsumi Japan’s Ocean Surveillance and Coastal Defence Capabilities (2015, ANU Press) http://press-files.anu.edu.au/downloads/press/p309261/pdf/book.pdf?referer=444.
C.
Further interesting comment on SOSUS and RAP/FDS arrays are at John Keller Editor, MiltaryAerospace(dot)com’s Navy to take a page from commercial undersea cable industry for new ocean surveillance technology of June 9, 2014. I have bolded some words for emphasis
“...U.S. Navy undersea warfare experts are trying to tap into the commercial undersea cable industry to find recent technological advances that might be useful in maritime surveillance systems (MSS)....”
“...The term maritime surveillance generally refers to sonar listening arrays installed on the ocean bottom in strategic areas like the Greenland-Iceland-United Kingdom (GIUK) gap, the Straits of Florida and Yucatan Channel gateways to the Gulf of Mexico, and the Strait of Malacca [between Malaysia and] Indonesia.”
“...Navy fixed-site undersea sensor systems today include the Fixed Distributed System (FDS) and the Sound Surveillance System (SOSUS), which are deployed in strategic ocean choke points...”
Pete