India and other Quads Tracking Chinese Submarines

Pete, inspired by geo-strategic legend's, Ghalib Kabir's comments, on August 14 and 16 2021, writes: 

The Indo-Pacific undersea Sensor Array (which not only features audio, but other undersea sensor technologies) is successfully picking up Chinese nuclear and conventional submarines emissions in the Indo-Pacific. This array (map below) is maintained and ultimately weaponized by Quadrilateral (Quad) members. 

 
Map above 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) at http://press-files.anu.edu.au/downloads/press/p309261/pdf/book.pdf?referer=444. 
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Parts of the Array are of the well known deep water passive sonar (SOSUS) technology for detection of submarines. Other seafloor sections (some called Fixed, Distributed System (FDS) nodes) consist of a higher concentration of sensors to detect submarines and other vessels passing over them in shallower water littorals and narrows. 

The "Quad" Sensor Array can detect minute submarine emissions. This includes Chinese nuclear and conventional submarines. Many, maybe most, of the conventional Chinese Song-class and Yuans rely on Stirling engine AIP. This AIP technology has relatively noisy moving parts (compared to Fuel Cell AIP) and also suffers from a working depth limitation which cannot exceed 200m. Even Chinese subs using AIP while sitting on the seafloor create tell-tale emissions.

Chinese Type 093 SSNs even use the Sunda Strait rather than the Malacca Strait to pass through the Pacific to Indian Oceans and vice versa. 

Regarding the South China Sea, which Chinese subs are trying to control, it is shallower than most seas. This makes the South China Sea relatively transparent to Western optical, LIDAR and magnetic anomaly sensors.

Indian Ocean Legs of the Quad Array

Several years ago the Quad Sensor Array was expanded from Port Blair in India's  Andaman Island territory, Indian Ocean, to Chennai communications intelligence node (map below) on the east coast of India. 

(See more details of Chennai and other 2017- onwards Indian intelligence-communication network matters)
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The Indian Navy continues to expand an undersea (passive sonar) and surface-sea (long-range radar and satellite) detection network to the Seychelles and elsewhere in the Indian Ocean (see yellow stars indicating Indian bases in the map below). India is also negotiating to build an airstrip near Mauritius for P-8 maritime patrol aircraft. Chinese SSNs are major targets for detection. 

(See more details)
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