Is India building its own SOSUS?

The ability to operate undetected has traditionally been a submarine's greatest defence. For nuclear-armed states, concealment is particularly important: an undetected submarine can preserve the ability to retaliate even after other nuclear forces may have been destroyed. India's underwater surveillance project raises the prospect that parts of the Indian Ocean could become progressively less opaque. India's Defence Research and Development Organization (DRDO) has initiated development of an Underwater Fibre Optic Sensing System (UFOSS). Led by its Naval Physical and Oceanographic Laboratory, the proposed system would place sensor nodes on the seabed, connect them through undersea fibre-optic cables and transmit data to a shore station for monitoring and analysis. India in the short to medium term may not achieve a geographical reach or technological maturity of United States Cold War Sound Surveillance System, better known as SOSUS. But UFOSS follows a similar operational logic: persistent seabed sensing can narrow the enormous search area supporting anti-submarine forces. That distinction matters. SOSUS did not make the oceans transparent. Its strategic value lay in detecting acoustic contacts, accumulating information on Soviet submarine signatures and cueing aircraft, warships and attack submarines towards smaller search areas. It became progressively more useful as fixed sensors were integrated with mobile anti-submarine warfare assets. Cold War operations reveal how consequential such tracking could become. For instance, in 1969, the US attack submarine USS Lapon covertly followed a Soviet Yankee-class ballistic-missile submarine throughout a 47-day deterrent patrol. Similarly, in 1978, USS Batfish tracked another Soviet Yankee-class submarine for approximately 50 days and for nearly 9,000 nautical miles. The Soviet Union reportedly remained unaware of the latter operation until it was declassified years later. These missions allowed the US to study Soviet operating patterns and acoustic characteristics without revealing that the submarines had been compromised. India increasingly possesses the components needed to develop a comparable, although geographically smaller, architecture. Alongside UFOSS, the Indian Navy operates P-8I maritime patrol aircraft, MH-60R helicopters, submarines and ASW-capable surface vessels. The result could eventually be a network where a fixed sensor provides an initial detection and mobile forces perform the much harder tasks of localisation, classification and potential tracking. According to Indian media reports, persistent monitoring and the cueing of anti-submarine warfare assets are among UFOSS's central functions. India does not need to monitor the entire Indian Ocean. The Andaman and Nicobar Islands overlook approaches connecting the Pacific and Indian Oceans. Chinese submarines entering the Indian Ocean would have to navigate a limited number of passages through Southeast Asia. Selectively positioned sensors could therefore function as barriers around strategically valuable approaches rather than attempting continuous surveillance across millions of square kilometres. The Interpreter noted in 2017 that growing Chinese submarine presence had encouraged Indian interest in a line of undersea sensors in the eastern Indian Ocean. Chinese submarines were already using deployments to acquire experience with the Indian Ocean's distinctive hydrology, bathymetry and thermoclines. UFOSS potentially gives this older strategic requirement a more concrete indigenous technological foundation. The longer-term advantage would be cumulative. A sensor contact that cannot be continuously tracked can still reveal when a submarine entered an area, its probable direction and elements of its acoustic behaviour. Repeated observations could contribute to signature libraries and patterns of deployment. Integrated with satellite surveillance, maritime patrol aircraft and increasingly sophisticated data analysis, individual detections may result in a much larger intelligence picture. Public reporting on UFOSS specifically identifies both Chinese submarines and Pakistan's Hangor programme among the developments driving India's requirement. Persistent surveillance could therefore acquire nuclear significance. A submarine suspected of carrying nuclear weapons may interpret Indian ASW activity very differently from a conventionally armed vessel. During a crisis, attempts to locate such submarines could be viewed not simply as maritime surveillance but as an effort to hold a retaliatory capability at risk. However there is another consequence that deserves greater attention. An Indian underwater surveillance system would not hear only Chinese and Pakistani submarines. The Indian Ocean is visited by several nuclear navies. In October 2022, for example, the US took the unusual step of publicly revealing that the Ohio-class ballistic-missile submarine USS West Virginia had visited Diego Garcia during an extended deterrence patrol. The deployment placed one of US most sensitive strategic assets in the same wider oceanic space in which India seeks greater undersea awareness. This creates a paradox for India's Indo-Pacific partners. The US, Australia and Japan may have strong reasons to welcome greater Indian capacity to monitor Chinese naval activity. Indian and partner surveillance capabilities can contribute to a more favourable maritime balance as the PLA Navy becomes increasingly active in the Indian Ocean. But acoustic sensors are politically indifferent. A network capable of collecting information on Chinese submarine movements could also encounter American, British and eventually Australian nuclear-powered submarines operating in the wider region. For Australia, for instance, this question will become increasingly relevant as it moves towards operating nuclear-powered submarines under AUKUS. Undersea cooperation with India could offer considerable strategic value, but greater Indian acoustic awareness also raises questions about data ownership, information sharing and protection of sensitive submarine signatures. This also demonstrates why undersea information is unusually sensitive. Repeated detections can reveal more than the presence of a submarine; over time, they can contribute to understanding operating patterns and acoustic characteristics. Nor will India's competitors remain passive. China can respond through quieter submarines, alternative transit patterns, deception, unmanned underwater systems and efforts to map or circumvent fixed sensors. Pakistan's AIP-equipped submarines are designed for prolonged stealth operations. UFOSS would therefore initiate another round in the longstanding competition between detection and concealment rather than end it. SOSUS became strategically important not because it could see everything, but because persistent sensing progressively reduced uncertainty and allowed US forces to exploit data collected on Soviet submarines. UFOSS suggests India may be moving towards the same logic.