Closing the Undersea Surveillance Gap in Southeast Asia

In recent years, the People’s Republic of China (PRC) has intensified its use of gray zone operations in the contested waters of Southeast Asia. These tactics, often conducted beneath the surface by submarines and unmanned underwater vehicles (UUVs), exploit key vulnerabilities in the maritime domain awareness (MDA) of Southeast Asian countries, particularly their limited capacity to detect and track undersea activity. This technological shortfall not only enables covert maneuvers by the People’s Liberation Army Navy (PLAN) but also undermines regional sovereignty and stability. Deploying targeted seabed sensor networks in critical international straits presents a feasible and effective solution for deterring these activities. By closing the undersea gap, Southeast Asian countries could bolster their security posture against gray zone tactics and reinforce their maritime rights for a more stable regional order.

Undersea Sensors as Deterrence

The deployment of undersea sensor systems in Southeast Asia’s strategic waterways could significantly alter the operational functions for the PRC’s submarine and UUV activity. Hydroacoustic arrays, used to detect and track undersea vessels in real time, would dramatically improve situational awareness for regional navies, imposing persistent surveillance that increases both the risk and operational cost to any covert maneuvers by the PLAN. Such heightened monitoring not only deters unauthorized incursions but also improves attribution to enable more credible response to gray zone activities. 

Legally, coastal states are fully within their rights to pursue undersea domain awareness (UDA) solutions in their Exclusive Economic Zones (EEZs) and through international straits in line with the United Nations Convention on the Law of the Sea (UNCLOS). Specifically, UNCLOS affirms the authority of coastal states to monitor and protect their EEZs, as well as to safeguard navigation and ecosystem security in transit points, providing a clear framework for installing and operating seabed sensor networks in these areas.

Several regional and global examples demonstrate the effectiveness of undersea sensor networks in improving maritime domain awareness and deterring submarine threats, providing valuable lessons for Southeast Asia. India, which faces increased PLA submarine activity in the Indian Ocean Region, is deploying an advanced underwater sensor network at strategic areas such as the Andaman and Nicobar Islands, the Bay of Bengal, and the Ninety East Ridge. This network integrates hydroacoustic arrays with satellite data and regional maritime information centers, enabling real-time tracking of PLA submarines and UUVs. The Indian Navy’s initiative, leveraging technologies developed by its Defence Research and Development Organisation (DRDO), aims not only for early detection and tracking but also for data fusion across multiple platforms, which enhances situational awareness and response.

A model of multinational collaboration exists in the U.S.-Japan “Fish Hook Undersea Defense Line,” which employs seabed sensor networks, satellite integration, and extensive data fusion to monitor submarine movement across the Western Pacific. This network exemplifies successful regional cooperation, persistent surveillance, and intelligence sharing that Southeast Asia could emulate to cover the Malacca and Sunda Straits, critical maritime areas vulnerable to covert operations. Furthermore, the U.S. Navy’s legacy undersea surveillance systems, such as the Sound Surveillance System and the Surveillance Towed Array Sensor System, have historically provided unmatched detection capabilities through large hydrophone arrays and towed sonar arrays, offering a benchmark for persistent, wide-area undersea monitoring.

Together, these examples highlight how integrating seabed sensor arrays with satellite and regional data centers develops persistent undersea awareness and data fusion, creating a robust deterrence posture. Southeast Asia can adapt these models to its strategic straits by fostering multinational collaboration on sensor deployment, data sharing, and artificial intelligence-enabled threat classification, thus mitigating the PRC’s gray zone undersea tactics.

The following three-phase framework offers one path to improve UDA in Southeast Asia. States could start with hardware investments or focus first on capacity building and partnerships. While regional cooperation provides broader coverage, individual countries like Indonesia, Vietnam, or the Philippines could unilaterally monitor key straits and EEZs with even limited efforts laying groundwork for future collaboration.

Implementation: Phase One – Sensor Deployment

The initial phase of developing undersea domain awareness in Southeast Asia could consider prioritizing deploying hydroacoustic sensor arrays in critical maritime areas, notably the Malacca Strait, Sunda Strait, and approaches to the South China Sea. These straits are among the world’s busiest and most strategically sensitive waterways, making them vulnerable to covert undersea activities by the PLAN engaged in gray zone operations; these activities include covert undersea maneuvers and persistent monitoring and underwater acoustic surveillance and data collection. Establishing cost-effective, modular hydroacoustic detection grids in these areas leverages passive sonar technologies proven to detect and track undersea vessels through sound propagation in variable underwater environments. These arrays exploit natural acoustic channel characteristics to provide real-time surveillance without requiring extensive infrastructure or continuous power supply, making them a financially viable option for Southeast Asian coalition efforts.

Deploying such sensor networks offers persistent monitoring capabilities essential to deterrence, aligning with existing undersea surveillance practices observed in the Indian Ocean and Pacific regions. By focusing resources on these priority straits, Southeast Asian countries could create a critical surveillance backbone that increases operational risk for adversarial undersea maneuvers and strengthens regional maritime security.

Implementation: Phase Two – Data Integration

The second phase is enhancing Southeast Asia’s undersea domain awareness by integrating data collected from disparate sensor networks into centralized regional MDA hubs. Regional centers like Singapore’s Information Fusion Centre – IOR (ISE) provide an existing collaborative framework for aggregating, processing, and disseminating maritime intelligence from various sensor platforms, including hydroacoustic arrays deployed in strategic areas such as the Malacca Strait, Sunda Strait, and the South China Sea. Feeding sensor outputs into these hubs facilitates real-time situational awareness and supports coordinated responses among Association of Southeast Asian Nations (ASEAN) member states and allied partners.

The integration phase could leverage AI and machine-learning algorithms to handle vast, complex acoustic data efficiently. AI-assisted threat classification can automate detection, identification, and anomaly recognition of submarine and UUV activities beneath dense maritime traffic and acoustic noise. These algorithms improve the accuracy and speed of distinguishing genuine threats from benign signals, reduce human operator burden, and enable predictive threat assessments.

This fusion of regional MDA hubs with AI-enhanced data analytics has the potential to create a robust, multi-layered maritime situational picture. Such integration not only increases detection certainty but also facilitates timely decision-making and cross-border intelligence sharing.

Implementation: Phase Three – Capacity Building

The third phase focuses on developing robust partnerships to build and sustain Southeast Asian countries’ capabilities in undersea domain awareness and maritime security. The United States and Japan play a pivotal role through technology transfer initiatives, providing advanced maritime surveillance platforms, sensor technologies, and operational know-how to regional partners. Japan’s Free and Open Indo-Pacific strategy emphasizes strengthening regional resilience through capacity sharing and joint technological development supporting hydroacoustic sensor deployments and data integration systems. Australia enhances regional preparedness by offering extensive training pipelines that improve ASEAN navies’ anti-submarine warfare (ASW) competencies, seamanship, and maritime intelligence sharing skills. India contributes by organizing joint ASW exercises with Southeast Asian navies, sharing expertise gained from its own sensor networks in the Indian Ocean, thus fostering interoperability and coordinated defense postures.

Complementing bilateral initiatives, as well as annual multilateral exercises such as the U.S.-Philippines Balikatan drills and the Southeast Asia Cooperating and Training (SEACAT) maritime exercises, provide critical platforms for practicing integrated ASW operations, sensor network management, and information sharing protocols. These exercises could enhance interoperability between ASEAN member states and their external partners, improving the ability to respond collectively to undersea gray zone threats and reinforcing deterrence. They also serve as venues for testing emerging technologies and doctrine adaptations relevant to complex underwater environments. Sustainable capacity building through coordinated partnerships and recurrent joint training ensures the persistence and effectiveness of undersea surveillance structures in these strategically sensitive waterways.

Reinforcing Sovereignty Through MDA

Closing the undersea surveillance gap in Southeast Asia’s critical waterways has the potential to deter coercive maritime tactics and reduce the ambiguity that enables gray zone operations. Investing in hydroacoustic sensor networks, integrating data through regional MDA hubs, and applying AI-assisted analytics are promising steps toward more persistent monitoring and transparency. While significant challenges remain, such as cost-sharing, political sensitivities, and technical interoperability, these strategies offer Southeast Asia multiple paths to strengthen sovereignty, safeguard maritime territories, and preserve the freedom of navigation. Exploring such options could help build a more secure and stable maritime environment for the region.

Julia Rocio Gatdula is a former research intern with the Southeast Asia Program at the Center for Strategic and International Studies in Washington, D.C.