An active detection regime: ASW on the noisy future battlefield

The ability to remain quiet has been a cardinal virtue for both submariners and anti-submarine warfare (ASW) operators for most of history. This has stemmed from the fact that active detection poses a considerable risk of counter detection and, thus, destruction by an opponent. Consequently, for most of the Cold War, competitors raced both to quieten their nuclear attack submarines and also to field increasingly quiet anti-submarine warfare frigates. However, in the contemporary operating environment, the focus may be changing.

The emergence of long-endurance uncrewed underwater vehicles (UUVs) has fundamentally changed the dynamics of undersea water as they relate to emissions control. A number of UUVs now have the endurance to operate at sea for extended periods of time. The CETUS UUV being trialled for the Royal Navy, for example, is meant to have an endurance of five days while the Solus-LR has an advertised range of 2,000 km.[1] These platforms have the range and, potentially, the payload to accompany and support submarines, but they are very noisy. UUVs like the US Navy’s Remus 300 typically use brushless motors which likely result in noise levels of around 120 dB.[2] These noise levels would be high, verging on unacceptable, for a modern nuclear-powered attack submarine (SSN).

However, if UUVs are paired with submarines, the multiplication of the number of contacts could pose a challenge for ASW operators. Whilst they might be able to detect contacts more easily, they might find it harder to classify them using passive sensors. If there are multiple contacts, some faint and others loud, this challenge is simplified. However, if contacts are comparably loud, the challenge is complicated considerably. An implication of this would be that submariners have an incentive to become louder to hide within the noise. In effect, the emergence of UUVs on the modern battlefield might mean that older and louder SSNs could receive a lease of life. This would be of particular importance in the Pacific, where Chinese SSNs such as the Type 093 still lag western and Russian counterparts by a significant margin.

Equally, increasing quietness poses a diametrically opposite problem with convergent ramifications for passive sonar. The increasing quietness of modern SSNs means that they will be increasingly difficult to detect and classify through passive means alone (except at exceedingly short distances, where in all likelihood the pursuing frigate may be at greater risk than its quarry).[3] This is especially true of modern Russian submarine classes such as the Project 885 ‘Yasen’ series.

In effect, submariners have two options. Operators of louder submarines, such as a Chinese Type 093, might well opt to employ larger volumes of clutter to mask their signatures, thereby making submarines which might have previously been deemed subpar more usable. Quieter submarines such as a ‘Yasen’ or Virginia (SSN-774) class boat might, by contrast, continue to be employed in ways optimised to evade passive detection. For ASW operators, the challenge of detection is thus compounded with one of classification.

On the other side of the ledger, the trend towards expanding use of uncrewed vehicles cuts both ways. One of the primary reasons that ASW operators avoid the regular use of active sonar is the risk of counter-detection by submarines, which in many cases outrange ASW frigates. In effect, active sonar might simplify the tasks of both detection and classification, but it can only be used intermittently. However, uncrewed systems can act as towed array passive receivers for vessels emitting active sonar (extending an emitting vessel’s range by reducing transmission distances) or as sources of noise that might be used to mask vessel signatures.

Uncrewed vehicles might also be used to carry non-acoustic sensors such as light emitting diodes, which might usefully be paired with sonar in an increasingly cluttered operating environment. Additionally, machine learning could significantly reduce the impact of false positives on active sonar returns, which have been a particular challenge for the employment of low frequency active sonar as an enabler of detection over long ranges.[4] Frigates, too, could thus have an incentive to get noisier.

In a context where the competition between submarines and ASW assets becomes more noisy, a number of principles regarding vessel design and employment might change. In particular, a greater focus on mass and the ability to saturate an area with active sensors, rather than to employ smaller numbers of passive systems, might become an increasingly important feature of ASW.

 Case study: Evolving competitor approaches to ASW

The approaches taken by competitors such as China and Russia, whose ASW operators have always been somewhat disadvantaged in a passive detection regime, might provide a glimpse of what future antisubmarine warfare might look like when all parties have to shift partially from passive detection.

China: Over the past two decades, China’s People’s Liberation Army Navy (PLAN) has dedicated substantial resources toward building a maritime force capable of challenging US and allied naval supremacy across the western Pacific. While progress in many domains has been swift, the undersea domain has remained a comparative weak point. However, that gap is beginning to narrow. Though PLAN submariners and ASW operators still lack the operational experience and finesse of their Western counterparts, the broader force has begun assembling a layered system of ASW tools that might, over time, mitigate this deficiency.

This emerging approach does not depend solely on refined tactics or elite operator skill. Instead, the PLAN is embracing a strategy rooted in sensor saturation and rapid response, particularly within the boundaries of the first island chain. While this evolving system may still struggle with expeditionary operations or to withstand the challenge of stealthy US attack submarines far from China’s coastline, its growing density and reach pose an increasingly formidable obstacle to adversarial sub-surface activity.

The PLAN’s central method for countering the acoustic stealth of adversary submarines relies on widespread use of active sonar systems. Contrary to Western doctrine – where passive listening remains the norm due to concerns about revealing one’s position – the PLAN prioritises active emissions, especially in the constrained environments of the East and South China Seas. By prioritising simplicity and coverage over subtlety, China aims to negate the acoustic advantages held by quieter foreign submarines.[5]

Key to this strategy is the widespread deployment of the Type 056A corvette — an ASW-optimised vessel equipped with variable depth sonar (VDS). With approximately 50 vessels in service, this class forms the bedrock of China’s coastal ASW operations. Unlike Western operators, who might use VDS cautiously, PLAN doctrine encourages aggressive active search, with authors in Chinese defence publications such as Modern Ships explicitly stating the belief that such an approach can deny submarines any real chance of concealment.[6] Complementing the Type 056A’s sonar capabilities is its integration with the Z-9 helicopter, which – despite its limited payload – provides an aerial vector for sonobuoy deployment and threat detection. Where the Z-9 falls short in payload, the Yu-8 missile (a vertically launched rocket-assisted torpedo) extends the corvette’s reach, enabling standoff engagement of contacts without requiring the helicopter to carry its own torpedoes.

In addition to conventional surface combatants, the PLAN is turning to both manned and unmanned undersea vehicles to broaden its sensor web. While older platforms like the Type 035G submarine contribute little in high-intensity conflict, it has been suggested that they may serve as active emitters employed either for detection or forcing engagements that expose enemy submarine positions.[7] Unmanned underwater vehicles (UUVs) such as the HSU001, which boasts long endurance and modular sensor payloads, are being developed for this purpose.

Russia: Another example of the shift to a likely active detection regime is Russia’s Harmony network, an analogue to the United States’ SOSUS and IUSS. While the details of Harmony are not publicly known, the network is understood to be partially powered by undersea nuclear reactors. There are several rationales for such an approach but one potential explanation is that Russia has opted to rely more heavily on low frequency active sensors for detection, which would align with stated Russian concerns regarding its ability to detect quiet SSN. A shift to an active detection regime would necessarily be power consuming, which might explain the employment of ATGU atomic generators.

In effect Harmony would represent a variant of the reliable acoustic path systems which employ a large number of distributed sensors (as contrasted with the large, highly sensitive fixed arrays of western networks such as SOSUS). The advantage of these systems has been a lower requirement for high sensor fidelity but they have been challenged by their relatively limited range. This can be resolved with low frequency active sonar but given the power consumption of sonar (detection of a submarine type target at 10 km absorbs half a million watts) this has historically been challenging. However fixed arrays powered by underwater power stations largely circumvent this challenge and may become an increasingly prominent feature of maritime combat.

While both of these Russian and Chinese solutions are primarily related to the issue of quietness rather than a noisy battlefield – both competitors faced challenges in their ability to track western submarines – the set of solutions arrived at may provide a glimpse of what future competition below the ocean surface might look like.

Conclusion

The observations made in this article are not a suggestion that quietness and stealth will lose their value on future battlefields. Rather, the argument is that – in much the same way as combat in the air domain may increasingly come to be conducted by ‘high/low mixes’ of capability – the inclusion of large numbers of relatively noisy UUVs in the future operating environment may have the effect of reviving the fortunes of older and nosier classes of submarine. This in turn will result in a growing premium being placed on the ability to saturate an area with active sensors, as well as non-acoustic methods of detection. This will impact both how ASW assets are designed and how they are employed.

Much as the emergence of low cost strike munitions like the HESA Shahed has not removed the threat of capabilities such as cruise missiles, new undersea platforms and increasingly usable older submarines may not end the role of the quiet SSN. However, they will make it one part of a more complex battlefield.

Dr Sidharth Kaushal

 

Author: Dr Sidharth Kaushal is a Senior Research Fellow at the military sciences team within the Royal United Services Institute (RUSI). His specialisms include sea power and integrated air and missile defence.

 

[1] UK’s CETUS XLUUV Launched by MSUBS in Portsmouth. Naval News. https://www.navalnews.com/naval-news/2025/03/uks-cetus-xluuv-launched-by-msubs-in-plymouth/; Cellula Robotics. Solus-LR Autonomous Underwater Vehicle. https://www.google.com/search?q=solus+LR&rlz=1C1GCEB_enGB1099GB1099&oq=solus+LR&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTINCAEQABiGAxiABBiKBTINCAIQABiGAxiABBiKBTINCAMQABiGAxiABBiKBTIHCAQQABjvBTIKCAUQABiiBBiJBTIKCAYQABiiBBiJBdIBCDE2OThqMGo0qAIAsAIA&sourceid=chrome&ie=UTF-8

[2] Author calculation based on the known power of the REMUS 300s motos, an assumed acoustic efficiency of 10^7 and an assumed quieting effect of 20 db – the latter two being relatively generous assumptions

[3] Author interview with Officer who served on several European ASW Frigates

[4] Matthias Buß. False Alarm Reduction for Active Sonars using Deep Learning Architectures. https://cdn.asp.events/CLIENT_Clarion__96F66098_5056_B733_492B7F3A0E159DC7/sites/udt-2024/media/libraries/platform-design/4—Matthias-Buss–Slides.pdf

[5] Sidharth Kaushal. Chinese Antisubmarine Warfare: A Blunt but Evolving Tool. RUSI Defence Systems. 25 July 2022. https://www.rusi.org/explore-our-research/publications/rusi-defence-systems/chinese-anti-submarine-warfare-capabilities-blunt-evolving-tool

[6] Lyle Goldstein. China Is Improving its anti-Submarine Warfare Capabilities. The National Interest. https://nationalinterest.org/blog/reboot/china-improving-its-anti-submarine-warfare-capabilities-197071

[7] Sidharth Kaushal. Chinese Antisubmarine Warfare: A Blunt but Evolving Tool.