Airborne ASW – Past and Future
Airborne assets have long been a critical component of Anti-Submarine warfare. Capabilities such as maritime patrol aircraft have had a central role in hunting subsurface threats since the second world war, when units such as the RAF’s 461 squadron worked to contain U-Boats in the Bay of Biscay.
During the Cold War, NATO campaign plans to contain Soviet submarines at chokepoints such as the GIUK gap involved the coordinated use of air and sea based assets. The airborne component of this effort included maritime patrol aircraft laying sonobuoy fields and shipborne helicopters equipped with dipping sonar.
The centrality of air-based assets to anti-submarine warfare (ASW) remains, but a number of factors may change its modalities. This will be of particular salience to Western navies, which lack the mass to operate the numbers of ASW barriers that they envisioned during the Cold War. However, it will also be driven by other factors. The nature of the challenge faced by ASW forces will also evolve in important ways. A tendency towards equipping submarines with long range precision strike capabilities- with examples including the Russian Yasen class SSGN and the Chinese Type 093- means that to perform certain missions submarines may not need to transit chokepoints. The use of Unmanned Underwater Vehicles (UUVs) – which will pose a challenge for both acoustic and non-acoustic detection – may also challenge existing approaches to airborne ASW. Finally, we might consider climate change, for example the heating of the world’s oceans and changes to salinity, both of which impact the passage of sound through water.
Equally, technological change – both in areas such as the application of autonomous capabilities to war and emerging capabilities such as quantum magnetometry, but also more evolutionary developments in multistatic sonar – will also have a substantial impact on how airborne ASW is conducted. The net effect of adaptations that may emerge in the area of airborne ASW, as well as ASW more broadly, may be to shift countries’ focus from the control of areas such as chokepoints, to forward defence.
The Current State of Play
Presently, airborne ASW is based around three layers which developed during the Cold War. First, air and space based capabilities including Electronic Intelligence (ELINT) satellites and aircraft provide early warning regarding submarines’ movements. Other sources of data include tripwire networks of hydrophones such as the US SOSUS, as well as ships equipped with long-range, low-frequency active sonar such as the US Navy’s T-AGOS ships.
Based on early warning regarding a submarine’s potential routes of transit, maritime patrol aircraft such as the P-8A Poseidon are tasked to form a second layer of defence. These aircraft can lay sonobuoy fields across a submarine’s likely route of travel. In the 1970s and 1980s, the US navy innovated the use of the AN/SSQ-77 directional frequency analysis and recording (DIFAR) sonobuoys, alongside the AN/SSQ-62 directional command activated sonobuoy system (DICASS). The former, a passive system, shaded its array to form beams in the horizontal direction and thus increase transmission gains. DICASS systems such as the AN/SSQ-62 are active systems, which emit omnidirectionally in azimuth and directionally in elevation. The purpose of directionality was to reduce the effects of ambient noise, an adaptation needed to cope with increasingly quiet Soviet submarines. Today, both DIFAR and DICASS sonobuoys such as the AN/SSQ-62 are still operated by both the US and allied forces such as Australia’s RAAF. Other nations such as Russia operate a similar mix of sonobuoys on their maritime patrol aircraft such as the Il-38N and the Tu-142.
Russian MPAs carry the RGB-57, RGB-75, RGB-15 and RGB-55 sonobuoys which are, respectively, passive omnidirectional sonobuoys, a Low-Frequency Array (LOFAR) sonobuoy, and a directional active sonobuoy. This is likely to change, however, in part due to the systems’ limited range of 5.55 km (3 NM), and the fact that their sensitivity is impacted by factors such as reverberation, which will be a particular challenge in littoral spaces and when dealing with increasingly quiet submarines more broadly. As a result, we should expect to see a growing emphasis on multistatic sonobuoys, a subject that will be discussed in more depth in subsequent sections.
Maritime patrol aircraft (MPA) can also use other sensors to track targets. Modern MPA such as the P-8 are typically equipped with Magnetic Anomaly Detectors (MADs), which use magnetometers to detect distortions in the Earth’s magnetic fields caused by the presence of a submarine. As large ferromagnetic objects (such as submarines) pass through the Earth’s magnetic field, they cause distortions in these fields around them, producing anomalies. Other sensors include radar, such as the AN/APS-154, and forward located infrared (IR) sensors which can detect features such as periscopes and masts. MPA are typically armed with both torpedoes and depth charges to engage targets.
Beyond MPA, the other major component of airborne ASW consists of helicopters. ASW helicopters are typically equipped with a dipping sonar, and helicopters such as the Royal Navy’s Merlin MK II can carry up to 30 sonobuoys as well. They can engage targets directly using torpedoes, such as the Sting Ray torpedo carried on the Merlin. The major limitations of helicopters are their endurance and their capacity to coordinate large numbers of offboard data sources. They are thus typically used as a point defence asset around critical platforms such as aircraft carriers, or as a final means of prosecuting ASW against targets cued by an external source such as an MPA.
The Evolution of Airborne ASW: Beyond Barriers
A number of factors are likely to combine to incentivise the evolution of airborne ASW. Key drivers are likely to be (among others) resource tradeoffs, evolutions to both the capabilities and Concept of Operations (CONOPS) of submarines being hunted, and technological shifts in areas such as non-acoustic submarine detection and the use of autonomous capabilities. Cumulatively, these shifts will incentivise a more forward-leaning approach to ASW than has previously been the case, and one that relies more heavily on surface and subsurface platforms. In this context, one might expect the role of manned airborne ASW assets to evolve into one that emphasises command and control rather than the laying of sensor fields or direct target engagement.
First, the subsurface threat is likely to evolve in a number of ways. Long range prompt strike capabilities are increasingly ubiquitous on modern SSNs. The US Virginia class Block V will carry a payload module that will enable it to launch, among other things, the US Navy’s Conventional Prompt Strike missile, which has a range of approximately 2,736 km (1700 miles). Similarly, the Russian Yasen class submarine can carry a range of missiles, from the Kalibr to the 3M22 Zircon. To be sure, there is nothing new about SSGNs, which have existed for some time. What is new is that the distinction between attack submarines and SSGNs is increasingly blurred, with most nuclear powered boats and many diesel electric boats such as the Chinese Yuan class also having an ASCM launch capability. What this means is that submarines may in many instances need not transit chokepoints in order to prosecute strike missions against targets both at sea and ashore. Moreover, both deep water threats and those found in littoral spaces are becoming increasingly quiet, which poses a challenge for passive detection. In littoral spaces this will be driven by the emergence of relatively quiet long endurance diesel electric submarines such as the Chinese Yuan class, which is equipped with air-independent propulsion. The environment within which submarines are likely to operate will also change in coming years. For example, in the high North, melting permafrost will impact both the temperature and salinity of water, thereby impacting sound transmission.
This poses a major challenge to airborne ASW. Maintaining barriers on station may not be viable, while operating assets such as maritime patrol aircraft close to the well-protected bastions from which submarines launch precision strike capabilities will expose them to both ground and sea based surface to air missile (SAM) systems. There is an additional challenge, maintaining and reseeding sonobuoy fields represents a capacity challenge, especially when tracking submarines at scale. During the Cold War, large numbers of SSNs and maritime patrol aircraft were needed to maintain multiple barriers across the likely routes of egress that submarines might take. Tasking MPA to track individual SSNs to cue assets such as a friendly SSN to trail a target involves multiple assets being engaged to provide information on any likely object of interest. This requires a resource commitment that is feasible in peacetime but may strain states’ wartime capacity. This is not an exclusively Western conundrum, with opponents such as Russia facing significant shortfalls in MPA capacity.
One way in which ASW forces might seek to shift the balance of cost in their favour is through the use of unmanned platforms. Unmanned Surface Vehicles (USVs) equipped with active and passive sensor payloads could, in principle, offer more enduring coverage of an area than a sonobuoy. Experimentation with the control of USVs using helicopters has been conducted using US Navy T24 and T38 USVs being tasked using Seahawk helicopters. In a similar vein, aircraft like the P-8 could be leveraged as control platforms for USVs and UUVs (the latter likely needing to communicate by coming close to the surface intermittently). Secondly, Unmanned Aerial Vehicles (UAVs) – which are currently being experimented with for ASW roles – could be used to expand the endurance of fixed wing ASW assets, thereby reducing the burden on manned MPA. Unmanned assets, if sufficiently cheap, could also be used to harry submarines in highly contested areas. Aerial platforms, which can communicate over the horizon more easily than surface assets, might make logical command platforms for these assets. The premise behind the proposal to use UAVs or USVs is that advances in areas such as machine learning will make it viable to layer historical data with sensor input to detect submarine tonals on the basis of limited acoustic data. Other suggestions include the use of UAVs and USVs in a multistatic array, with expendable assets being used as active emitters and more expensive ones acting as passive receivers. While promising, such forward-looking CONOPS are still nascent.
Secondly, new non-acoustic methods of detection can help offset the impact of quieting submarines. For example, the Peoples Liberation Army Navy (PLAN) has trialled the use of a Superconducting Quantum Interference Device (SQUID), capable of measuring extremely subtle changes in magnetic fields. The capability, if realised, would lead to a step change in the range of magnetic anomaly detectors, with a SQUID having ranges of up to 8 km – well in excess of what a contemporary MAD can accomplish.
Thirdly, we might expect to see more evolutionary changes, many of which are already occurring. For example, the US Navy is shifting from DIFAR and DICASS sonobuoys to the use of the AN/SSQ-125 and the AN/SSQ-101 sonobuoys which form the active and passive components of a multistatic sonobuoy system. Multistatic sonobuoys mitigate some of the effects of sound reverberation, and thus extend the range of a given sonobuoy to approximately 8 km, up from the 2-3 km that might be expected using existing bistatic and monostatic systems.
Finally, remotely activated mines such as the US Navy’s Hammerhead can be deployed from the air to act as active components of an anti-submarine barrier.
Ultimately, the future of airborne ASW holds out both challenges and opportunities for its practitioners. A combination of increasingly quiet and long endurance threats and the fact that many threats will operate within contested bastions may pose a challenge to existing approaches. This will be compounded by the issue of diminishing mass, which means that even as capabilities of platforms increase, the capacity available to Western navies may diminish. In this context, purely reactive approaches to ASW will become ever more difficult. A number of evolutions in the area of ASW, particularly those leveraging unmanned assets and longer-range modes of non-acoustic detection, could lead to a renaissance of more forward-leaning approaches to ASW. In this context, airborne ASW assets would retain many of their core functions, but would increasingly act as controllers for other assets, including surface- and subsurface-based unmanned vessels.
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