The threat posed to vessels at sea by anti-ship missiles (ASMs) is not a new one. Since the Israeli destroyer Eilat was sunk by a Soviet made P-15 Termit (NATO: SS-N-2 Styx) missile launched from an Egyptian Komar class patrol vessel during the Six-Day War, the potential for missiles to hold large surface vessels at risk was understood by navies around the world.
The lesson was reinforced by conflicts such as the Falklands War, which saw the Argentinian armed forces use the Exocet missile to good effect against the Royal Navy, as well as Western late Cold War naval planning which would have seen Western maritime power applied to force its way into Soviet bastions defended by layers of cruise missile equipped platforms. Equally, surface combatants and tactical formations developed a range of countermeasures against missile threats, from the interception of launch platforms to the fielding of capable shipboard air and missile defence systems such as the US Navy’s Aegis system.
The missile threat to surface combatants has evolved in recent decades in several ways. First, Anti-Ship Cruise Missiles (ASCMs) have proliferated widely, with even non-state actors like the Houthis and Hezbollah fielding capabilities like the Chinese made C-801 and C-802 as well as, in the latter case, the supersonic Russian Yakhont ASCM. Secondly, ASCMs have been joined by Anti-Ship Ballistic Missiles (ASBMs) such as the Chinese DF-21D, DF-26 and YJ-21. Missiles like the DF-21D are capable of operating at theatre ranges of up to 2000 km, carrying payloads capable of inflicting ‘mission kills’ or even outright sinking aircraft carriers. They can be operated from ashore in positions that may be relatively difficult to identify, and, by virtue of their ballistic trajectories, present maritime air and missile defences with an additional challenge.
Finally, the missile threat to vessels will likely be exacerbated by the fielding of hypersonic missiles both in the form of Hypersonic Glide Vehicles (HGVs) such as the Chinese DF-17 and Hypersonic Cruise Missiles (HCMs) such as the Russian 3M22 Zircon. Hypersonic weapons, which can travel at speeds over Mach 5, combine the characteristics of high speed and manoeuvrability, imposing substantial strains on the reaction times of shipboard air and missile defence systems. Moreover, they could in principle impart particularly high levels of kinetic energy on impact, which is noteworthy because kinetic energy, more than warhead size is the best predictor of lethality against large surface combatants.
Despite qualitative changes in the range, speed and manoeuvrability of the ASMs that can target surface combatants, however, it would be a mistake to assume that large surface combatants are white elephants. The complexity of the kill chains that enable missiles to be used over long ranges and the physical constraints of missiles themselves can curtail their effectiveness. Moreover, existing and emergent shipboard air defences – both active ‘hard kill’ defences and ‘soft kill’ methods – mean that large surface combatants will enjoy significant protection. This being said, however, there is an undeniable cost asymmetry between missiles and the vessels that they target, which means that an attacker can sustain more failures than a defender can sustain successful attacks. When high value naval platforms are deployed in contested environments, it will be on the assumption that they may be lost. For shrinking Western navies this can represent a substantial challenge, but in many ways it is one that mirrors a perennial lesson of naval combat – that fleet size and the capacity to sustain losses matter.
The Missile Threat
A number of factors work to constrain the operational effectiveness of missiles against moving targets, especially at very long ranges of over 1000 km (the ranges at which most ASBMs and HGVs would be used). Given the expanse of the areas in which a vessel or group of vessels such as a carrier task group might be at these ranges, it is difficult to maintain surveillance over them using assets with narrow fields of view. Even Optoelectronic and Synthetic Aperture Radar (SAR) -equipped satellites cannot, by themselves, provide sufficient levels of persistent ISR over a given area to enable the tracking and engagement of a target. They have substantial swath widths, of up to 100 km, but in a theatre such as the western Pacific or the North Atlantic, they would be surveying even larger expanses with a requirement for rapid revisit over an area. To use an example, it has been estimated that the revisit rate of China’s Yaogan satellite constellation of Optoelectronic and SAR satellites was around 2.9 days in 2017.
In order to mitigate this challenge, states need to rely on assets that can survey a wider area, albeit with lower resolution. This can include Naval Surveillance satellites which the USSR used to cue in its Legenda radar-equipped satellites. It could also include Over the Horizon – Surface Wave (OTH-SW) and Over the Horizon – Backscatter (OTH-B; also known as ‘Skywave’) radar. China operates both types, known examples include an OTH-SW on the coast at Ruian, Zhejiang province, and an OTH-B at Xiangyang, Hubei Province. The challenge faced by these capabilities is one of resolution – OTH-B radars, for example, face a substantial challenge when surveying oceans due to the effects of oceanic backscatter. Moreover, they have an error radius of around 40-170 km due to the very large size of the radar’s resolution cell. Similarly, ELINT satellites are potentially vulnerable to spoofing, as was practiced by the US navy during Operation Haystack. Moreover, relatively few nations maintain large constellations of Optical Surveillance and ELINT satellites. Russia, for example, has struggled to field its Liana and Persona satellites, which will substantially impact its ability to cue long range missiles like the KH-47M2 at their maximum ranges. While nations can rely on other sources of information, such as maritime patrol aircraft, this will come at a cost in capacity – drawing these assets from other missions.
There are additional challenges faced by countries seeking to cue ASBMs and hypersonic weapons at very long ranges. Bottlenecks, both technical and organizational, can limit the speed at which data can be passed from early warning assets to those that can provide targeting and finally to the platform launching a missile. These bottlenecks can be further exacerbated by adversary disruption in the form of either kinetic or electronic warfare against space-based assets, cyberattacks and the destruction of ground-based command and control nodes. None of this is to suggest that these problems are absolutely irresolvable, and countries can also receive intermittent targeting data from other sources. China, for example, could conceivably use longer-range UAVs like the GJ-11 for reconnaissance roles. States can even rely on non-traditional data sources. For example, the PLAN appear to be equipping fishermen with satellite communications and furnishing them with the basic training needed to identify targets of interest. Finally, the area covered by the seeker of a ballistic missile warhead dropping near vertically from altitude is substantial. Chinese estimates suggest that the ‘kill radius’ (here meaning the distance a target can deviate from its position and still likely be hit) of a terminally-guided ASBM such as DF-21D is around 20 km at a conservative estimate. This means that ballistic missiles, especially if launched in large numbers, can cope with a certain degree of imprecision in initial targeting data, though it would still take an especially large salvo to operate on the basis of ELINT or data provided from an OTH radar alone. It is nonetheless the case, however, that the threat posed by ASBMs to vessels will be somewhat attenuated by the complexities of the associated kill chain.
ASCMs are somewhat easier to cue, as their sources of data are on board the launch platform. For example, most surface vessels would track targets using their own radar, such as the Chinese Type 364B AESA radar on China’s Type 055 cruiser. This is not always the case, however. For example, cruise missile-armed nuclear submarines (SSGNs) can typically gather information on target range and bearing from their organic acoustic sensors, but rely on offboard sources for more granular targeting. For example, during the Cold War, Soviet Oscar Class submarines would have received data from satellites. This does pose certain challenges, such as high data latency and the fact that methods of communication such as a trailing wire can be spotted by Anti-Submarine Warfare (ASW) assets such as maritime patrol aircraft. Similarly, ground-based coastal missile systems like the Russian Bastion-P rely on a combination of organic Monolit-B radar and Ka-32 helicopters to triangulate target locations, and the loss of the latter could restrict the range at which a battery could accurately classify a target. The challenge of poor granularity regarding target location can be partially mitigated through the use of large salvos of cruise missiles to cover a search area with seekers. For example, the P-700 Granit fired by the Oscar were programmed to swarm – with one cruise missile rising to a high altitude to conduct a sweep and pass data to lower-flying missiles.
There is a second question regarding kill chains, namely that of coordinating assets from across multiple platforms. The gold standard of missile attacks is what Wayne Hughes described as simultaneous salvos, as opposed to dispersed salvos (the occasional single missile) or sequential salvos (grouped salvos in sequence). Many contemporary authors, including many Chinese ones, describe similar approaches. For example, one Chinese article in a journal associated with the PLA posited that six cruise missiles flying on different trajectories and one ASBM would be needed to overwhelm an Aegis destroyer. However, this represents a substantial coordination challenge given that these assets may be held on different platforms and indeed under different services. Notably, this challenge could be partially resolved by ever larger vessels. The Chinese Type 055 cruiser which can launch a YJ-21 ASBM from its large cold launch capable Vertical Launch System (VLS) cells (which have cells with a 60% greater volume than those of a US Ticonderoga class) could in principle coordinate such a launch independently. However, in most cases, coordinating salvos will remain a challenge.
Beyond the kill chains needed to sustain them, there are other more physical limitations faced by missiles. For example, hypersonic bodies travelling through the atmosphere form a plasma ‘sheath’ around the body which has the downside of effectively precluding offboard communication, but also has the benefit of making detection via radar much more difficult by lowering the effective Radar Cross Section (RCS) of the body. This plasma sheath effect also means that in order to operate their seekers they need to slow to speeds well short of their top speeds in their terminal phase. Hypersonic cruise missiles face an additional challenge – in order to compress air for their scramjet engines, they need to fly at altitudes of over 20 km, meaning that they cruise relatively high and cannot sea skim until late in their trajectories. The plasma sheath effect is also a challenge for the DF-21D which has to perform a ‘pull up’ manoeuvre in terminal phase to reduce its speed, identify a target, and then steer itself towards the target. The difficulty of doing this accurately likely increases the odds of missing, and may also provide a window for interception.
Defences and Countermeasures
The second consideration when discussing the vulnerabilities of vessels is their defences and countermeasures against incoming missiles, both those currently fielded and those which can reach maturity. Modern air defence vessels, such as the Aegis-equipped destroyers (which form the lynchpin of a US carrier strike group’s air defence picket), or comparable contemporaries like the UK’s Type 45 destroyer possess credible air defence suites. Most modern air defence platforms are adopting active electronically scanned array (AESA) radar which can emit beams on multiple frequencies simultaneously, reducing inaccuracies caused by factors such as interference between systems and enabling the simultaneous tracking of a large number of targets. Moreover, battle management systems such as Aegis can enable engagement windows (the process between tracking and engagement) to be closed at pace – especially when they are operating in fully autonomous mode. Layered air defences aboard vessels typically include ballistic missile defences such as the SM-3, long-range assets such as the SM-2 or Aster-30 (which would typically be used against air breathing threats) and shorter-range interceptors such as the ESSM or Aster-15. Some interceptors, like the US Navy’s SM-6 can be used both for area defence against air-breathing threats, and as point defence against ballistic threats. As such, overwhelming these systems, especially with cruise missiles, represents a challenge and so far there are no cases of an alert vessel with credible hard and soft kill capabilities being sunk by an ASCM. However, against less than alert vessels, or when crews are taken by surprise, the chances of missiles leaking through the defences are greater. Notable examples include the sinking of the HMS Sheffield and the Moskva, as well as damage to the INS Hanit in the 2006 war in Lebanon. Surprise may also depend on situational awareness regarding the proximity of launch platforms. For example, SSGNs, which can potentially surprise targets at relatively close ranges, can surprise their prey if launching from closer ranges, where they do not require offboard cueing.
Two factors can complicate air and missile defence against cruise missiles. The first is the increasing speed of incoming targets, such as Russia’s 3M22 Zircon hypersonic cruise missile. However, as previously noted, the nature of scramjets may make it necessary for these missiles to operate on a higher altitude trajectory – meaning that while they are faster than a slow sea skimming missile, they are also spotted earlier by a ships radar or electronic support measures (ESM). Moreover, both HGVs and HCMs likely have to slow substantially in their terminal phase in order to use their seekers- meaning that they are potentially vulnerable to interception in the final stages of their trajectories.
A more substantial challenge for ship self-defence may be a lack of VLS capacity – especially if VLS cells have to be divided between offensive strike capabilities and defensive interceptors. Adversaries can exacerbate the challenge through the use of UAVs as decoys, or dedicated decoys such as the US’ ADM-160 Miniature Air-Launched Decoy (MALD). The challenge of the cost curve can be partially solved by more effective close-in defences, particularly those involving Directed Energy Weapons (DEW). With their large powerplants, ships are a relatively easy platform onto which to incorporate DEWs, and though their effectiveness may be partially influenced by climatic factors, they could help shift cost curves in the defenders direction. Ballistic missiles can be intercepted during the midcourse, much like land attack missiles. In terminal phase, the pull up manoeuvres they perform to slow down and activate their seekers may make tracking and generating a fire control solution against them more difficult, but will also provide defenders with additional reaction time. Countermeasures and decoys, as well as other penetration aids can be used to confound a defender. However, in many ways the challenge is a fairly traditional BMD task.
Beyond hard-kill measures, vessels can also rely on soft kill methods such as dispensing chaff or decoys, and electronic jamming to confound missiles. The Israeli raid on Latakia during the 1973 Yom Kippur war illustrated how outranged Naval vessels equipped with capable electronic warfare suites could nonetheless outmanoeuvre their opponents. A vessel does however need a directional track on a missile’s seeker in order to jam its signal, which makes this a relative last resort against missiles. Decoys by contrast have shown their effectiveness, with the Nulka Digital Radio Frequency Memory (DRFM) active decoys being used to good effect during the Houthi attack on the USS Mason in the Bab al Mandeb strait. DRFM decoys effectively detect a missile’s active radar seeker frequency and then radiate it back to the missile. They can be launched to safe distances away from the vessel by rocket propulsion. To be sure, solutions do exist for attackers. Millimetric wave seekers, for example, are far less easy to defect and reflect back to their source using decoys – but this comes at a cost in the seeker’s target detection range. Dual-mode seekers combining, for example, active radar and infra-red (IR) can also represent a potential adaptation – albeit one that increases, cost, complexity, size, and forces design trade-offs.
Conclusions – Neither Invulnerable Nor Sitting Ducks
Ultimately, neither hard nor soft kill methods can provide guaranteed protection for naval platforms – vessels are at risk in an age of proliferating missile threats. However, they are far from sitting ducks. The complexity of cuing large salvos of missiles, especially when dealing with particularly long range missiles, as well as the capabilities of layered shipboard air and missile defences, make sinking a vessel a resource-consuming task for the attacker. Moreover, launch platforms such as ships, submarines and aircraft are themselves vulnerable to interdiction left of launch.
That said, the cost of protecting high-value vessels in terms of the dense layered air defences they require, and the disproportionate costs of losses to fragile and shrinking maritime force structures do mean that the political constraints on deploying vessels may grow. In many ways, however, this is a reflection of shrinking force structures which make navies more loss-averse, and not the fact that missiles, though lethal, have necessarily upended the balance between offence and defence at sea.
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