Warding Off New Threats – Maritime counter UAV and USV technologies

Unmanned aerial vehicles (UAVs) and unmanned surface vehicles (USVs) have emerged as a potent new threat to military ships. This requires a re-evaluation of ship-defence technology and tactics. 

For decades anti-ship missiles (ASMs) have arguably posed the dominant threat to surface vessels. However, a new, asymmetrical threat emerged in October 2000 when a small, explosive-laden boat attacked the destroyer USS Cole (DDG-67) in the port of Aden, killing 17 crew members and severely damaging the ship. While the Cole attack was perpetrated by suicidal terrorists, the same tactic can now be carried out by remotely-controlled or even autonomous unmanned boats laden with explosives or carrying anti-ship or anti-tank missiles. UAVs – including so-called loitering munitions that can transition from a de facto unmanned aircraft to an attack missile – have also provided less-sophisticated combatants a viable tool for holding even highly advanced warships at risk. They are low cost and easy to acquire or produce, and can be launched in mass in an effort to overwhelm defences. Both UAVs and USVs are small, fast and manoeuvrable, making them difficult to detect or target. Even if UAVs are unlikely to sink a destroyer, one or more could render the ship ‘hors de combat’ by damaging or destroying sensor and communications arrays.

The ongoing standoff between western naval forces and Houthi rebels in the Red Sea illustrates these developments. The Houthis have deployed a broad-based arsenal including anti-ship cruise missiles, UAVs, and explosive-laden USVs to attack both civilian and military shipping. Swarm attacks have included as many as 28 UAVs at one time, requiring the combined efforts of several warships to stop the incoming threat. While the attacks have, to date, failed to penetrate the defences of the military vessels, they have damaged several commercial ships. These include the Liberian-flagged bulk carrier MV Tutor, which sank after being struck by a USV (and later, probably, by an ASM) on 12 June 2024. In principle, warships remain vulnerable to such attacks, as demonstrated by a January 2017 Houthi strike on a Saudi frigate south of the Bab el-Mandeb Strait. While the damaged vessel was able to return to port under its own power, two crewmembers were killed.

 Two multinational missions – the US-led Operation Prosperity Guardian and the EU-led Operation Aspides – have been mounted to secure shipping through the Bab el-Mandeb Straits and the Red Sea. The coalition forces have been forced to adapt their tactics to prevent being overwhelmed by the volume of attacks. Concepts of operation are being revised and optimised. In a way, the Red Sea crisis provides an opportunity to learn and prepare for future unmanned campaigns by significantly larger and more sophisticated state opponents, such as Iran or China. It has become clear that UAV and USV based campaigns will require a combined arms approach utilising a variety of weapon systems, including some which are still in development.

 Kinetic weapons

The coalition ships’ initial response was to deploy precision guided munitions not only against the Houthi’s anti-ship missiles but against all incoming threats. It soon became obvious that this is not an optimal strategy. To begin with, utilising high-performance missiles designed to intercept manned aircraft, cruise missiles and/or ballistic missiles at long range are pure ‘overkill’ when deployed against small to medium sized UAVs. The cost-ratio calculation of using SM-2 missiles costing USD 1.2 million each against a USD 5,000 drone is justifiable as a short term measure to protect a ship, but is not sustainable as a long-term strategy. Finally, shipboard missile arsenals are finite. A determined enemy can continue to launch unmanned systems until the arsenal of defensive missiles is depleted, forcing the ships to withdraw. Worse, the UAV can be deployed in a calculated strategy to deplete the defensive arsenal, leaving the vessel vulnerable to attack by ASMs or (in the case of a state actor) manned aircraft.

In the Red Sea, coalition vessels quickly began using their deck guns against both airborne and surface targets including both UAVs and USVs. Rapid fire guns such as the OTO 76/62 Super Rapid can achieve a rate of fire of 120 rounds per minute. Depending on munition choice, targets can be engaged at up to 40,000 metres distance. The combination of radar and optical precision guidance on the one hand and high-explosive pre-fragmented munitions on the other can place multiple rounds very near a manoeuvring target at considerable distance from the ship. Northrop Grumman is developing a new, manoeuvring 57 mm artillery round designed specifically for the Mk 110 gun mounting used on US Navy warships. Aided by an on-board seeker, the guided high explosive round will continue to autonomously adjust its flight path as it approaches the target, then self-select between proximity detonation or point detonation mode in order to maximise the odds of target destruction. It is designed specifically to defend against fast-moving surface threats, drones, and swarming threats. If surface or aerial threats evade this fire, they can be engaged by close-in weapon systems such as the 20 mm Phalanx Gatling gun and/or heavy machine guns.

Electronic warfare

Shipboard electronic warfare (EW) systems can also be deployed against both UAVs and USVs in order to disrupt radio-control frequencies or satellite navigation. This has proven effective in the short-term in other current drone-intensive conflicts, such as the Ukraine war. However, whether on land or at sea, attackers can utilise various techniques – such as frequency hopping or inertial and optical guidance systems – to overcome EW. As such, it is not a definitive solution.

Directed energy weapons 

Directed energy weapons (DEWs) have increasingly been considered the potential optimal solution to shipboard defence against unmanned aircraft, as well as small fast-attack craft, both manned and unmanned. The primary categories of DE systems are lasers and microwave weapons.

 US Navy laser programmes: Numerous nations are pursuing high energy laser (HEL) weapon technology for their naval fleets. The US Navy has one of the oldest and largest research and development programmes, although progress has lagged behind original estimates. The maritime environment is particularly challenging for laser weapons as the moist and salt-laden air can interfere with beam cohesion, exacerbating the general difficulty of maintaining beam contact on one spot of a manoeuvring target.

Several different laser systems are currently being evaluated under the umbrella term Navy Laser Family of Systems (NLFoS). The most promising for near-term applications against UAVs and USVs are the Optical Dazzling Interdictor, Navy (ODIN) and the High-Energy Laser with Integrated Optical Dazzler and Surveillance (HELIOS). To date the US Navy has received seven ODIN and one HELIOS system; they are currently installed on warships for evaluation.

ODIN is a purely non-destructive system designed to neutralise UAVs by disabling their optical sensors. The goal is not to destroy the unmanned aircraft but to prevent is from observing vessels and gathering intelligence which could be used, among other purposes, for targeting. In the same vein, disabling its sensors could prevent an explosive laden USV from acquiring a targeting fix on a ship, but additional kinetic measures would still be necessary to destroy the boat and completely eliminate the threat. This underscores ODIN’s major drawback; first conceived before armed UAVs and USVs were considered a serious threat, the system is not designed to repel attacks.

HELIOS, by contrast, is a dual-capable system. Its optical dazzler can blind a drone’s sensors to prevent targeting, but its 60 kW high-energy laser is also capable of physically destroying a UAV or USV. This would be accomplished by directing the beam against a key element, such as the engine or a hull section, until it catches fire. HELIOS is currently being evaluated aboard the destroyer USS Preble (DDG-88). According to Navy Secretary Carlos Del Toro, the testing has made good progress. “We are at a point now, where…we are a bit beyond the experimentation point,” Del Toro said at the Surface Navy Association (SNA) symposium in January 2024. “We will be continuing to do experiments [but] over the course of the next year, even less, that will fully flush out how we can employ this incredibly transformative system.”

Despite this, US Navy leadership is not entirely satisfied with the state of affairs. “I am not content with the pace of directed energy weapons. We must deliver on this promise that this technology gives us,” said Vice Admiral Brendan McLane, head of US Naval Surface Forces, during the same SNA symposium. He is pressing for accelerated fielding of both lasers and high-powered microwave weapons throughout the fleet. Developing these capabilities will be a major effort for the surface force, he said. “I really want to put a lot of effort into accelerating that, because that that gives you so much [capability] when it comes to magazine capacity and speed and distance [for target engagement.]” Secretary Del Toro indicated that funding for DE development will be increased in upcoming budget requests.

European laser programmes:

Other NATO nations are on the same track. In January 2024 the United Kingdom’s DragonFire laser directed energy weapon (LDEW) development programme reached a major milestone, achieving the UK’s first high-power firing of a laser weapon against aerial targets. The engagement range remains classified, with the Ministry of Defence (MoD) only commenting that the precision required would equate to hitting a coin at a distance of one kilometre. The beam can cut through metal “leading to structural failure or more impactful results if the warhead is targeted,” according to a MoD statement. The system had previously proven the ability to track moving air and sea targets with very high accuracy at range.

In April 2024 the MoD announced it was accelerated the fielding of DragonFire with the Royal Navy (RN). The first ships will be equipped with the LDEW in 2027, five years earlier than previously planned. “In a more dangerous world, our approach to procurement is shifting with it. We need to be more urgent, more critical and more global,” said then British Defence Secretary Grant Shapps. The UK sees DragonFire as a long-term low-cost alternative to missiles for such tasks as destroying attack drones. In addition to operational effectiveness, expected benefits include simplified logistics through reduced need to stockpile some munitions, and a very low cost per engagement; the MoD estimate less than GBP 10 for a ten-second shot. Presuming the LDEW performs as promised, fielding DragonFire before the end of the decade could place the RN in the vanguard for maritime laser weaponry.

Germany is also pursuing a maritime laser weapon suitable for countering the threat from drones, drone swarms, and speedboats. A 20 kW HEL naval laser weapon demonstrator (LWD) jointly developed by MBDA Deutschland GmbH and Rheinmetall was integrated aboard the frigate FGS Sachsen in June 2022 and evaluated at sea over nearly a year.  The LWD performed over a hundred test firings onboard the Sachsen. As described by the working group, the trials consisted of six campaigns testing the combat effectiveness of the LWD in increasingly complex scenarios, under realistic operating conditions and against different target types. This included detection and tracking (including highly agile targets); the interplay of sensors, command and weapon engagement systems, and effectors; possible rules of engagement; and successful engagement of targets with a high-energy laser beam. Following removal from Sachsen in late 2023 the LWD was transferred to the Bundeswehr Technical Centre 91 in Meppen for in-depth evaluation. The test results will inform the development of an operational laser weapon system.

High power microwave weapons: High power microwave (HPM) weapons emit intense pulses of electromagnetic energy which can disable UAVs, loitering munitions, and USVs in two ways. The electromagnetic pulse overloads and damages the on-board electronic components necessary for communications, sensors, and flight control or navigation; aircraft can be forced down, while USVs can be left dead in the water. Additionally, the microwaves generate intense heat within the target, which can melt or distort sensitive components such as circuit boards, antennas and power supplies, also disabling the target. While lasers must focus on one target at a time, and maintain the beam on target for sufficient time to do damage, microwave bursts can be fired either as pulsed-wave HPMs or as continuous-wave HPMs. Pulsed-wave HPMs are narrowly focussed, high-power, short-duration pulses which can provide precise targeting at longer range. Continuous-wave HPMs stream energy over a wider area but have a shorter effective range. They are well suited to area denial missions such as defeating incoming UAV or USV swarms.

Overall, HPMs are considered to have great force-protection potential, especially for warfare scenarios in confined waters which favour operations by smaller unmanned systems, or in a major war scenario against an adversary such as China, which is developing a large and sophisticated unmanned arsenal. The US Navy plans to begin at sea testing of the Meteor HPM in 2026. The weapon is expected to “provide capability with low cost-per-shot, deep magazine, tactically significant range, short time engagement for multi-target approach, dual deception and defeat capability,” according to fiscal year 2025 budget documents.

Here, too, other nations are pursuing their own efforts. In May 2024 the UK MoD announced that field testing of the Radio Frequency Directed Energy Weapon (RFDEW) will begin in the summer of 2024. While this testing will be conducted by British Army air defence personnel, the RFDEW is intended for both the land forces and the RN. The weapon is designed to detect, track, and engage multiple targets on land, sea, and in the air at a range up to one kilometre; range is to be increased over time. The RFDEW can defend against drone swarms and is designed for extensive automation, allowing operation by a single person.  A single electromagnetic pulse over a wide arc is expected to consume only ten pence worth of electricity.

Overlapping coverage required 

A viable, long-term defensive strategy against unmanned aerial and surface attacks will require a combination of kinetic and directed energy systems, deployed to provide layered and overlapping coverage at long, medium and short ranges. Significant research, development and testing efforts relating to weapons in both categories are underway. Smart munitions for ship’s artillery will optimise precision interceptions at long to medium range. Scalable high energy lasers will provide flexibility to warn off, disable or destroy threats; they will also offer commanders the option of non-destructive response to perceived threats, if de-escalation or minimising collateral damage are considerations. Microwave weapons will also provide significant flexibility, and enhance defence against swarming threats or attacks emanating from various directions simultaneously. In some scenarios coordinated measures by several defensive systems will be required to ensure that a threat has not simply been disabled but destroyed. Ultimately there will be no ‘golden bullet’ or optimal end-state; as adversaries continue to refine the capabilities of their offensive systems, fleets will continue to pursue evolutionary and revolutionary UAV/USV countermeasures indefinitely.

Sidney E. Dean