Sweden and Saab complete first sea trials of new LUUV undersea vehicle to protect Baltic Sea infrastructure.

Saab, the Swedish Defence Materiel Administration (FMV), and the Swedish Armed Forces announced the completion of initial sea trials for Sweden's Large Uncrewed Undersea Vehicle (LUUV) demonstrator. Developed under a SEK 60 million contract awarded in August 2025, the autonomous underwater vehicle utilizes Saab's Autonomous Ocean Core control system to conduct underwater reconnaissance, seabed surveillance, and critical infrastructure protection in the Baltic Sea. The platform is designed to perform forward surveys in high-risk areas prior to deploying crewed assets, including Sweden's Gotland-class and future Blekinge-class submarines.

Sweden and Saab completed the initial sea trials of the Large Uncrewed Undersea Vehicle (LUUV), an autonomous underwater drone developed to conduct reconnaissance, monitor seabed infrastructure, and support submarine operations in the Baltic Sea. (Picture source: Swedish MoD)

Sweden's Saab, Defence Materiel Administration (FMV), and Armed Forces announced on October 16, 2026, the completion of initial sea trials of the Large Uncrewed Undersea Vehicle (LUUV), an autonomous underwater vehicle (AUV) intended for submarine reconnaissance, seabed surveillance, and underwater infrastructure protection in the Baltic Sea. Developed under a SEK 60 million contract awarded on August 29, 2025, the LUUV demonstrator incorporates Saab's Autonomous Ocean Core software as well as technology from its Autonomous Ocean Drone (AOD), which measures 7 m, weighs 6.5 tonnes, and has a published range exceeding 1,111 km at 7.4 km/h. The first development phase is now complete, but the Swedish vehicle's validated endurance, operating depth, and sonar performance remain undisclosed.

FMV project manager Martin Nyqvist confirmed that further trials will evaluate autonomous functions, sensors, and operations in representative environments. Sweden's operational requirement for a LUUV is the possibility to investigate potentially dangerous waters before committing its three Gotland-class submarines, HSwMS Gotland, HSwMS Uppland, and HSwMS Halland, commissioned between 1996 and 1997. These submarines displace approximately 1,500 tonnes submerged, measure 60.4 m in length, and use Stirling air-independent propulsion, while two Blekinge-class submarines are under construction. On August 29, 2025, Tobias Söderblom of the Swedish Naval Warfare Centre identified reconnaissance ahead of crewed submarines as a potential LUUV mission. In practical terms, the LUUV could inspect a designated approach corridor for mines, seabed surveillance equipment, and underwater obstacles while the crewed submarine remains outside the area.

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This could preserve the vessel's availability for other assignments, as a submarine conducting preliminary reconnaissance must allocate time to navigation, sonar observation, and potentially repeated passes, reducing the time available for intelligence collection or other assigned missions. Logically, buried mines, passive sensors, and objects concealed by seabed relief or outside the surveyed corridor would remain potential hazards even after a completed survey. The LUUV concept combines dimensions suitable for coastal deployment with endurance intended for missions lasting several days, as FMV's preliminary LUUV requirements included a length of 6.5 m, a width of 1.4 m, a maximum speed of 14 km/h, and an operating depth of 500 m. Saab's related AOD reference design is 7 m long, 1.4 m in diameter, and weighs 6.5 tonnes, and uses lithium-ion batteries to achieve a range exceeding 600 nautical miles at 4 knots. Covering 600 nautical miles at that speed requires 150 hours, equivalent to six days and six hours.

A mission involving 100 nautical miles outbound, 300 nautical miles of surveillance, and 100 nautical miles returning would consequently require 125 hours, leaving 100 nautical miles of nominal range before reserves and additional sensor consumption. The AOD also incorporates directional thrusters and an internal payload compartment with weight compensation for equipment deployment. As the Swedish LUUV demonstrator's actual battery capacity, maximum submerged endurance, acoustic signature, and validated operating depth have not been disclosed, the AOD specifications should not be treated as simultaneously demonstrated LUUV capabilities. To perform several underwater reconnaissance functions during the same deployment, the planned AOD sensor configuration combines multi-aperture sidescan sonar, forward-looking sonar, intercept sonar, flank-array sonar, a multibeam echo sounder, and Doppler Velocity Log/Inertial Measurement Unit navigation.

Sidescan sonar identifies mine-like objects, cables, and unfamiliar seabed equipment on or above the seabed, while the multibeam sonar maps bottom contours and the forward-looking sonar detects obstacles during submerged movement. Flank-array and intercept sonars provide passive acoustic sensing for underwater contacts and acoustic emissions, potentially supporting submarine surveillance without active transmissions. DVL/IMU navigation estimates the vehicle's movement and position without continuous satellite access, although errors could logically accumulate during long submerged operations. Sonar detection ranges, acoustic frequencies, classification accuracy, and false-alarm rates remain undisclosed. Consequently, the planned sensor arrangement does not yet establish a validated LUUV capability to detect and track operational submarines. 

The Baltic Sea covers 377,000 km², averages 55 m in depth, and reaches 459 m at Landsort Deep, while its shallow coastal areas, salinity gradients, and shipping traffic complicate underwater acoustic surveillance. Sweden's priority areas include the Gotland island, situated approximately 90 km from the mainland, and the southern approaches toward the Danish Straits, while submarine cables, power connections, and pipelines link Sweden with Finland, Poland, and the Baltic states. The September 2022 Nord Stream explosions and October 2023 Balticconnector damage demonstrated that underwater infrastructure can be disrupted without an immediate surface-level indication of the cause. Therefore, a LUUV could conduct repeated inspections of selected corridors, comparing sidescan imagery and seabed profiles to detect displaced objects, cable exposure, damaged installations, or changes in bottom conditions.

For illustration, 300 nautical miles of survey movement equals 555.6 km of track, providing 55.6 km² of nominal coverage with a 100 m sonar swath, or 111.1 km² with a 200 m swath, although actual coverage would be reduced by overlapping tracks and manoeuvring losses. These widths are hypothetical, since the LUUV's actual sonar coverage has not been disclosed. Therefore, Saab's Autonomous Ocean Core system reduces the need for continuous remote control through four manoeuvring functions: Mission Manoeuvre for waypoints and payload activation, Vector Manoeuvre for precise positioning, Direct Manoeuvre for fallback control, and Position Hold for maintaining a designated operating state. Its Local Safety Mode supports supervised operation, the External Safety Mode automatically stops the vehicle following communication loss, and the Assault Mode permits autonomous missions without continuous connectivity.

This software architecture has also been used in Saab's Enforcer 3.1 in San Diego and Enforcer 3.2 in Sweden. Underwater operation introduces additional constraints because satellite communications require surfacing, while acoustic links provide limited bandwidth and can expose the vehicle's presence. The LUUV may therefore need to store high-resolution sonar imagery and execute navigation tasks onboard, transmitting information after surfacing or recovery. This is manageable for scheduled infrastructure inspections but creates delays when detecting moving contacts whose positions may change before operators receive the information. Sweden is also examining whether LUUV technology could eventually operate from its two Blekinge-class A26 submarines, the future HSwMS Blekinge and HSwMS Skåne.

Their Multi-Mission Portal is intended to accommodate divers, underwater equipment, and autonomous vehicles, while the AOD's dimensions were selected with potential submarine compatibility in mind. In March 2026, Rear Admiral Fredrik Lindén, FMV's Director Naval Systems, identified the LUUV demonstrator as a means of establishing requirements for future A26-compatible underwater drones. Submarine deployment would nevertheless require qualified launch procedures, safe separation, submerged rendezvous, accurate relative navigation, and recovery with sufficient battery reserves. Shore or surface-vessel deployment would avoid several of these constraints. The AOD's internal payload compartment could also accommodate temporary seabed sensors, allowing the vehicle to perform tasks beyond reconnaissance, although weapons were explicitly excluded from the first Swedish development phase.

The SEK 60 million development contract is not a production unit price, and no operational procurement quantity, submarine recovery qualification, or service-entry date has been announced. Sweden joined NATO on March 7, 2024, and the LUUV's development coincides with increased allied underwater surveillance activity following the launch of Baltic Sentry in January 2025. Task Force X-Baltic also tested 70 aerial and maritime drones between March and October 2025, and the LUUV would complement these systems by inspecting submerged infrastructure and collecting acoustic information that surface radar, aerial sensors, and automatic identification systems cannot obtain.

Saab's AOD is smaller than the U.S. Orca XLUUV and Australia's Ghost Shark, but Israel's BlueWhale could provide a closer comparison for acoustic surveillance. A meaningful comparison with crewed submarine operations would require measuring the number of kilometres or square kilometres surveyed per mission, the proportion of relevant objects detected, mission-completion rates, maintenance hours, and the cost of producing usable intelligence. It would also require establishing how many submarine operating days could actually be reassigned, rather than assuming that every autonomous mission replaces a submarine patrol.

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Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, South Korea, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.