Atlantic Bastion plans advancing as autonomous submarine XV Excalibur handed to Royal Navy
Source: gov.uk (2025-12-11)

Excalibur is a 12-metre experimental vessel. At a displacement of 19 tonnes, it’s the largest uncrewed underwater vessel ever trialled by the Royal Navy.

It’s been a busy year for the SDA’s Autonomy Unit who have achieved several milestones with XV Excalibur, the UK’s first uncrewed submarine.

Officially classified as an Extra-Large Uncrewed Underwater Vehicle (XLUUV), Excalibur is a 12-metre experimental vessel. At a displacement of 19 tonnes, it’s the largest uncrewed underwater vessel ever trialled by the Royal Navy.

The culmination of Project Cetus, it’s been developed in under three years by the SDA in partnership with MSubs Ltd. Sponsored by the Royal Navy, Project Cetus aims to build trust in Autonomy and be a testbed to assess military payloads and missions.

Official naming ceremony

In May 2025, Excalibur was named and unveiled at a ceremony in His Majesty’s Naval Base (HMNB) Devonport. It’ll now spend the next two years carrying out extensive sea trials, which will help understand the opportunities and challenges of operating an uncrewed vessel of this size.

Exercise Talisman Sabre

Excalibur’s long-distance interoperability was demonstrated just months after the ceremony, in Exercise Talisman Sabre. During the exercise in August the Royal Navy successfully controlled Excalibur in UK waters from a remote operating centre in Australia – more than 10,000 miles from the vessel’s home in Plymouth.

Part of AUKUS Pillar 2, the “advanced capability” component of the AUKUS trilateral partnership, the exercise marked the first time the UK and Australia, have demonstrated XLUUV interoperability as a single fighting force.

Quantum optical atomic trials

This was followed with a world-first trial, as Excalibur went to sea with a quantum optical atomic “Tiqker” clock on board developed by Infleqtion, a UK company leader in Quantum Technology. This marked the first time such a device has been operated at sea in an underwater vessel.

Unlike surface vessels, submarines cannot rely entirely on Global Positioning System (GPS) for navigation. Traditional microwave-based clocks provide stability but can drift over time, making them less accurate.

The use of quantum technology like Tiqker enhances a submarine’s ability to maintain accurate timing and navigation and reduces the need for external signals. This allows the submarine to stay submerged and covert for longer periods.

This trial was not only delivered successfully by the SDA’s Autonomy Unit, MSubs, Infleqtion and Royal Navy, but also took place six months ahead of schedule.

Handover to the Royal Navy

Since launching in February 2025, Excalibur has completed many acceptance trials, surpassing several original design specifications.

Excalibur has now officially been handed over to the Royal Navy. The SDA’s Autonomy Unit will continue to support the Navy on Excalibur’s test and evaluation programme, which aims to learn more about her capability and how we can effectively introduce autonomy to operational use.

This year’s progress heralds an exciting future in which autonomous underwater vehicles and nuclear submarines work in unison to provide exceptional underwater capability for the Royal Navy.

Excalibur’s capability positions the UK as a leader in underwater autonomous technology and showcases the SDA’s ability to exploit the latest technologies in the interests of national security.

SDA’s Autonomy Unit

SDA’s Autonomy Unit is a team of engineers who develop some of the cutting-edge technology that will support the Royal Navy’s submarine service in the future. They focus on innovation in the autonomous space, which refers to vehicles which can operate remotely and without direct human input.