QinetiQ's Cerberus will detect a diver or swimmer at up to 800m. Its dome-shaped module can be suspended beneath a hull to provide 360? cover, or fitted into a hull or onto the seabed.

The system's sonar-based location is married to a tracking system that helps define the nature of an approaching object. It is said to be able to distinguish between, say, fish and divers – and even between divers on open-circuit scuba and those on rebreathers.

Sources such as ship wakes and flotsam in the sea can also be recognised and, if required, eliminated from the alarm activation programme.

An immediate market is reported to be with ultra-wealthy superyacht owners, who might have cause for concern over possible intruders. Two units were used at an America's Cup ranking event at Valencia in Spain this summer, the sailing teams being as sensitive as Formula One outfits in protecting the secrets of their designs.