The Type 45 destroyers take advantage of some Horizon development work and use the Sea Viper air-defence system and the SAMPSON radar. The ships are built by BAE Systems Maritime – Naval Ships, originally created as BVT Surface Fleet by the merger of the surface shipbuilding arms of BAE Systems and VT Group. These two companies previously built the ships in collaboration. BAE's two Glasgow shipyards and single Portsmouth shipyard are responsible for different "blocks". BAE's Govan yard is responsible for Block A (stern to edge of helicopter hangar). The Scotstoun yard builds Blocks B/C (a 2600 tonne section which contains the Rolls-Royce WR-21 gas turbines, starts with the helicopter hangar to the bridge section) and Block D (bridge section itself). BAE's Portsmouth shipyard is responsible for Blocks E/F (bridge to the bow) and the funnels and masts. For ships 2 to 6 blocks A-D were assembled in the Ships Block and Outfit Hall of the Govan shipyard, and taken fully outfitted to the Scotstoun berth. The masts and funnels were also fitted before launch.