OVERVIEW: The Mk 251 “Siren” is a 130 mm active parachute-deployed RF decoy round introduced in 2004. It is designed to entice incoming radar-guided missiles away from naval vessels by emitting deceptive radar signals.

DETAILS: Siren is launched from 130 mm Sea Gnat-compatible launchers, including the Royal Navy’s trainable DLH/Centurion systems. Upon deployment, a low-G rocket propels the round to approximately 500 m from the ship, after which a two-stage parachute rig unfolds into a parasail wing. The onboard electronics detect missile guidance signals and emit tailored I‑ and J‑band deception or barrage jamming waveforms. It entered service in 2004 and is fitted across major Royal Navy platforms from Type 22 and 23 frigates to destroyers and carriers.

FONCTION:
Fired from 130 mm decoy tubes after radar threat detection.
Deploys parachute/wing and begins RF signature transmission.
Lures radar missiles toward the decoy by emulating ship-like radar signals.

SOURCE: Royal Navy “Seagnat” system documentation ; Defense‑Aerospace article “Royal Navy Deploys Active Decoy Round” Feb 2004 ; Navy Lookout – “Royal Navy upgrades its passive decoy launchers” ; ThinkDefence – Chemring Centurion/Siren overview