OVERVIEW: The S2170 “Sea Sentor” (also known as SSTD – Surface Ship Torpedo Defence) is a towed and expendable acoustic decoy system used by the Royal Navy. It is designed to detect, classify, and divert incoming torpedoes through active sonar sensing and deployable decoys.

DETAILS: Sea Sentor entered service in 2004, developed by Ultra Electronics as the UK’s primary shipboard torpedo defence system. The system incorporates a variable-depth towed passive acoustic array along with one-port and one-starboard expendable decoy launchers, holding a total of 16 torpedo-decoy rounds. It automatically classifies incoming torpedo threats using AI-powered processing to recommend countermeasure deployment or evasive manoeuvres, replacing the earlier AN/SLQ-25 “Nixie” system. Sea Sentor remains in active service, with over 65 systems fitted across Royal Navy surface combatants, and has been exported under the same name for use by allied navies.

FONCTION:
Passive towed array continuously monitors for torpedo contacts.
System auto-classifies threats and calculates needed countermeasures.
Fires expendable acoustic decoys or suggests manoeuvres to divert torpedoes.

SOURCE:
Wikipedia – “SSTD” article ; Naval‑Technology – “UK explores next generation Surface Ship Torpedo Defence” ; Deagel – S2170 Sea Sentor specifications ; UK Defence Journal – “Contract placed for Type 31 Frigate torpedo defence system” ; Navy‑Matters blog – overview of RN torpedo defence evolution