Continuous At-Sea Deterrent (CASD)

Continuous At-Sea Deterrent is the United Kingdom’s posture of keeping at least one armed Trident SSBN on patrol at all times, an unbroken cycle the Royal Navy has maintained since the first Polaris patrol in 1969 under Operation Relentless. Four Vanguard-class boats sustain it: one on patrol, one in deep maintenance, one working up, and one in transit or training. CASD rests on the boat staying undetected for the whole patrol so its missiles always survive a first strike. The Dreadnought-class will replace the Vanguards from the early 2030s.