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Action Stations is the general signal to the personnel of a warship (usually British or Commonwealth) that combat with a hostile attacker (contact) is imminent or deemed probable. The alarm is also used as a preventative measure: for example, just before dawn when cruising in hostile waters and the possibility of air or submarine attack is deemed particularly high, although no active contacts exist. The term is also used in Royal Navy installations and bases, as well as Royal Air Force airbases and installations. Action Stations is equivalent to the United States Navy's 'General Quarters', and replaces the 'beat to quarters' instruction common during the age of sail.
Aboard ship, Action Stations is indicated by an alarm signal issued over the ship's bells or horns. This is usually followed by an announcement giving specific information, then a repeat of the alarm signal, after which the alarm is silenced. The alarm is distinguished from a ship’s readiness state and Chemical, biological, radiological, and nuclear (CBRN) condition, which refers to the manning of equipment and the closing of openings (both hatches and ventilators) respectively. When sounded, all personnel immediately stow any non-essential gear, take their life belts and action kit, and immediately report to their pre-assigned post or station. As personnel report to their stations, all weapons systems are readied to fire on command, the damage control centre is manned, and watertight doors are shut or sentries placed on those that must remain open. On aircraft carriers, the flight deck is made ready, any alert craft are prepared for launch, fuel lines are purged, and hangars secured. On ships with CBRN citadels, these are secured against the outside atmosphere and a positive pressure established.