Sequential Events Control Subsystem
The Sequential Events Control Subsystem (SECS) is responsible for controlling events which must happen in a pre-programmed sequence, for example parachute deployment during landing, or jettisoning the Service Module.
Earth Landing System
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The Earth Landing System (ELS) is responsible for the sequential events required to land the crew safely on Earth, for example jettisoning the apex cover and deploying the parachutes.
Since these events are irreversible and could be fatal if they occurred at the wrong time or didn't occur at the correct time, NASA went to a significant amount of trouble to ensure that the ELS couldn't be activated accidentally and would operate reliably when it was activated.
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