There are five general purpose computers (GPCs) on the Orbiter. They control the space shuttle vehicle systems. Up to four of them are working together at any one moment during a mission. They make up the Primary Avionics Software System (PASS). Each computer constantly monitors how the others are functioning. When one of the computers provides information that seems incorrect, the others will vote on its Òmental health.Ó If they decide it isnÕt working properly, they will ignore it.

The fifth computer houses the backup flight system.