"A simplex GPC is one in run and not a member of the redundant set, such as the BFS (Backup Flight System) GPC. Systems management and payload major functions are always in a simplex GPC."

"Even though the four primary avionics software system GPCs control all GN&C functions during the critical phases of the mission, there is always a possibility that a generic failure could cause loss of vehicle control. Thus, the fifth GPC is loaded with different software created by a different company than the PASS developer. This different software is the backup flight system. To take over control of the vehicle, the BFS monitors the PASS GPCs to keep track of the current state of the vehicle. If required, the BFS can take over control of the vehicle upon the press of a button. The BFS also performs the systems management functions during ascent and entry because the PASS GPCs are operating in GN&C. BFS software is always loaded into GPC 5 before flight, but any of the five GPCs could be made the BFS GPC if necessary."