page_title: Space Shuttle
section_title: Flight systems
hierarchical_section_title: Space Shuttle / Description / Orbiter / Flight systems
caption_attribution_description: English: NASA Space Shuttle General Purpose Computers (GPC) IBM AP-101 "The shuttle GPCs were upgraded in 1991, and the system that used to take two boxes, right, could run on one, left."
context_page_description: The Space Shuttle was a partially reusable low Earth orbital spacecraft system that was operated from 1981 to 2011 by the National Aeronautics and Space Administration as part of the Space Shuttle program. Its official program name was Space Transportation System, taken from a 1969 plan for a system of reusable spacecraft of which it was the only item funded for development. The first of four orbital test flights occurred in 1981, leading to operational flights beginning in 1982. Five complete Space Shuttle orbiter vehicles were built and flown on a total of 135 missions from 1981 to 2011, launched from the Kennedy Space Center in Florida. Operational missions launched numerous satellites, Interplanetary probes, and the Hubble Space Telescope; conducted science experiments in orbit; and participated in construction and servicing of the International Space Station. The Space Shuttle fleet's total mission time was 1322 days, 19 hours, 21 minutes and 23 seconds.
context_section_description: The orbiter was equipped with an avionics system to provide information and control during atmospheric flight. Its avionics suite contained three microwave scanning beam landing systems, three gyroscopes, three TACANs, three accelerometers, two radar altimeters, two barometric altimeters, three attitude indicators, two Mach indicators, and two Mode C transponders. During reentry, the crew deployed two air data probes once they were travelling slower than Mach 5. The orbiter had three inertial measuring units (IMU) that it used for guidance and navigation during all phases of flight. The orbiter contains two star trackers to align the IMUs while in orbit. The star trackers are deployed while in orbit, and can automatically or manually align on a star. In 1991, NASA began upgrading the inertial measurement units with an inertial navigation system (INS), which provided more accurate location information. In 1993, NASA flew a GPS receiver for the first time aboard STS-51. In 1997, Honeywell began developing an integrated GPS/INS to replace the IMU, INS, and TACAN systems, which first flew on STS-118 in August 2007
  While in orbit, the crew primarily communicated using one of four S band radios, which provided both voice and data communications. Two of the S band radios were phase modulation transceivers, and could transmit and receive information. The other two S band radios were frequency modulation transmitters, and were used to transmit data to NASA. As S band radios can operate only within their line of sight, NASA used the Tracking and Data Relay Satellite System and the Spacecraft Tracking and Data Acquisition Network ground stations to communicate with the orbiter throughout its orbit. Additionally, the orbiter deployed a high-bandwidth Kᵤ band radio out of the cargo bay, which could also utilized as a rendezvous radar. The orbiter was also equipped with two UHF radios for communications with air traffic control and astronauts conducting EVA.
  The Space Shuttle's fly-by-wire control system was entirely reliant on its main computer, the Data Processing System (DPS). The DPS controlled the flight controls and thrusters on the orbiter, as well as the ET and SRBs during launch. The DPS consisted of five general purpose computers (GPC), two magnetic tape mass memory units (MMUs), and the associated sensors to monitors the Space Shuttle components. The original GPC used was the IBM AP-101B, which used a separate central processing unit (CPU) and input/output processor (IOP), and non-volatile solid-state memory. From 1991 to 1993, the orbiter vehicles were upgraded to the AP-101S, which improved the memory and processing capabilities, and reduced the volume and weight of the computers by combining the CPU and IOP into a single unit. Four of the GPCs were loaded with the Primary Avionics Software System (PASS), which was Space Shuttle-specific software that provided control through all phases of flight. During ascent, maneuvering, reentry, and landing, the four PASS GPCs functioned identically to produce quadruple redundancy, and would error check their results. In case of a software error that would cause erroneous reports from the four PASS GPCs, a fifth GPC ran the Backup Flight System, which used a different program and could control the Space Shuttle through ascent, orbit, and reentry, but could not support an entire mission. The five GPCs were separated in three separate bays within the mid-deck to provide redundancy in the event of a cooling fan failure. After achieving orbit, the crew would switch some of the GPCs functions from guidance, navigation, and control (GNC) to systems management (SM) and payload (PL) to support the operational mission. The Space Shuttle was not launched if its flight would run from December to January, as its flight software would have required the orbiter vehicle's computers to be reset at the year change. In 2007, NASA engineers devised a solution so Space Shuttle flights could cross the year-end boundary.
  Space Shuttle missions typically brought a portable general support computer (PGSC) t
caption: AP-101S (left) and AP-101B general purpose computers
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