The IBM System/4 Pi is a family of radiation hardened avionics computers used, in various versions, on the B-52 Stratofortress bomber, the F-15 Eagle fighter, E-3 Sentry, NASA's Skylab space station, MOL, and the Space Shuttle, as well as other aircraft. It descends from the System/360 mainframe family of computers.
The top-of-the-line 4 Pi is the AP-101, used in the B-52. The U.S. Navy used a similar variant in the carrier based A-6E/A-6E TRAM medium attack aircraft. The Shuttle is controlled by five AP-101s, four of which are arranged in a redundant configuration, with the fifth as backup. Skylab employed the model TC-1, which had a 16-bit word length in contrast to the AP-101's 32 bits.
The name of the system is derived from the fact that the angular measure of a complete sphere (solid angle) is 4π steradians, while the angular measure of a complete circle is 360 degrees; hence System/4 Pi and System/360. This implies that System/4 Pi is a version of the IBM System/360 for the three-dimensional world of avionics.
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