IBM System/4 Pi

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The IBM AP-101S Space Shuttle General Purpose Computer is a member of the System/4 Pi family

The IBM System/4 Pi is a family of avionics computers used, in various versions, on the F-15 Eagle fighter, E-3 Sentry, AWACS, Harpoon Missile, NASA's Skylab, MOL, and the Space Shuttle, as well as other aircraft. It descends from the approach used in the System/360 mainframe family of computers in that members of the family were intended for use in many varied user applications. Previously custom computers had been designed for each aerospace application which was extremely costly.

The Skylab space station employed the model TC-1,[1] which had a 16-bit word length and 16,384 words of memory with a custom input/output assembly.[2]

The top-of-the-line 4 Pi is the AP-101, used in the AWACS. The U.S. Navy used a similar variant, the AN/ASQ-155, in the carrier based A-6E/A-6E TRAM medium attack aircraft. The Shuttle is controlled by five AP-101 computers, three of which are arranged in a Triple Modular Redundant configuration, one was kept powered up as a "hot spare" and one was maintained powered down as a redundant "cold spare".

The name of the system is derived from the fact that the angular measure of a complete sphere (solid angle) is 4π steradians,[3] 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. This was only for marketing purposes because neither the electronics, architecture nor software were related to the IBM commercial 360 line.


  1. ^ Jenkins, Dennis (April 5, 2001). "Advanced Vehicle Automation and Computers Aboard the Shuttle". NASA History Homepage. NASA. Retrieved 27 October 2013. 
  2. ^ "Skylab Space Station". eoPortal. ESA. Retrieved 27 October 2013. 
  3. ^ Technical Description of IBM System/4 Pi Computers. Owego, NY: Federal Systems Division of IBM. 1967. Retrieved 27 October 2013. 

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