Gemini Guidance Computer

The Gemini Guidance Computer (sometimes Gemini Spacecraft On-Board Computer (OBC)) was a digital, serial computer designed for Project Gemini, America's second manned space project.[2] The computer, which facilitated the control of mission maneuvers, was designed by the IBM Federal Systems Division.[3]

Gemini Guidance Computer

Gemini Guidance Computer in National Air and Space Museum
Invented by	IBM Federal Systems Division
Manufacturer	IBM Federal Systems Division
Introduced	1965 (1965)
Discontinued	1966 (1966)
Type	Avionics Guidance Computer
Processor	Discrete IC RTL based
Frequency	7.143 kilohertz clock
Memory	39-bit words memory, each composed of three 13-bit syllables, 4096 words of memory, in a ferrite core array.
Ports	Modular Display Keyboard (MDK), Modular Display Readout (MDR), Attitude Control and Maneuver Electronics (ACME), Inertial Measurement Unit (IMU), Horizon Sensors, Time Reference System (TRS)[1]
Power consumption	28V DC
Weight	58.98 pounds (26.75 kg)
Dimensions	18.9"(H)×14.5"(W)×12.75"(D)

Functionality

Project Gemini was the first with an on-board computer, as Project Mercury was controlled by computers on Earth.[4] The Gemini Guidance Computer was responsible for the following functions:[3][2]

• Ascent – serves as a backup guidance system. The switchover is manually controlled by the astronauts
• Orbital flight – provides a navigation capability to the astronauts to determine the time of retrofire and to select the landing site for safe reentry in the case of an emergency. (on extended missions ground data may become unavailable when ground data network rotates out of the orbital plane.)
• Rendezvous – serves as primary reference by providing guidance information to the astronauts. The orbit parameters are determined by the ground tracking which are then sent to the spacecraft; the guidance computer was responsible for processing the information along with sensed spacecraft attitude. The information was presented to the astronauts in terms of spacecraft coordinates.
• Reentry – feeds commands directly to the reentry control system for automatic reentry or provides the guidance information to the astronauts for manual reentry.

Specs

• The computer was architecturally similar to the Saturn Launch Vehicle Digital Computer, in particular in the instruction set;[5] however its circuit integration was less advanced.[6] The GGC weighed 58.98 pounds (26.75 kg) and was powered by 28V DC. During a short power outage it could be powered by the Auxiliary Computer Power Unit (ACPU)
• 39-bit words memory, each composed of three 13-bit syllables[5]
• Ferrite core memory of 4096 words[7]
• Two's complement integer arithmetic
• 7.143 kilohertz clock (140 μs per instruction); all instructions took a single cycle except for multiplication and division

References

1. http://www.ibiblio.org/apollo/Gemini.html#Peripheral_Devices
2. McDonnell Corporation, NASA Project Gemini Familiarization Manual, 1965, vol. 2, pp. 8.7,8.45.
3. 8. J. C. Hundley and R. A. Watson, "A Digital Computer in Orbital Flight," TR 63-825-892, IBM Federal Systems Division, Owego, New York, October 1964.
4. Tomayko, James E. (1988). "The Gemini Digital Computer: First Machine in Orbit". Computers in Spaceflight: The NASA Experience. Washington, D.C.: National Aeronautics and Space Administration. Retrieved 2015-01-04.
5. "Gemini Spacecraft Computer (OBC): Layout of Memory Words". 2016 [2011]. Archived from the original on 2016-05-28. Retrieved 2016-05-28.
6. LVDC board reverse engineering project
7. Computers in Spaceflight: The NASA Experience, chapter one

External links

• Gemini Spacecraft On-Board Computer (OBC)
• Gemini Program Overview
• IBM and the Gemini Program

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