Gemini Guidance Computer Explained

Gemini Guidance Computer
Invent-Name:	IBM Federal Systems Division
Manufacturer:	IBM Federal Systems Division
Type:	Avionics Guidance Computer
Processor:	Discrete Components ^[1]
Frequency:	7.143 kilohertz clock
Memory:	39-bit words memory, each composed of three 13-bit syllables, 4,096 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)^[2]
Dimensions:	(H)×(W)×(D)

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

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:^[5]

• Ascent – serves as a backup guidance system. The switchover is manually controlled by the astronauts
• Orbital flight – gives the astronauts the capacity to navigate, allowing them to choose a safe landing spot in an emergency and calculate the timing of retrofire (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; 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^[7]
• Ferrite core memory of 4,096 words^[8]
• Two's complement integer arithmetic
• 7.143 kilohertz clock (140 μs per instruction); all instructions took a single cycle except for multiplication and division

See also

• Apollo Guidance Computer

External links

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

Notes and References

1. Web site: Ch1-2. March 1988 . Tomayko . James E. .
2. Web site: The Gemini Spacecraft Computer.
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. Book: Tomayko . James E. . 1988 . Computers in Spaceflight: The NASA Experience . The Gemini Digital Computer: First Machine in Orbit . National Aeronautics and Space Administration . Washington, D.C. . 2015-01-04.
5. McDonnell Corporation, NASA Project Gemini Familiarization Manual, 1965, vol. 2, pp. 8.7,8.45.
6. http://www.frantone.com/designwritings/design_writings_LVDC.html LVDC board reverse engineering project
7. Web site: Gemini Spacecraft Computer (OBC): Layout of Memory Words . 2016 . 2011 . 2016-05-28 . live . https://web.archive.org/web/20160528114015/http://www.ibiblio.org/apollo/Gemini.htp . 2016-05-28.
8. https://history.nasa.gov/computers/Ch1-2.html Computers in Spaceflight: The NASA Experience