Gemini Guidance Computer

Functionality

• 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.
• 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. The GGC weighed 58.98 pounds and was powered by 28V DC. During a short power outage it could be powered by the Auxiliary Computer Power Unit
• 39-bit words memory, each composed of three 13-bit syllables
• Ferrite core memory of 4096 words
• Two's complement integer arithmetic
• 7.143 kilohertz clock ; all instructions took a single cycle except for multiplication and division