Gemini Guidance Computer

• Name — Gemini Guidance Computer • Invent Name — IBM Federal Systems Division • Manufacturer — IBM Federal Systems Division • Type — Avionics Guidance Computer • Processor — Discrete Components • 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) • Dimensions — (H)×(W)×(D)

• Illustration — Locations of the Gemini Guidance System Project Gemini was the first with an on-board computer, as Project Mercury was controlled by computers on Earth. The Gemini Guidance Computer was responsible for the following functions: • 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.

• 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 (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 • Ferrite core memory of 4,096 words • Two's complement integer arithmetic • 7.143 kilohertz clock (140 μs per instruction); all instructions took a single cycle except for multiplication and division