page_title: Apollo PGNCS
section_title: Versions
hierarchical_section_title: Apollo PGNCS / Versions
caption_attribution_description: English: Diagram of the one of the three rate integrating gyroscopes used it the Apollo guidance system's IMU
context_page_description: The Apollo primary guidance, navigation, and control system was a self-contained inertial guidance system that allowed Apollo spacecraft to carry out their missions when communications with Earth were interrupted, either as expected, when the spacecraft were behind the Moon, or in case of a communications failure. The Apollo command module and lunar module, were each equipped with a version of PGNCS. PGNCS, and specifically its computer, were also the command center for all system inputs from the LM, including the alignment optical telescope, the radar system, the manual translation and rotation device inputs by the astronauts as well as other inputs from the LM systems.
  PGNCS was developed by the MIT Instrumentation Laboratory under the direction of Charles Stark Draper. The prime contractor for PGNCS and manufacturer of the inertial measurement unit was the Delco Division of General Motors. PGNCS consisted of the following components:
  an inertial measurement unit
  the Apollo Guidance Computer
  resolvers to convert inertial platform angles to signals usable for servo control
  optical units, one for the CM and a different one for the LM
context_section_description: The CM and LM used the same computer, inertial platform and resolvers. The main difference was the optical unit. The navbase was different for each spacecraft as well, reflecting the differing mounting geometries. The LM's rendezvous radar was also connected to its navbase.
  There were two versions of PGNCS—Block I and Block II—corresponding to the two generations of the CM. After the Apollo I fire, which occurred in a Block I CM, NASA decided that no further crewed missions would use Block I, though uncrewed missions did. Major differences between Block I and Block II PGNCS included replacing electromechanical resolvers with an all electronic design and replacing the Block I navbase, which was machined from beryllium, with a frame built out of aluminum tubing filled with polyurethane foam. The Block II navbases were lighter, cheaper, and just as rigid.
  Another major difference between Block I and Block II was repairability. An original goal for the Apollo program was for the astronauts to be able to make repairs to the electronics. Accordingly, the Block 1 PNGCS was designed with many identical modules that could be replaced with spares, if necessary, in flight. However high humidity conditions inside the crew compartments and accidents in handling body fluids during the Gemini 7 mission made having unsealed electrical connections undesirable. The repairability goal was eliminated in Block II and all units and electrical connections were sealed. The fatal Apollo 1 fire reenforced this concern.
  Components from PGNCS were used by Draper for the U.S. Navy's deep-submergence rescue vehicle (DSRV).
caption: Apollo gyroscope (IRIG)
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