Apollo PGNCS

The Apollo Primary Guidance, Navigation and Control System (PGNCS) (pronounced pings) 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 (CM) and Lunar Module (LM), 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 Kollsman Instrument built 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. The Prime Contractor for PGNCS and manufacturer of the Inertial Measurement Unit, IMU was the Delco Division of General Motors. Development was under the direction of Charles Stark Draper and MIT Draper Labs and consisted of the following components:

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 manned missions would use Block I, though unmanned 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.

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