Apollo PGNCS
Apollo spacecraft guidance system
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) each carried a version of PGNCS. PGNCS, and specifically its computer, also served as the command centre for all system inputs from the CM and the LM, including the optical units, the radar system, the manual translation and rotation device inputs by the astronauts, and other inputs from the CM and LM systems.
The MIT Instrumentation Laboratory developed PGNCS under the direction of Charles Stark Draper (the Instrumentation Laboratory was later named after him). The prime contractor for PGNCS and manufacturer of the inertial measurement unit (IMU) was the Delco Division of General Motors. PGNCS consisted of the following components:
an inertial measurement unit (IMU)
the Apollo Guidance Computer (AGC)
resolvers to convert inertial platform angles to signals usable for servo control
optical units, a Space Sextant for the CM and an Alignment Optical Telescope for the LM
a mechanical frame, called the navigation base (or navbase), to rigidly connect the optical devices and, in the LM, the rendezvous radar to the IMU
the AGC software
Versions
The CM and LM used the same computer, inertial platform and resolvers. The main difference was the optical unit. Each spacecraft also had a different navbase, reflecting their different mounting geometries. The LM's rendezvous radar was also connected to its navbase.
PGNCS had two versions—Block I and Block II—corresponding to the two generations of the CM. After the Apollo 1 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 aluminium tubing filled with polyurethane foam.
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