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 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 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, one for the CM and a different one 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