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:
- an inertial measurement unit (IMU)
- the Apollo Guidance Computer (AGC)
- resolvers to convert inertial platform angles to signals usable for servo control
- an optical unit
- a mechanical frame, called the navigation base (or navbase), to rigidly connect the optical device and, in the LM, the rendezvous radar to the IMU
- the AGC software
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.
Inertial measurement unit
The IMU was gimbaled on three axes. The innermost stable member, a 6-inch beryllium cube, had three gyroscopes and three accelerometers mounted in it. Feedback loops including the resolvers used signals from the gyroscopes to control motors at each axis. This servo system kept the stable member fixed with respect to inertial space. The IMU was derived from the guidance system developed by Draper for the Polaris missile.
Inertial guidance systems are not perfect and Apollo system drifted about one milliradian per hour. Thus it was necessary to "realign" the inertial platform periodically by sighting on stars.
The CM had a fixed sextant, the AOT, which could measure angles between stars and Earth or Moon landmarks and planetary horizons. The unit included a scanning telescope for star sightings, and could be used to determine position and orientation in space. In contrast, the LM had an alignment optical telescope, and could only determine the craft's orientation. The outer element of the AOT was a sun-shielded prism that could be rotated to one of three fixed positions relative to the LM, in order to cover a large portion of the lunar sky. When rotated, the AOT's position was readable by the AGC; by pointing the reticule at several different stars, the computer could determine the craft's orientation .
The onboard guidance software used a Kalman filter to merge new data with past position measurements to produce an optimal position estimate for the spacecraft. The key information was a coordinate transformation between the IMU stable member and the two reference coordinate systems, one centered on Earth and one centered on the Moon. In the argot of the Apollo program this matrix was known as REFSMMAT (for "Reference to a Stable Member Matrix").
Despite the word "primary" in its name, PGNCS data was not the main source of navigation information. Tracking data from NASA’s Deep Space Network was processed by computers at Mission Control, using least squares algorithms. The position and velocity estimates that resulted were more accurate than those produced by PGNCS. As a result, the astronauts were periodically given state vector updates to enter into the AGC, based on ground data. PGNCS was still essential to maintain spacecraft orientation, to control rockets during maneuvering burns, including lunar landing and take off, and as the prime source of navigation data during planned and unexpected communications outages. PGNCS also provided a check on ground data.
The lunar module had a third means of navigation, the abort guidance system (AGS), built by TRW. This was to be used in the event of failure of PGNCS. The AGS could be used to take off from the Moon, and to rendezvous with the Command Module, but not for landing. During Apollo 13, after the most critical burn near the Moon the AGS was used in place of PGNCS because it required less electrical power and cooling water.
Two PGNCS alarms (1201 "No VAC areas available" and 1202 "Executive alarm, no core sets") were relayed to mission control on the first lunar landing attempt on July 20th 1969. The computer system overload was caused by the simultaneous capture of landing radar data and rendezvous radar data.