Project Apollo Digest: "Guidance and Navigation" ~ 1966 NASA

more at http://quickfound.net/

Originally a public domain film from the National Archives or Library of Congress Prelinger Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.

The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

https://en.wikipedia.org/wiki/Apollo_PGNCS

Wikipedia license: http://creativecommons.org/licenses/by-sa/3.0/

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...

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...

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.

Optical unit

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...

Software

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").

Navigational information

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...