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 PNGCS.

PGNCS was developed by the MIT Instrumentation Laboratory under the direction of Charles Stark Draper and consisted of the following components:

• an Inertial Measurement Unit (IMU)
• the Apollo Guidance Computer
• 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

Versions

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 the PGNCS, Block I and Block II, corresponding to the two generations of command module. After the Apollo I fire, which occurred in a Block I CM, NASA decided than no further manned missions would use Block I, though further 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 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.

Components from PGNCS were used by Draper for the U.S. Navy's Deep Submergence Rescue Vehicle (DSRV).

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 the stars. 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 miliradian per hour. Thus it was necessary to ??realine?? the inertial platform periodically by sighting on stars.

Optical unit

CM had a space sextant that could measure angles between stars and Earth or Moon landmarks and planetary horizon. It also had a scanning telescope for star sightings. This arrangement permitted for navigational fixes that included position and orientation in space. The LM had an Alignment Optical Telescope that only permitted star sightings. so just orientation could be determined. The LM?s Alignment Optical Telescope was essentially a wide field of view periscope. The outer element was a prism that could be rotated to one of three fixed positions relative to the LM, to cover a large portion of the lunar sky. The astronauts could rotate an illuminated reticule, whose position was readable by the AGC. In this way star positions relative to the LM and hence the IMU's stable ember, could be recorded. Because of concerns that direct sunlight falling on the outer prism would scatter enough light into the AOT to make stars difficult to see, a sun shade was added that rotated with the prism.

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 reference coordinate system (there were two, actually, one centered on Earth and one centered on the Moon). In the argot of the Apollo program this matrix was known as REFSMMAT.

Who's in charge?

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) and the position and velocity estimates that resulted proved more accurate than what PGNCS could produce. As a result, the astronauts were periodically given numbers to enter into the AGC to update REFSMMAT 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, to be used in the event of failure of the PGNCS. The AGS could be used to take off from the moon, and to rendezvous with the command module, but not for landing.