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. An inertial guidance system consists of an Inertial Measurement Unit (IMU) combined with control mechanisms, allowing the path of a vehicle to be controlled according to the position determined by the inertial navigation system. ... Apollo Spacecraft: Command Module, Service Module, Lunar Module. ... The Command/Service Module (CSM) was a spacecraft built for NASA by North American Aviation. ... Description Role: Lunar landing Crew: 2; CDR, LM pilot Dimensions Height: 20. ...

PGNCS was developed by the MIT Instrumentation Laboratory under the direction of Charles Stark Draper and consisted of the following components: The Charles Stark Draper Laboratory, Inc. ... Charles Stark Draper (October 2, 1901 – July 25, 1987) is often referred to as the father of inertial navigation. ...

• 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

The Apollo Guidance Computer (AGC) was the first recognizably modern embedded system, used in real-time by astronaut pilots to collect and provide flight information, and to automatically control all of the navigational functions of the Apollo spacecraft. ...

Versions

LM (click to enlarge). Note PGNCS components at top-left.

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. Download high resolution version (500x698, 26 KB)line drawing of lunar module spacecraft This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... Download high resolution version (500x698, 26 KB)line drawing of lunar module spacecraft This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This long range radar antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll[1]. Radar is a system that uses radio waves to detect, determine the distance of, and map, objects such...

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 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. Apollo One is the name given to the Apollo/Saturn 204 (AS-204) spacecraft after it was destroyed by fire during a training exercise on January 27, 1967, at Pad 34 atop a Saturn IB rocket. ... This article or section is missing references or citation of sources. ... General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Atomic mass 9. ... Aluminum is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. ... A polyurethane is any polymer consisting of a chain of organic units joined by urethane links. ...

Components from PGNCS were used by Draper for the U.S. Navy's Deep Submergence Rescue Vehicle (DSRV). The United States Navy (USN) is the branch of the United States armed forces responsible for naval operations. ... A Deep Submergence Rescue Vehicle (DSRV) is a type of Deep Submergence Vehicle used for rescue of downed submarines and clandestine missions. ...

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. Mid-19th century tool for converting between different standards of the inch An inch is an Imperial and U.S. customary unit of length. ... This article or section does not cite its references or sources. ... A depiction of an accelerometer designed at Sandia National Laboratories. ... The term servo can refer to: Servomechanism - usually just shortened to servo, is a device used to effect mechanical motion for a specified distance. ... The Polaris Missile was a submarine-launched ballistic missile (SLBM) carrying a nuclear warhead developed during the Cold War for the United States Navy. ...

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 radian (symbol: rad, or a superscript c ( half circle)) is the SI unit of plane angle. ...

Optical unit

The CM had a fixed sextant which could measure angles between stars, and Earth or Moon landmarks and planetary horizon. 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 [1]. A sextant is a measuring instrument used to measure the angle of elevation of a celestial object above the horizon. ...

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"). The Kalman filter is an efficient recursive filter which estimates the state of a dynamic system from a series of incomplete and noisy measurements. ... See Cartesian coordinate system or Coordinates (elementary mathematics) for a more elementary introduction to this topic. ...

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. The position and velocity estimates that resulted were more accurate than those produced by the PGNCS. As a result, the astronauts were periodically given REFSMMAT 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. Deep Space Network (DSN) is an international network of radio antennas that supports interplanetary spacecraft missions, and radio and radar astronomy observations for the exploration of the solar system and the universe. ... Least squares is a mathematical optimization technique which, when given a series of measured data, attempts to find a function which closely approximates the data (a best fit). It attempts to minimize the sum of the squares of the ordinate differences (called residuals) between points generated by the function and...

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 the PGNCS. The AGS could be used to take off from the moon, and to rendezvous with the command module, but not for landing. TRW Incorporated was an American corporation involved in a number of businesses, mostly defense-related, but including automotive supply and credit reporting. ...