Apollo PGNCS

Last updated
Apollo Command Module primary guidance system components Apollo Command Module primary guidance system locations.png
Apollo Command Module primary guidance system components
Apollo Lunar Module primary guidance system components Apollo Lunar Module primary guidance system locations.png
Apollo Lunar Module primary guidance system components
Apollo Inertial Measurement Unit Apollo Inertial Measurement Unit.png
Apollo Inertial Measurement Unit

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.

Contents

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:

Versions

Apollo gyroscope (IRIG) Apollo block II IRIG diagram.png
Apollo gyroscope (IRIG)
Apollo accelerometer (PIPA) Apollo PIPA, model D, size 16.png
Apollo accelerometer (PIPA)

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 crewed missions would use Block I, though uncrewed 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.

Another major difference between Block I and Block II was repairability. An original goal for the Apollo program was for the astronauts to be able to make repairs to the electronics. Accordingly, the Block 1 PNGCS was designed with many identical modules that could be replaced with spares, if necessary, in flight. However high humidity conditions inside the crew compartments and accidents in handling body fluids during the Gemini 7 mission made having unsealed electrical connections undesirable. The repairability goal was eliminated in Block II and all units and electrical connections were sealed. [1] The fatal Apollo 1 fire reinforced this concern.

Components from PGNCS were used by Draper for the U.S. Navy's deep-submergence rescue vehicle (DSRV). [2]

Inertial measurement unit

Apollo IMU Apollo IMU at Draper Hack the Moon exhibit.agr.jpg
Apollo IMU

The IMU was gimbaled on three axes. The innermost part, the stable member (SM), was a 6-inch beryllium cube, with three gyroscopes and three accelerometers mounted in it. Feedback loops used signals from the gyroscopes by way of the resolvers to control motors at each axis. This servo system kept the stable member fixed with respect to inertial space. Signals from the accelerometers were then integrated to keep track of the spacecraft's velocity and position. 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.

Optical units

CM space sextant Apollo CM Sextant at Draper Hack the Moon exhibit.agr.jpg
CM space sextant
Apollo CM optical unit Apollo CM optical unit assembly.png
Apollo CM optical unit

The CM optical unit had a precision sextant (SXT) fixed to the IMU frame that could measure angles between stars and Earth or Moon landmarks or the horizon. It had two lines of sight, 28× magnification and a 1.8° field of view. The optical unit also included a low-magnification wide field of view (60°) scanning telescope (SCT) for star sightings. The optical unit could be used to determine CM position and orientation in space.

LM alignment optical telescope Apollo Alinement Optical Telescope.png
LM alignment optical telescope

The LM instead had an alignment optical telescope (AOT), essentially a periscope. The outer element of the AOT was a sun-shielded prism that could be rotated to one of six fixed positions relative to the LM, in order to cover a large portion of the lunar sky. Each position had a 60° field of view. When rotated, the AOT's position was readable by the AGC; by pointing the reticule at two different stars, the computer could determine the craft's orientation. [3]

Apollo 11 Command Module Pilot Michael Collins noted that the visibility through the optics was sub-standard, and it was difficult to see through in certain lighting conditions.

The sun shade was added late in the program, in 1967, after tests and modeling determined that the astronauts might not be able to see stars on the lunar surface due to direct sun light or light scattered by near-by parts of the LM impinging on the outside prism. Adding the sun shade also allowed increasing the number of view positions from three to six. [1] :p. 41 ff

AOT sun shade on Apollo 9 Lunar Module AS09-19-2963 AOT sun shade on Apollo 9.jpg
AOT sun shade on Apollo 9 Lunar Module

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. In the argot of the Apollo program this matrix was known as REFSMMAT (for "Reference to Stable Member Matrix"). There were two reference coordinate system used, depending on the phase of the mission, one centered on Earth and one centered on the Moon.

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.

Apollo 11

During the Apollo 11 mission, two PGNCS alarms (1201 "No VAC areas available" and 1202 "Executive alarm, no core sets") were relayed to mission control as the first lunar landing was being attempted on July 20, 1969. The computer system overload was caused by the simultaneous capture of landing radar data and rendezvous radar data. Support staff at Mission control concluded that the alarms could be safely ignored and the landing succeeded. [4] [5]

See also

Related Research Articles

Apollo 8 First crewed space mission to orbit the Moon

Apollo 8 was the first crewed spacecraft to leave low Earth orbit, and also the first human spaceflight to reach another astronomical object, namely the Moon, which the crew orbited without landing, and then departed safely back to Earth. These three astronauts—Frank Borman, James Lovell, and William Anders—were the first humans to witness and photograph an Earthrise.

Apollo 7 First crewed flight of the Apollo space program

Apollo 7 was the first crewed flight in NASA's Apollo program, and saw the resumption of human spaceflight by the agency after the fire that killed the three Apollo 1 astronauts during a launch rehearsal test on January 27, 1967. The Apollo 7 crew was commanded by Walter M. Schirra, with command module pilot Donn F. Eisele and lunar module pilot R. Walter Cunningham.

Apollo 9 3rd crewed mission of the Apollo space program

Apollo 9 was the third human spaceflight in NASA's Apollo program. Flown in low Earth orbit, it was the second crewed Apollo mission that the United States launched via a Saturn V rocket, and was the first flight of the full Apollo spacecraft: the command and service module (CSM) with the Lunar Module (LM). The mission was flown to qualify the LM for lunar orbit operations in preparation for the first Moon landing by demonstrating its descent and ascent propulsion systems, showing that its crew could fly it independently, then rendezvous and dock with the CSM again, as would be required for the first crewed lunar landing. Other objectives of the flight included firing the LM descent engine to propel the spacecraft stack as a backup mode, and use of the portable life support system backpack outside the LM cabin.

Apollo Lunar Module Lander used in the Apollo program

The Apollo Lunar Module, or simply Lunar Module, originally designated the Lunar Excursion Module (LEM), was the Lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed spacecraft to operate exclusively in the airless vacuum of space, and remains the only crewed vehicle to land anywhere beyond Earth.

Apollo Guidance Computer Guidance and navigation computer used in Apollo spacecraft

The Apollo Guidance Computer (AGC) is a digital computer produced for the Apollo program that was installed on board each Apollo command module (CM) and Apollo Lunar Module (LM). The AGC provided computation and electronic interfaces for guidance, navigation, and control of the spacecraft.

Apollo 5 First uncrewed test flight of the Apollo Lunar Module

Apollo 5, also known as AS-204, was the uncrewed first flight of the Apollo Lunar Module (LM) that would later carry astronauts to the surface of the Moon. The Saturn IB rocket bearing the LM lifted off from Cape Kennedy on January 22, 1968. The mission was successful, though due to programming problems an alternate mission to that originally planned was executed.

A guidance system is a virtual or physical device, or a group of devices implementing a controlling the movement of a ship, aircraft, missile, rocket, satellite, or any other moving object. Guidance is the process of calculating the changes in position, velocity, altitude, and/or rotation rates of a moving object required to follow a certain trajectory and/or altitude profile based on information about the object's state of motion.

AS-201 1966 uncrewed, suborbital test flight within the Apollo program

AS-201, flown February 26, 1966, was the first uncrewed test flight of an entire production Block I Apollo command and service module and the Saturn IB launch vehicle. The spacecraft consisted of the second Block I command module and the first Block I service module. The suborbital flight was a partially successful demonstration of the service propulsion system and the reaction control systems of both modules, and successfully demonstrated the capability of the command module's heat shield to survive re-entry from low Earth orbit.

Apollo (spacecraft) Saturn V-launched payload (including adapter and escape tower) that took men to the Moon

The Apollo spacecraft was composed of three parts designed to accomplish the American Apollo program's goal of landing astronauts on the Moon by the end of the 1960s and returning them safely to Earth. The expendable (single-use) spacecraft consisted of a combined command and service module (CSM) and an Apollo Lunar Module (LM). Two additional components complemented the spacecraft stack for space vehicle assembly: a spacecraft–LM adapter (SLA) designed to shield the LM from the aerodynamic stress of launch and to connect the CSM to the Saturn launch vehicle and a launch escape system (LES) to carry the crew in the command module safely away from the launch vehicle in the event of a launch emergency.

Saturn V instrument unit Ring-shaped structure

The Saturn V instrument unit is a ring-shaped structure fitted to the top of the Saturn V rocket's third stage (S-IVB) and the Saturn IB's second stage. It was immediately below the SLA (Spacecraft/Lunar Module Adapter) panels that contained the Apollo Lunar Module. The instrument unit contains the guidance system for the Saturn V rocket. Some of the electronics contained within the instrument unit are a digital computer, analog flight control computer, emergency detection system, inertial guidance platform, control accelerometers, and control rate gyros. The instrument unit (IU) for Saturn V was designed by NASA at Marshall Space Flight Center (MSFC) and was developed from the Saturn I IU. NASA's contractor to manufacture the Saturn V Instrument Unit was International Business Machines (IBM).

Apollo command and service module Component of the Apollo spacecraft

The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship, which carried a crew of three astronauts and the second Apollo spacecraft, the Apollo Lunar Module, to lunar orbit, and brought the astronauts back to Earth. It consisted of two parts: the conical command module, a cabin that housed the crew and carried equipment needed for atmospheric reentry and splashdown; and the cylindrical service module which provided propulsion, electrical power and storage for various consumables required during a mission. An umbilical connection transferred power and consumables between the two modules. Just before reentry of the command module on the return home, the umbilical connection was severed and the service module was cast off and allowed to burn up in the atmosphere.

Flight controller Person who aids in spaceflight activities

Flight controllers are personnel who aid space flight by working in such Mission Control Centers as NASA's Mission Control Center or ESA's European Space Operations Centre. Flight controllers work at computer consoles and use telemetry to monitor various technical aspects of a space mission in real time. Each controller is an expert in a specific area and constantly communicates with additional experts in the "back room". The flight director, who leads the flight controllers, monitors the activities of a team of flight controllers, and has overall responsibility for success and safety.

Lunar escape systems Series of proposed emergency spacecraft for the Apollo Program

Lunar escape systems (LESS) were a series of emergency vehicles designed for never-flown long-duration Apollo missions. Because these missions were even more hypothetical than the planned cancelled Apollo missions, the designs were never constructed. This concept was an outgrowth of the Lunar Flying Vehicle designed by Bell Aerospace.

Jack Garman

John Royer "Jack" Garman was a computer engineer, former senior NASA executive and noted key figure of the Apollo 11 lunar landing. As a young specialist on duty during the final descent stage on 20 July 1969 he dealt with a series of computer alarms which could have caused the mission to be aborted.

Apollo Abort Guidance System

The Apollo Abort Guidance System was a backup computer system providing an abort capability in the event of failure of the Lunar Module's primary guidance system during descent, ascent or rendezvous. As an abort system, it did not support guidance for a lunar landing.

Transposition, docking, and extraction Maneuver done by the Apollo command and service module

Transposition, docking, and extraction was a maneuver performed during Apollo lunar landing missions from 1969 to 1972, to withdraw the Apollo Lunar Module (LM) from its adapter housing which secured it to the Saturn V launch vehicle upper stage and protected it from the aerodynamic stresses of launch. The maneuver involved the command module pilot separating the Apollo Command and Service Module (CSM) from the adapter, turning the CSM around, and docking its nose to the Lunar Module, then pulling the combined spacecraft away from the upper stage. It was performed shortly after the trans-lunar injection maneuver that placed the Apollo spacecraft on a three-day trajectory to the Moon. The docking created a continuous, pressurized tunnel which permitted the astronauts to transfer internally between the CSM and the LM.

Albert L. Hopkins Jr. was an American computer designer. He worked at the US MIT Instrumentation Laboratory during the development of the Apollo Guidance, Navigation, and Control System, or the GN&C. The system was designed in two forms, one for the command module and one for the lunar module. The CM version included an optical system with an integrated scanning telescope and sextant for erecting and correcting the inertial platform. Albert Hopkins received a Ph.D. from Harvard University under Howard Aiken, he then joined the MIT Instrumentation Lab where he was Assistant Director; together with Ramon Alonso, and Hugh Blair-Smith he was a member of the group that designed the computer, designated AGC for Apollo Guidance Computer, identical in the CM and LM.

Inertial navigation system Continuously computed dead reckoning

An inertial navigation system (INS) is a navigation device that uses a computer, motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate by dead reckoning the position, the orientation, and the velocity of a moving object without the need for external references. Often the inertial sensors are supplemented by a barometric altimeter and sometimes by magnetic sensors (magnetometers) and/or speed measuring devices. INSs are used on mobile robots and on vehicles such as ships, aircraft, submarines, guided missiles, and spacecraft. Other terms used to refer to inertial navigation systems or closely related devices include inertial guidance system, inertial instrument, inertial measurement unit (IMU) and many other variations. Older INS systems generally used an inertial platform as their mounting point to the vehicle and the terms are sometimes considered synonymous.

Unified S-band

The Unified S-band (USB) system is a tracking and communication system developed for the Apollo program by NASA and the Jet Propulsion Laboratory (JPL). It operated in the S band portion of the microwave spectrum, unifying voice communications, television, telemetry, command, tracking and ranging into a single system to save size and weight and simplify operations. The USB ground network was managed by the Goddard Space Flight Center (GSFC). Commercial contractors included Collins Radio, Blaw-Knox, Motorola and Energy Systems.

Don Eyles is a retired computer scientist who worked on the computer systems in the Apollo Lunar Module vehicle. As a young engineer during the lunar landing on Lunar Module Eagle on 20 July 1969 he assisted with a series of computer alarms caused by data overflow from the radar, which could have caused the mission to be aborted.

References

  1. 1 2 Holley, M. D. (May 1976). "Apollo Experience Report--Guidance And Control Systems: Primary Guidance Navigation And Control System Development, NASA TN D-8287" (PDF). Lyndon B. Johnson Space Center, United States. National Aeronautics and Space Administration.
  2. The Silent War: The Cold War Battle Beneath the Sea, John Pina Craven, Simon and Schuster, 2002, p.120
  3. The Apollo Lunar Module Alignment Optical Telescope, Apollo Lunar Surface Journal
  4. Eyles, Don (2004-02-06), Tales from the Lunar Module Guidance Computer , retrieved 2017-10-01
  5. "Apollo 11 Lunar Surface Journal: Program Alarms". www.hq.nasa.gov. Retrieved 2017-04-16.