Project Apollo: The Beginnings
| Mission Planning
| Landing Site Selection
| Earthbound Support Systems
Astronaut Selection and Training | The Saturn V | The Saturn 1B | The Apollo Spacecraft
Guidance and Navigation | Command and Service Modules | The Lunar Module
Assembling and Launching | Pathfinders | The Early Missions | Apollo 11, The First Landing
The Intermediate Missions | Apollo 15 Exploration | Apollo 16 Exploration | Apollo 17 Exploration
Skylab and Apollo-Soyuz | Conclusion
While many of the aspects of the construction of the Apollo and Saturn hardware was based on existing technology, one aspect of the project, the spacecraft's guidance and navigation required extensions into new disciplines. To place a spacecraft into orbit and track it around the earth was one thing, but to send it off beyond the gravitational influence of the earth and have it arrive at a precisely defined point just 60 miles in front of another planetary body, at a precise speed and time would require some original thinking.
It was considered desirable to provide the crew with the facility to input their own navigational data in the event of an emergency situation or loss of contact with the ground. Some of the manoeuvres necessary for a manned landing by the LM and initiation of the Trans Earth Injection (TEI) manoeuvre would have to be initiated by the crew out of sight of the earth. This would require a degree of autonomous control and the need for an on-board computer to be capable of controlling the navigation, flight guidance and control computations without earthbound assistance.
As both the CSM and the LM were at times required to operate independently of each other during the mission, each craft was equipped with its own independent means of navigation. Although the individual computers were practically identical, the command module's computer would handle guidance between the earth and the moon and in lunar orbit, while the lunar module's provided guidance during the lunar landing, ascent and rendezvous. The software code differed for the separate purposes and that used in the command module was named 'Colossus' and a second program, 'Luminary' was used in the lunar module.
Command Module (GandN)
The G and N system of the Apollo spacecraft comprised three main subsystems, computer, inertial and optical. The Apollo Guidance Computer (AGC), kept a running log on the spacecraft's velocity and position as it changed due to gravitational effects of the earth and moon, matching it with a pre-planned flight trajectory. The AGC received data from its own Inertial Measurement Unit (IMU), measuring rotation of the spacecraft around the three axes of pitch, roll and yaw and from accelerometers which measured changes in the spacecraft's velocity during engine firings.
Inevitably, due to precession in the IMU's gyroscopes, alignment tended to drift off and required periodic resetting. This was achieved through the spacecraft's optical system (OPS), by taking two star sightings through its sextant and comparing readings with a known list of 37 stars who's positions were stored in the computer's memory. This fix, when input to the AGC would establish the spacecraft's attitude.
The computations and control commands during a lunar flight were to be carried out by ground based equipment at MCC and uplinked to the spacecraft. By measuring the time taken to transmit and return a ranging signal and its Doppler shift, the spacecraft's range and speed could be determined with a high degree of accuracy. This data was included in a mathematical computation, a 'state vector', which was transmitted to the spacecraft and input to its guidance computer which enabled the establishment of the spacecraft's position and speed.
Crew input into the AGC was through the Display and Keyboard system (DSKY) a 21 digit display panel and 19 pressbutton keyboard, referred to as the 'disky'. Selection of programmed manoeuvres and their initiation was controlled by the crew's input of two figure numerical commands, which called up the required program from the computer and the action required.
CM Flight Control
The G and N system was directly linked to the spacecraft's Stabilisation and Control System (SCS). Computations made by the AGC provided a 'thrust vector' to align the main engine for major velocity and direction changes and to control the spacecraft's rotation about its axes. The SCS also permitted manual control of spacecraft attitude through two translation (hand) controllers which allowed the CM Pilot to fire external thrusters to manoeuvre the spacecraft.
Lunar Module Primary Guidance Navigation and Control (PGNCS)
Once separated from the CSM, the LM had to navigate its own way to the lunar surface and on return, manoeuvre to a rendezvous with the waiting CSM. For this purpose it carried two guidance systems to manage these complex manoeuvres. The Primary Guidance Navigation and Control System (PGNCS), referred to as 'pings', utilised its own Lunar Module Guidance Computer (LGC), which was essentially a duplication of the AGC computer used in the command module. Manual input to the computer was through a DSKY, located at the commander's station.
To manoeuvre the spacecraft, the PGNCS computer automatically controlled thrusters mounted externally on the ascent stage and the descent engine's thrust and alignment to maintain the desired flight path, through a series of programs entered by the crew through the DSKY. The LGC received data from its own inertial guidance platform that could be updated through its optical star sighting system. During landing the computer also received data from a landing radar sub-system, which became operative from approximately 40,000 feet above the lunar surface, providing height and rate of descent of the LM. During the LM's ascent, a separate rendezvous radar updated the LGC after locating the CSM at ranges of up to 350 miles and provided range and rate of closure with its target.
The landing radar, rendezvous radar and stabilisation and control system were supplied by Aerospace Communications and Control, a division of the Radio Corporation of America (RCA). RCA designed and manufactured the rendezvous radar and bought in the landing system from Ryan Aeronautical Co who had previously developed the landing radar for the Surveyor soft landing lunar probes.
The Abort Guidance System (AGS)
In the lunar module the PGNCS was backed up by a separate, independent computer, the Abort Guidance System (AGS) referred to as 'aggs'. The AGS incorporated its own strapped-down inertial system and with data from the landing or rendezvous radar providing a running check on the PGNCS function. In the event of a PGNCS failure it provided guidance and control for the ascent stage back to rendezvous with the CSM, or an orbit from which it could be rescued. Input to the AGS was through its own DSKY at the lunar module pilot's position. It was never found necessary to use the AGS on any of the lunar missions except under test conditions.