The Impact of Risk Management: An Analysis of the Apollo and cev guidance, Navigation and Control Systems
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- Apollo GNC Computer Hardware
- Lunar Module Landing System Architecture
- PGNCS Architecture
Apollo GNC SystemThe MIT Instrumentation Laboratory under Charles Stark (Doc) Draper received the contract to provide the primary navigation, guidance, and control for Apollo in August of 1961. At the time, NASA was still debating the decision on how to land on the moon. Whether one large rocket or a small lunar module that descended to the moon, one of the first major technical decisions made was the need to have the ability to autonomously guide the spacecraft to the moon, land it safely, and return the astronauts back to Earth. The Instrumentation Lab was the pioneer of inertial guidance, navigation, and control. Doc Draper had first applied the use of gyros on the Mark 14 gun sight during WWII. The effectiveness of the system led to more advanced applications, including self-contained inertial systems on aircraft and missiles. By the mid 1950's, the Instrumentation Lab was working on a number of applications of inertial guidance including the Air Force's Thor missile, the Navy's Polaris missile, and a robotic Mars Probe [HALL40]. The Apollo requirements for self-contained guidance, navigation, and control were similar to the projects completed at the Instrumentation Lab, but it would also be a lot more complex. Apollo would require a much more powerful computation system than any of their previous projects. This computer could be either analog or digital. The decision to use a digital computer was one of the first major decisions made and one with many risk-associated implications. While it is conceivable that an analog computer could have accomplished the requirements of Apollo, the system would have been much bigger and heavier than the eventual digital computer developed by MIT [HHBS]. An analog computer would also have been much more difficult to program, and the tasks it performed would have been much more limited, with consequences for the design of the rest of the spacecraft and mission. The engineers at MIT had a very good reason for choosing digital over analog; they had gained a lot of experience with digital computers from their previous projects. To apply the guidance and control equations for the Polaris missile, MIT had developed a set of relatively simple equations that were implemented using digital differential analyzers. The digital differential analyzer designed by MIT was nothing more than some memory registers to store numbers and adders that produced the result of the incremental addition between two numbers. Although simple by computational standards, the work on the Polaris digital system provided the necessary base of technology needed for the Apollo Guidance Computer (AGC). Wire interconnections, packaging techniques, flight test experience, and the procurement of reliable semiconductor products were all required for the successful delivery of the AGC [HALL44]. In the late 1950's, the Instrumentation Lab was granted a contract to study a robotic mission to Mars. The mission would involve a probe that would fly to Mars, snap a single photo, and return it safely to Earth [BAT]. The requirements for the proposed probe led to the development of the Mod 1B computer. The computer would have been responsible for navigation and control of the probe through its mission had it been launched. The resulting computer used core-transistor logic and core memories. It was a general-purpose computer, meaning it could be programmed, unlike the Polaris system. While the Polaris computer could only calculate one set of equations, the Mod 1B computer could be programmed to perform any number of calculations. Although the Mars probe was canceled before it was built, the computer continued to evolve and provided the necessary knowledge and experience needed for the design of the AGC hardware. Apollo GNC Computer HardwareTwo identical computers were used on Apollo. One was used on the Command Module (CM) and the other in the Lunar Module (LM). The hardware on each was exactly the same, as required by NASA. This requirement meant that the design of the computer was more difficult as the computer had to interface with different and unique equipment for the CM and LM. In addition, since different contractors built the CM and LM, any changes to the computer meant that North American, Grumman, and MIT had to agree to the changes. The primary advantages of having the same computers on both spacecraft were simplified production and testing procedures. Lunar Module Landing System ArchitectureThe systems involved with the LM landing system consisted of several major components. Among them were the Primary Guidance, Navigation and Control System (PGNCS), the Abort Guidance System (AGS), the landing radar, the LM descent engine, reaction control system (RCS) jets, and various crew interfaces. The PGNCS included the IMU for inertial guidance, and the digital computer. Within the computer was a digital autopilot program (DAP) and manual control software. The AGS, to be discussed further in section xxx, was responsible for safely aborting the descent and returning the LM ascent stage back to lunar orbit if the PGNCS were to fail. Although it was never used in flight, the AGS served to mitigate some of the risk associated with the single-string primary computer. 
There were several crew interfaces required during landing, which will be covered in more detail. Among these were the DSYK (discussed in detail in section x), which is used by the astronauts to call various programs stored on the computer, a control stick to perform manual control of the spacecraft, and a grid on the commander's forward window called the Landing Point Designator (LPD). The window was marked on the inner and outer panes to form an aiming device or eye position. The grid was used by the astronaut and computer to steer the LM to a desired landing site. By using a hand controller, the commander could change the desired landing spot by lining up a different target as seen through the grid on his window [BEN]. PGNCS ArchitectureThe Primary Guidance, Navigation, and Control System (PGNCS) architecture on board the LM included two major components (See Figure 39 HALL). The AGC was the centerpiece of the system. It was responsible for calculating the state vector (position and velocity) of the vehicle and interfaced with the crew and other systems on board. The second part of the PGNCS was the Inertial Measurement Unit (IMU). The IMU provided inertial measurements from gyros and accelerometers. These measurements were integrated to derive the vehicle's position and velocity. 
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