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The technological marvel that facilitated the Apollo missions to the Moon was the on-board computer. In the 1960s most computers filled an entire room, but the spacecraft’s computer was required to be compact and low power. Although people today find it difficult to accept that it was possible to control a spacecraft using such a ‘primitive’ computer, it nevertheless had capabilities that are advanced even by today’s standards.

This is the first book to fully describe the Apollo guidance computer’s architecture, instruction format and programs used by the astronauts. As a comprehensive account, it will span the disciplines of computer science, electrical and aerospace engineering. However, it will also be accessible to the ‘space enthusiast’. In short, the intention is for this to be the definitive account of the Apollo guidance computer.

Frank O’Brien’s interest in the Apollo program began as a serious amateur historian. About 12 years ago, he began performing research and writing essays for the Apollo Lunar Surface Journal, and the Apollo Flight Journal. Much of this work centered on his primary interests, the Apollo Guidance Computer (AGC) and the Lunar Module. These Journals are generally considered the canonical online reference on the flights to the Moon. He was then asked to assist the curatorial staff in the creation of the Cradle of Aviation Museum, on Long Island, New York, where he helped prepare the Lunar Module simulator, a LM procedure trainer and an Apollo space suit for display. He regularly lectures on the Apollo computer and related topics to diverse groups, from NASA's computer engineering conferences, the IEEE/ACM, computer festivals and university student groups.



0. The state of the art

The dizzying pace of technological change is so ingrained into our consciousness, it is impossible to remember what was considered “the state of the art” as recently as a few decades ago. Consider the first practical computer – the room sized, vacuum tube ENIAC. In the years since its creation in 1946, the power of perhaps a million ENIACs has shrunk to an attractive package that comfortably fits in a briefcase. In such a 21st century laptop, with its multiple processors and gigabytes of memory, there is rarely any thought given to its internal complexity.

1. The AGC hardware

In our technological society, we define the concept of computer “processing” by the task we need to perform. Producing a video, optimizing an engine’s performance or fulfilling an online book order all fall under the generic definition of “processing”, but say nothing about how that magic is performed. Stripped of their elaborate interfaces, clever optimization tricks and marketing hype, all computers contain the same three major components: a central processing unit or CPU, memory, and input and output devices. The processor performs all the numerical and logical operations of the computer, and is the “traffic cop” that coordinates the movement of data within the system.

2. The Executive and Interpreter

In the most elementary computers, executing a program is a relatively easy task. Without having to accommodate multiple programs with potentially conflicting requirements, the user only has to load the instructions into memery, set the starting address, and watch it run. Indeed, for early computers (and specialized controller chips today), this “load and go” methodology works acceptably well. For the first decade of the computer age, the modern concept of an operation system simply did not exist. Programmers were required to write software to operate tape drives, printers and other peripherals in addition to the logic required to solve the business or scientific problem at hand. Over time, primitive ‘executive’ programs were introduced to reduce the burden of managing hardware. Rather than directly commanding a device such as a card reader, a program would invoke a pre-written routine to operate the external hardware on its behalf. Delegating such ‘low level’ operations to standardized and (hopefully) reliable interfaces improved productivity tremendously by eliminating the tiresome chores of system management.

3. The basic of guidance and navigation

Alone by itself, the computer is blind to the world around it, lacking the most basic information on orientation and the accelerations that the spacecraft undergoes. Without a source for this data, the AGC’s role of guidance computer is invalidated. Two additional components are necessary for a completely integrated guidance, navigation and control package: an Inertial Measurement Unit (IMU) to provide both attitude and acceleration data, and an optics system to initialize the platform and perform navigation fixes using stellar observations.

4. Mission programs and operations

Up to this point, the discussion of the AGC has necessarily taken the approach of an anatomy lesson. Each organ is placed in full view, described and dissected, and its interactions with other body parts are described. Now that the major components of the AGC are introduced, it is possible to take the next step and consider how the perform real mission functions. Of particular interest is to extend the scope outside the narrowly defined world of the AGC, its platform and optics, and the DSKY.


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