NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.


NASA SP-504: Space Shuttle Avionics System

Preface

THE SPACE SHUTTLE avionics system represents a significant advance in avionics system technology. The system was conceived in the early 1970's, developed throughout that decade, and became operational in the 1980's. Yet even today in 1988, it remains the most sophisticated, most advanced, most integrated avionics system in operational use in the aerospace arena. Some of the more significant "firsts" achieved by the system include the following.

Such pioneering innovations and concepts are remarkable in that they emerged in a design environment which would be considered archaic by today's standards. For instance, the data processing state of the art has turned over at least four times since the Space Shuttle design was conceived. In 1974, there were no off-the-shelf microcomputers, large-scale integrated-circuit technology was emerging but immature, and the use of data buses for critical functions was considered to be radical and of high risk. Prior to the Space Shuttle, aerospace systems were made up of an essentially independent collection of subsystems, organized along disciplinary lines such as flight control, guidance and navigation, communications, and instrumentation. Each subsystem typically had its own dedicated controls, displays, and command and signal paths. The Space Shuttle avionics system not only integrated the computational requirements of all subsystems in one central computer complex, but introduced the concept of multifunction controls, displays, and command/data paths.

The overall system design was driven by mission requirements and vehicle constraints never before encountered in a space program. Significant among these were the following.

A myriad of other mission, vehicle, and system requirements influenced or dictated various aspects of the design; however, the basic system concepts were derived from those described. 

The Space Shuttle avionics system which evolved features a five-computer central processing complex, which provides software services to all vehicle subsystems that require them. Each computer is connected to a network of 28 serial digital data buses, which distribute input/output commands and data to/from bus terminal units located throughout the vehicle. Dedicated hardware components, unique to the various subsystems, interface as necessary with bus terminal unit signal conditioning devices. During critical mission phases such as ascent and entry, the system is configured in four redundant, independent but synchronized strings, each controlling one-fourth of the redundant sensors and control effectors required for the operation in progress. A backup, simplex software package is installed in the fifth computer to be used if a generic error causes failure of the total redundant set. During more benign mission phases such as on-orbit, the computer complex can be configured, by loading the appropriate software programs, to perform a wide variety of mission and payload support functions.

The system includes more than 270 components, depending on the mission, and uses approximately 500 000 lines of software code. Although very complex and difficult to describe or understand, the system has proven to be reliable, durable, extremely versatile, and a tribute to the multitudes who contributed to its design, development, and verification.


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