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

Section 3  Payload Support

Because the primary purpose of the Space Shuttle is to deliver, recover, and otherwise service a wide variety of payloads, high priority was given early in the program to incorporating features in the avionics system which would support such operations. As indicated in the Data Processing section, one of the five computers in the data processing complex was originally designated for payload support. Even though this machine eventually became the residence of the backup system during ascent and entry, some payload servicing functions were retained. On orbit, where the redundant set was not required and only one or two of the machines were needed for Orbiter operations, the excess computational capacity was to be made available for payload use. The payload major function, one of the three major software partitions, was created to assemble all payload support programs. The intent was to store these programs in mass memory and to load them as appropriate into one of the unused computers during the on-orbit phase. Two data buses and two multiplexer/demultiplexers (MDM's) were allocated to payload services. A version of the MDM called a "flex MDM," which could easily be reconfigured, was developed especially to accommodate the unique interface requirements of various payloads.

For various reasons, these system features incorporated for payload support have not yet been fully exploited. The difficulty in deriving or distilling a reasonable set of support requirements from the payload community was discussed in the Communications section. In addition, the rigor required and the cost and lead time involved in developing and verifying flight software have discouraged any extensive tailoring of programs to specific payloads. A standard set of services, largely associated with the system management function, has evolved and is provided on each flight. These include caution and warning, limit sensing and status determination, sequencing, and command and telemetry formatting. The GNC function provides pointing information and state vector data as required.

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