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.

2.1.4 Data Processing

A large amount of data can be generated in flight by scientific experiments as well as by other spacecraft subsystems, such as guidance and navigation instrumentation. Depending upon the type of data and the extent of processing desired, the onboard computer speed and size requirements may be significantly altered. The engineering and scientific data generally have a high bandwidth and are available in real time only, corrupted by noise and not in a convenient form. To reduce this data to a manageable and comprehensible form, without losing the information content, it may be processed through an onboard computer. Such processing can include filtering, smoothing, estimating, averaging, compression, and formatting for display or transmission.

Even for small scientific spacecraft, an onboard computer can be used as the master control for collecting data and formatting it for telemetry. Since the power consumption of the computer is generally much lower than that of the telemetry transmitter, it is easy to justify substantial computation to achieve a moderate amount of data compression. For example, the use of a small central computer in place of the individual processors on the three IMP-F satellite experiments on Explorer 34 would have reduced the average power consumption by about 70%, without increasing the size or weight requirements (ref. 18). A general-purpose stored-program computer, the SDP-3, has been designed at the NASA Goddard Space Flight Center (GSFC) for use as the core of the data system of small scientific spacecraft and will be flown as an engineering experiment on IMP-1 (ref. 19). As an example of onboard data processing requirements, the reduction of 238 pieces of raw data from the IMP-F magnetic field autocorrelation experiment to one average value and nine other statistical values is estimated to require 150 memory words and 340 operations per second. Calculating a sum, a sum of squares, and their ratio for the plasma statistics experiment on that spacecraft (ref. 20) would require about 140 memory words and 140 operations per second (ref. 7).

The computer can also serve as the hub for data transfer through the spacecraft, such as the IBM primary processor and data storage system (ref. 21) on the Orbiting Astronomical Observatory (OAO). A more sophisticated, general-purpose central computer, the NASA/GSFC-Westinghouse onboard processor (OBP), was scheduled to be flown on the OAO-B (refs. 22 and 23). Had the launch been successful, the OBP would have taken a central role in the spacecraft's operation and control as well as data handling.

Reference 24 discusses some of the many ways onboard computers may be used to enhance experimentation in space and indicates the wide range of computer requirements which might be encountered. Estimates of the computer memory and speed requirements for several individual experiments are summarized in table 2 (from ref. 24).

TABLE 2. - Summary of Computer Requirements for Typical Experiments (ref. 24)

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