Computers in Spaceflight: The NASA Experience

Part Two: Computers On Board Unmanned Spacecraft
[135] Unmanned spacecraft computers differ from manned spacecraft computers in that they are designed to work much longer and use much less spacecraft resources. A typical manned mission lasts a week or less; the exception was Skylab, whose computers operated for 9 months straight and again later during its reactivation mission. Unmanned missions in earth orbit or to the outer planets can last a decade or longer. Manned spacecraft usually carry large auxiliary power units based on fuel cell technology, as power requirements for life support, experiments, and computers are high. Spacecraft in earth orbit are often dependent on solar cell arrays, which are by nature low-power generators. Interplanetary probes use either solar cells or small radioisotope generators. Clearly, these circumstances cause different requirements for computers.
Of the two types of unmanned spacecraft, one is designed for earth orbit operations and the other flies to the moon, planets, or deep space. Earth orbiters usually need no navigation after achieving orbit; space probes, however, are critically dependent on proper guidance. Earth orbiters can be commanded nearly instantaneously from the ground during the roughly 10% of the time they are "visible" to ground stations. Interplanetary probes need to be autonomous, at least capable of independent routine operation, due to speed of light delays in communication and longer periods out of earth control. Multiple missions and simple geography prevent interplanetary probes from being in constant contact with the three Deep Space Network stations. Therefore, the basis of fault handling on an interplanetary probe is failure detection and repair, whereas earth orbiters concentrate on "safing" the spacecraft until the ground stations can help out. For these reasons, computers became more sophisticated on spacecraft designed to leave the gravity pull of the earth.
Moreover, the different computers have distinct origins. Many near earth spacecraft used a variant of a single machine developed at the Goddard Space Flight Center, whereas the Jet Propulsion Laboratory (JPL) of the California Institute of Technology, a long time NASA contractor, has dominated computer construction for deep space flight, designing and building an evolving series of computers for the Agency's interplanetary probes. These two lines of development represent the most fruitful of NASA's forays into computer [136] research. Computers on manned spacecraft were generally developed from other computers (Apollo from Polaris; 4Pi from the System 360). Computers in ground operations were adapted from commercial machines. However, computers on unmanned spacecraft were custom designed. In these cases, NASA was not only a contract monitor but was actively involved in development.
The making of the first NASA Standard Spacecraft Computer, which has controlled a number of earth-orbiting missions, has been described elsewhere*. As can be inferred by its name, NASA designed this computer to fly on multiple, varying missions, which it has done to good effect. For example, both the Solar Maximum Mission and the Hubble Space telescope used the computer. Goddard Space Flight Center led development of the device over a 10-year period from the late 1960s to the late 1970s.

Figure B
Figure B: The NASA Standard Spacecraft Computer I in its packaging. (NASA photo)

In contrast, machines built at JPL have had a longer and more related history. Although some reuse has occurred, the, various space probes built at JPL carried mission-unique computers of increasing [137] power and complexity, paralleling the development of computers for manned spacecraft. Unlike the manned programs, however, JPL sponsored fundamental research into spacecraft computing, which was then translated into concepts that guided the development of flight systems. The result was a series of innovative and flexible on-board computers.

*See Raymond G. Hartenstein, Ann C. Merwarth, William N. Stewart, Thomas D. Taylor, and Charles E. Trevathan, "Development and Application of NASA's First Standard Spacecraft Computer," Commun. ACM, 27(9), 902-913 (September 1984.)

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