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Computers in Spaceflight: The NASA
Experience
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- - Chapter Six -
- - Distributed Computing On Board
Voyager and Galileo -
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- Future unmanned spacecraft
computers
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- [203] Distribution of computers aboard spacecraft
has now been done several times. Both Voyager spacecraft inherited
command computers from the Viking project. Computers for specific
functions such as attitude control and data formatting were added
in response to increased requirements. The result was a
functionally distributed system of processors. Galileo's project
managers also adopted the concept of functional distribution,
assigning microprocessors to control attitude and, in the
lower-level modules of the Command and Data Subsystem, to connect
to engineering and other instruments, including the scientific
experiments. Additional innovations on the Galileo spacecraft
centered on the development of virtual machine software, which
distributes functions over several processors.
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- Advancing microprocessor technology makes
the continuation of the concept of single function computers more
attractive. At JPL, plans are currently under way for the Mariner
Mark II, which will be the deep space version of the Multimission
Modular Spacecraft developed by Goddard Space Flight Center for
earth orbital operations. Using the same concepts of a standard
bus and modular equipment, JPL hopes to reduce mission costs to
$400 million each, about half the price of
Galileo147. The staff is exploring the use of the C
programming language, a very powerful tool, for the new
spacecraft. Future missions seem certain to use multicomputers,
with internal networks similar to Galileo's. In the 15 years since
the first primitive programmable sequencers flew with 128 word
memories, JPL spacecraft have grown to carry 2,500 times more
memory. Progressing from simple counting to complex coordinate
transformations in such a short time is remarkable, and the
application of computer power to each spacecraft function will
make for ever more remarkable gains.
