QUESTION:
What kind of design margins and fault tolerance do you build into the
computers in these spacecraft? Do you stretch the devices to their maximum
specs as we do in commercial designs, or have very conservative margins? Is
ther any special PCB lay-out, fabrication method?

ANSWER from Tim Gallagher on July 23, 1997:
The Mars Pathfinder's electronics are a combination of radiation-hardened
devices such as the IBM RISC processor (the main computer) and
radiation-tolerant military-spec circuits (i.e. higher temperature and
screening processes then a commercial device).

For example the IMP Camera control electronics uses a radiation-tolerant
(still susceptible to high-energy particles) but high temperature range
field-programmable gate array (FPGA) to perform most digital functions.  We
derated the FPGA during simulations for both radiation effects and
temperature effects.  On longer missions we also derated (added timing
margins) for end-of-life effects (total dosage radiation, age, etc.).  We
try to use good noise techniques in development of our PCB lay-outs such as
multiple ground planes, adequate bypass filtering, and control impedance
lines (for higher speed circuits) as example.