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Re: Flight Computers - the MIPS FLOPS POPS and HOPS fad



Assi,

>1. Build a slow and RELIABLE OBC. It could be slow, but it would run a well
>written bulletproof task to manage the spacecraft. Use an older CPU which is
>built using larger gate size, and thus more reliable.
>2. Add a funky high tech cpu which will enable you to run all the payload

I agree with the need to isolate the critical housekeeping features
from the other stuff. But using two separate computers is only one way
to provide that isolation. Today it may not even be the best way.

>Well, that's all nice, and for a while it was a hot item. But then everyone
>found out that when a mega MIPS OBC has to run housekeeping tasks plus a
>number of other complex tasks, it becomes really difficult to schedule, much
>less reliable, and alsmost impossible to manage.

The newer CPUs not only provide raw speed, which as you say may not be
all that important for simple housekeeping, but also the basic
protection features that could allow housekeeping and application
tasks to safely share the same computer.

There's a real cost to multiple, dedicated computers.  Every piece of
hardware in a spacecraft entails weight, volume and complexity. With
typical launch costs ranging from $10,000/kg (LEO) to $20,000/kg
(GTO), every module box, connector, cable harness, IC and PC board in
a spacecraft might as well be made from solid gold or platinum given
what it'll cost you to get it into orbit. That alone makes me want to
look very hard at the possibility of doing everything on a single
computer,

You may decide it's worth the extra cost to fly a second, redundant
computer.  But redundancy implies interchangeability; two computers
are actually worse than one if they can't do each other's job in the
event one fails, and if both are needed for the spacecraft to serve
its intended function as opposed to merely keeping its batteries
charged.

Phil



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