There have definitely been cubesats flown without rad-hard
architecture manufactured into the chips. They use the lock-step method
you've mentioned with lengthy lifespans ( for a cubesat). I believe the
Ardu-sat cubesats where such spacecraft. Those motherboards were just a
bunch of ATmega328P microcontrollers which are not in the least bit
rad-hardened. Both Ardusats lasted until reentry, about 8 months.
It's important to remember any lock step sequence of cpus is going to have
a rock solid Watch Dog Timer making sure if any of the cpus are hit with
radiation or a software bug hangs the system, the whole system is
restarted fresh, resetting all the pins. There's enough literature for
spacecraft in low earth orbit to give the impression lock-step rad
hardening can be trusted. Over long missions however, your going to run
into esoteric problems such as space radiation, electrical current and
zero-gravity working in concert to cause metal hairs similar to a mans
beard to growing off the circuit board causing short circuiting. Maybe the
lamination method mentioned above would help address the metal beard hair
problem.
On Fri, Jan 18, 2019 at 2:11 PM justin corwin <outlawpoet@xxxxxxxxx> wrote:
It's mildly more power efficient, at least in theory, to do a single CPU
with equivalent internal redundancy like the ARM in that kit, and they
further improve things by metal packaging and their proprietary HARDSIL
process, which sounds like design guidelines for geometry and some kind of
lamination process, which reduces the amount of redundancy required to hit
their metrics.
The advantage of full lockstep is that it addresses failures in the voting
and error correction process as well, and can be scaled, so you can get
much longer lifetimes at higher rads than is possible (you'd run out of
space for internal redundancy within a single core/chip). And of course you
can use rad-hardened chips in lockstep themselves, which improves your math
further.
On Fri, Jan 18, 2019 at 11:46 AM adam paul <pauladam316@xxxxxxxxx> wrote:
That's really interesting, thanks!
What is the advantage of a traditional rad-hard CPU versus a lockstep
solution? To the best of my knowledge products like Pumpkin Space's
rad-hard board (
https://www.pumpkinspace.com/store/p199/Vorago_Rad-hard_OBC_%2F_C%26DH_Avionics_Kit.html)
Uses a traditional method. Is it more efficient as commands only need to be
run once?
Thanks,
Adam
On Fri, Jan 18, 2019 at 2:18 PM justin corwin <outlawpoet@xxxxxxxxx>
wrote:
Lots of safety critical applications use multi core or multi chip in
lockstep to prevent bit errors from causing problems, that's not
particularly new. In fact, I'm almost certain a TI Hercules processor was
used in a cubesat, which is by default, a two ARM core lockstep solution.
SpaceX may be trying to save money by lockstepping or voting cheaper
CPUs and trusting their mean time to failure calculations to cut closer to
the bone, but it's an established technique you can apply at multiple
levels. In fact, some "radiation tolerant" controllers are just FPGAs who
are running lockstep IP blocks on a low level with error correction or
voting circuits built in at a low level.
On Fri, Jan 18, 2019 at 11:09 AM adam paul <pauladam316@xxxxxxxxx>
wrote:
Hi Everyone,
I was looking at the different ways that spacecraft have approached
radiation hardening for components in the past. It seems like the old way
to do things was to use components specifically designed to be radiation
tolerant, which often resulted in using components that were older and not
state-of-the-art.
However, it seems like SpaceX is using a new method of rad-hardening
Dragon by using multiple CPUs running code in parallel and comparing
results to avoid bit flips. My question is, are there other examples of
spacecraft doing this, and has it ever been tried in a cubesat? If not, why
not?
Thanks everyone,
Adam
--
Justin Corwin
outlawpoet@xxxxxxxxx
http://programmaticconquest.tumblr.com
--
Justin Corwin
outlawpoet@xxxxxxxxx
http://programmaticconquest.tumblr.com
