[ale] [OT] grounding in space

[Jim Kinney]

Thanks, Alex!

I'm not planning to build and launch microsatellites any time soon but always enjoy learning about new (to me) tech. OK. So I maybe a little bit want to launch my own sputnik :-)

The voyager probes must be electronic tanks. The circuits are built from components that are much larger so while they have larger radiation exposure cross section they have a much larger operational material cross section.

On August 28, 2017 9:38:45 PM EDT, Alex Carver <agcarver+ale at acarver.net> wrote:
>On 2017-08-28 13:55, Phil Turmel wrote:
>> On 08/28/2017 01:50 PM, Jim Kinney wrote:
>>> The (bs) EMP bags made me think (usually a good thing!) about
>circuit
>>> design (a field I'm not good in).
>>>
>>> For a spacecraft, the obvious choice for ground plane is the craft
>>> framework itself. Relative to the typical power loads in the various
>>> modules on ISS, the frame is a "near infinite supply or sink of
>>> electrons".
>>>
>>> But space weather is nasty. Solar flares are huge problems. How is a
>>> system designed to withstand an EM flux greater than the typical
>power
>>> throughput? On the ground, we use fat braided copper wires and
>deeply
>>> buried rods. That's not an option on a satellite. Thick skin can
>>> shield. What else?
>> 
>> You don't shield everything.  You shield the ICs with as small a
>shell
>> as possible, and every other circuit in the whole spacecraft is a
>> precision engineered tightly twisted pair w/ carefully balanced
>current
>> flows.  Both power and signal.  Plus optical isolation between any
>> devices grounded to different parts of the structure.
>
>No, everything is shielded unless the circuit can handle random induced
>currents (validated with EMI/EMC testing) or the effects of such random
>currents can be mitigated in other ways (putting circuits to sleep,
>using them only in benign environments).  All signal wires are shielded
>at all times unless the wire must be unshielded to operate (a Langmuir
>probe would fall in this category).  Power wires may or may not be
>shielded depending on instrument susceptibility (for example, wires for
>pyrotechnics would be shielded to prevent accidental activation).
>
>All electronics are fully enclosed in metal enclosures because this
>provides both electrical shielding and charged particle/ionizing
>radiation shielding (for low level radiation, high radiation always
>makes it through).  There's a limit to the amount of shielding you can
>actually put on and not just because of weight.  High energy particles
>can create radiation inside enclosures (called secondary
>radiation/particles or just "secondaries") which could potentially be
>worse than the original radiation.  Thin material reduces secondaries
>but leaves you exposed to more of the lower energy primary radiation.
>Thick material can quench low energy primary radiation but provides a
>significant radiation cross section for generating secondaries.
>
>
>So back to the first question of "what else?":
>
>Heavy filtration on power leads using reactive components (inductors
>and
>capacitors) can help reduce conducted interference (induced current).
>Shielding of the cables (even power) helps reduce radiated
>interference.
> (Note that this also helps prevent an instruments own internal noise
>from leaking into another instrument).  Clamping circuits can help with
>some voltage issues caused by charging or induced voltage/current
>(separate from EMI/EMC because induction is a low frequency process).
>If you can avoid shielding by using good design principles then you can
>save a lot of weight.
>
>The spacecraft is considered to be at a floating potential.  Depending
>on its size (like the ISS) a contactor may or may not be used.  The
>ISS,
>due to its immense size, uses a plasma contactor because the whole
>superstructure actually experiences an induced current as it travels
>through Earth's magnetic field.  The contactor prevents voltages from
>building up due to this and other processes.  Smaller satellites don't
>have this feature and rely on natural leakage from the bus to the
>environment.  Ground loops must be avoided if at all possible so
>shields
>are broken at one end of cables.  A star topology for grounding is
>used.
>
>Instruments must undergo environmental testing to determine if they are
>susceptible to any of these issues.  If shielding can not be used on
>the
>cables itself then "natural" shielding (by routing cables such that
>they
>are hidden by other components) or rearranging the components to
>minimize cable lengths (also affects mass) or crosstalk.
>
>Parts selection is the next option.  Choosing parts that can handle
>large input voltages without stress or adding additional clamping
>circuitry to prevent voltage excursions would be done.  These are not
>as
>ideal as shielding because they are accepting a more hostile
>environment
> but there may not be an alternative.
>
>Much further down the list is concept of operation.  This would include
>when and when not to operate equipment, automatic failsafes, live
>ground
>intervention, etc.  All of this requires some level of autonomy or
>human
>interaction and is the least favorable of all methods.  It does get
>used.  For example, when one of the space weather satellites detects an
>incoming coronal mass ejection (CME) an advisory is issued to all space
>customers.  They can then choose to put their hardware into safe mode
>(typically powering down many instruments), rotate the satellite into a
>more benign attitude or, for the ISS, evacuate the crew to a safer
>location inside the structure.
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