[ale] [OT] grounding in space
Alex Carver
agcarver+ale at acarver.net
Tue Aug 29 12:06:33 EDT 2017
The Voyagers were indeed built with a lot of discrete components but
they also do have integrated circuits. Data storage was to magnetic
tape which is then rewound for playback at a later time. This was very
important during the various planetary encounters since there was
significant maneuvering to do and collecting science data while
performing a flyby was very important especially with the data coming in
faster than the downlink could accommodate.
The thing to keep in mind is that the Voyagers were built in what could
be considered record time for a spacecraft fabrication process. Most
deep space spacecraft (or rovers, too) have a design life cycle of about
10 years starting from project approval. The Voyagers were designed and
built in five after approval (1972 to 1977). They are also highly
redundant. Nearly everything of the primary system components
(computer, storage, transmitters) is duplicated. Some items are not
like the cameras, spectrometers and magnetometer but those are then
built to more stringent requirements.
The Pioneer spacecraft explained a lot to engineers about what they
would see during the new mission. The Pioneers were damaged heavily by
Jupiter's massive radiation environment. No one knew how much radiation
was around Jupiter (a human would die of radiation exposure in about one
minute standing on the surface of Jupiter's moon Europa). So the
Voyagers were redesigned with massive shielding to protect from as much
of this radiation as possible (this is one of the rare exceptions to the
"more is not always better" shielding issue I mentioned previously --
Jupiter is just intensely hostile). All missions to Jupiter afterwards
benefited from Pioneer's data. Now they all fly there with vaults made
of aluminum and tantalum for the electronics (Galileo, Juno, and the
upcoming Europa Clipper). Shielding is also important for deep space
spacecraft since they're almost always powered by radioisotope thermal
generators (RTG) and these do cause some elevated background radiation.
Juno and Europa Clipper are exceptions, they are both be solar powered
despite the distance.
As for the exposure cross section, larger isn't always better.
Depending on the specific technology (bipolar transistors versus MOSFET
for example) a large cross section can be your enemy. Bipolar
transistors do have a large bulk of material to work with but the
junctions are bigger and that's where the damage typically takes place.
In the MOSFET, it's the gate oxide that suffers the most damage so a
large area gate can have more issues. In both cases large areas mean
that damage is most likely to happen somewhere (probability of being
struck by a radiation particle) and that leads to early onset of
radiation effects. On the other hand, a small cross section means that
the total survivable dose is reduced. The probability that it can go
for a period of time without being struck increases but the magnitude of
the effect once it is struck is greater. You have to strike a balance
between them and build the structures in such a way that they tolerate
radiation effects gracefully. A lot of work goes into radiation
hardening which is why spacecraft technology is always so far behind
terrestrial technology. Experiments are continuing though, with many
more advanced COTS parts being evaluated for use. A baby step example
is the use of modern 20 megapixel color CMOS imagers on the new Mars
2020 rover for some of the cameras. These are not mission critical
cameras, they will be evaluated for future use while the mission
critical cameras are older, well tested technologies. It's part of a
process called Technology Readiness Level (TRL) advancement. The TRL
scale is 1-9 with 1 being a concept and 9 being flight proven/heritage.
The newer cameras will be around TRL 6/7 when they launch and if all
goes well they will become TRL 8/9 in the future.
By the way, the record and record player stylus on each Voyager is
purely mechanical unlike the rest of the spacecraft. All instructions
for assembly and operation are included and the two pieces are built to
survive at least one billion years. The gold covers over the records
have pure uranium 238 inside them to serve as a chronometer so that a
distant civilization that finds them can determine the age of the
spacecraft. The RTG won't have any power left to operate the spacecraft
after about 2036. The Pioneers had plaques denoting their origins but
the Voyagers are the only ones that have records with sounds and images
stored on them.
On 2017-08-29 04:49, Jim Kinney wrote:
> 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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