<html><head></head><body>Thanks, Alex!<br>
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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 :-)<br>
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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.<br><br><div class="gmail_quote">On August 28, 2017 9:38:45 PM EDT, Alex Carver <agcarver+ale@acarver.net> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<pre class="k9mail">On 2017-08-28 13:55, Phil Turmel wrote:<br /><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;"> On 08/28/2017 01:50 PM, Jim Kinney wrote:<br /><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;"> The (bs) EMP bags made me think (usually a good thing!) about circuit<br /> design (a field I'm not good in).<br /><br /> For a spacecraft, the obvious choice for ground plane is the craft<br /> framework itself. Relative to the typical power loads in the various<br /> modules on ISS, the frame is a "near infinite supply or sink of<br /> electrons".<br /><br /> But space weather is nasty. Solar flares are huge problems. How is a<br /> system designed to withstand an EM flux greater than the typical power<br /> throughput? On the ground, we use fat braided copper wires and deeply<br /> buried rods. That's not an option on a satellite. Thick skin can<br /> shield. What else?<br /></blockquote> <br /> You don't shield everything. You shield the ICs with as small a shell<br /> as possible, and every other circuit in the whole spacecraft is a<br /> precision engineered tightly twisted pair w/ carefully balanced current<br /> flows. Both power and signal. Plus optical isolation between any<br /> devices grounded to different parts of the structure.<br /></blockquote><br />No, everything is shielded unless the circuit can handle random induced<br />currents (validated with EMI/EMC testing) or the effects of such random<br />currents can be mitigated in other ways (putting circuits to sleep,<br />using them only in benign environments). All signal wires are shielded<br />at all times unless the wire must be unshielded to operate (a Langmuir<br />probe would fall in this category). Power wires may or may not be<br />shielded depending on instrument susceptibility (for example, wires for<br />pyrotechnics would be shielded to prevent accidental activation).<br /><br />All electronics are fully enclosed in metal enclosures because this<br />provides both electrical shielding and charged particle/ionizing<br />radiation shielding (for low level radiation, high radiation always<br />makes it through). There's a limit to the amount of shielding you can<br />actually put on and not just because of weight. High energy particles<br />can create radiation inside enclosures (called secondary<br />radiation/particles or just "secondaries") which could potentially be<br />worse than the original radiation. Thin material reduces secondaries<br />but leaves you exposed to more of the lower energy primary radiation.<br />Thick material can quench low energy primary radiation but provides a<br />significant radiation cross section for generating secondaries.<br /><br /><br />So back to the first question of "what else?":<br /><br />Heavy filtration on power leads using reactive components (inductors and<br />capacitors) can help reduce conducted interference (induced current).<br />Shielding of the cables (even power) helps reduce radiated interference.<br /> (Note that this also helps prevent an instruments own internal noise<br />from leaking into another instrument). Clamping circuits can help with<br />some voltage issues caused by charging or induced voltage/current<br />(separate from EMI/EMC because induction is a low frequency process).<br />If you can avoid shielding by using good design principles then you can<br />save a lot of weight.<br /><br />The spacecraft is considered to be at a floating potential. Depending<br />on its size (like the ISS) a contactor may or may not be used. The ISS,<br />due to its immense size, uses a plasma contactor because the whole<br />superstructure actually experiences an induced current as it travels<br />through Earth's magnetic field. The contactor prevents voltages from<br />building up due to this and other processes. Smaller satellites don't<br />have this feature and rely on natural leakage from the bus to the<br />environment. Ground loops must be avoided if at all possible so shields<br />are broken at one end of cables. A star topology for grounding is used.<br /><br />Instruments must undergo environmental testing to determine if they are<br />susceptible to any of these issues. If shielding can not be used on the<br />cables itself then "natural" shielding (by routing cables such that they<br />are hidden by other components) or rearranging the components to<br />minimize cable lengths (also affects mass) or crosstalk.<br /><br />Parts selection is the next option. Choosing parts that can handle<br />large input voltages without stress or adding additional clamping<br />circuitry to prevent voltage excursions would be done. These are not as<br />ideal as shielding because they are accepting a more hostile environment<br /> but there may not be an alternative.<br /><br />Much further down the list is concept of operation. This would include<br />when and when not to operate equipment, automatic failsafes, live ground<br />intervention, etc. All of this requires some level of autonomy or human<br />interaction and is the least favorable of all methods. It does get<br />used. For example, when one of the space weather satellites detects an<br />incoming coronal mass ejection (CME) an advisory is issued to all space<br />customers. They can then choose to put their hardware into safe mode<br />(typically powering down many instruments), rotate the satellite into a<br />more benign attitude or, for the ISS, evacuate the crew to a safer<br />location inside the structure.<br /><hr /><br />Ale mailing list<br />Ale@ale.org<br /><a href="http://mail.ale.org/mailman/listinfo/ale">http://mail.ale.org/mailman/listinfo/ale</a><br />See JOBS, ANNOUNCE and SCHOOLS lists at<br /><a href="http://mail.ale.org/mailman/listinfo">http://mail.ale.org/mailman/listinfo</a><br /></pre></blockquote></div><br>
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