[Ground-station] Need some nuclear science help

[Bruce Perens]

Thanks. I would assume that a satellite for anything other than LEO that we
launch is expected to be long-lived and that the flight computer must be
rad-hard or it won't survive in orbit due to actual physical damage to the
component, as AO-10's RAM did not. This was back when RAM used much larger
silicon features than today, and was probably less vulnerable. I need to be
able to tell people who know even less than I about this why we can't use
consumer parts with shielding or redundancy as a strategy for anything but
low earth orbit. I still would like to have more numbers, rather than
simply relying on experts or waving my hands :-)

    Thanks

    Bruce

On Sun, Jun 3, 2018 at 12:52 AM, Phil Karn via Ground-Station <
ground-station at lists.openresearch.institute> wrote:

> Radiation was one of the first projects I did for AMSAT when I joined in
> 1980. The problem then was to estimate the lifetime of the 4116 16k x 1
> bit dynamic RAMs to be flown on AO-10.
>
> There were a *lot* of uncertainties. My final estimate was that the RAM
> would last anywhere between 1 and 6 years. It actually failed after 3.
>
> There are two basic kinds of ionizing particle radiation at issue here:
> solar wind particles trapped in the earth's magnetic field and cosmic
> ray particles.
>
> The trapped solar wind particles are better known as the Van Allen
> belts. There are two: an inner, nastier one composed mostly of protons
> and an outer one that's mostly electrons that are more easily stopped.
> Both are densest along the geomagnetic equator.
>
> The inner belt is densest at 1-2 earth radii; that's why you don't find
> very many satellites at those altitudes. Because the earth's magnetic
> field axis is at an angle to the rotational axis and offset from it,
> both latitude and longitude matter. Because of the offset, the inner
> belt essentially touches the atmosphere in the region between south
> America and Africa; this is the so-called South Atlantic Anomaly. The
> ISS (and other LEO sats) picks up most of its exposure in this region.
>
> The trapped particle energy distribution has a very long tail. As you
> add shielding the dose drops rapidly but you reach a point of
> diminishing returns. It's impractical to stop it all.
>
> Then you have cosmic rays, mostly hydrogen and helium nuclei but also
> heavier ions. These are much less numerous than trapped solar particles
> but they have MUCH higher energies. They are essentially impossible to
> stop. These are the major radiation source on the moon and in
> interplanetary space. The flux does vary about 2:1 with the solar cycle;
> a more active sun tends to shield the inner solar system from cosmic
> radiation.
>
> The bottom line is that while you can do a lot with a moderate amount of
> shielding, there's no way you can stop it all. Some of what gets through
> can have very high energies, so random errors and latchup are always a
> concern.
>
> Latchup can be handled with fast-acting, self-resetting circuit breakers
> in the power supply lines.
>
> ECC (paying attention to geometry) can fix the transient error problem
> with RAM but you can always have a transient error in some other part
> (like a CPU) where it's not so easy to correct.
>
> If one computer isn't reliable enough, some sort of system level
> redundancy seems essential. I seem to recall three computers arranged in
> a circular rock-scissors-paper arrangement, i.e, where computer 1 can
> monitor and reset computer 2, which can monitor and reset computer 3,
> which can monitor and reset computer 1. (Or maybe I just made that up.)
>
> That leaves total dose, which is what got AO-10. There's basically
> nothing you can do about it beyond using whatever rad-hard parts you can
> get, adding as much shielding as you can afford, and avoiding hotter
> orbits.
>
> Phil
>
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-- 
Bruce Perens K6BP - CEO, Legal Engineering
Standards committee chair, license review committee member, co-founder,
Open Source Initiative
President, Open Research Institute; Board Member, Fashion Freedom
Initiative.
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