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Re: Space Harding of birds - a lesson -

The short answer to your question is: Yes, Amsat’s satellite builders have
been mindful of radiation effects in the design of our satellites. 

The longer answer is somewhat more involved. I have not worked in this field
myself, but perhaps I can sum up some of the discussions that I have followed
over the past ten years or so.

During the 1960’s and thereabouts, the US government was a major buyer of
semiconductors for space and defense programs. The semiconductor industry was
motivated to provide radiation-hardened components to meet government
specifications. These parts were specifically designed and engineered to be
resistant to damaging effects of radiation, both for long term exposure in a
space environment and short term exposure that might have occurred when the
H-bombs were raining down. Thankfully their work was never tested under the
second scenario.

In today’s economy semiconductor companies make most of their money selling
chips for personal computers, video games and cellphones. Space and defense
programs are minor customers and many companies have found it unprofitable to
continue “rad-hard” product lines which are a very small part of their
overall sales. Today there are very few semiconductors that are specifically
rated by their manufacturers as being “rad-hard”, and those that still
exist are expensive and several generations behind current technology. A good
example of this would be the 1802 microprocessor in AO-40’s IHU which Amsat
obtained from a US government weapons laboratory. This chip is all but
obsolete and gone from the commercial computer market but the special
radiation hardened version was still quite useful to Amsat for building
AO-40’s main computer. Slightly out-of-spec rad-hard static RAM chips were
donated to AMSAT by an aerospace company for use on AO-13, these chips were
worth about $80,000 if I remember correctly. 

A newer trend in space systems design is to test and qualify commercial grade
chips after the fact to see which ones are more resistant to radiation damage.
 This is made more interesting by the fact that minor changes in a company’s
wafer fab process can greatly alter the radiation properties of its chips, so
that a part that tested OK in the last batch may be completely unusable in the
current batch. Some companies have put special programs in place to notify
customers of process changes that would normally not be announced outside of
the company. These chips are sometimes marketed as “rad-tolerant”.

Every agency and company that builds space hardware has been affected by this
change. Many satellites including the Hubble Space Telescope are affected by
radiation. The new science instruments that were installed on HST during the
second service mission in 1997 were found to be sensitive to single event
upsets in certain circuits. Most of these bad effects were corrected by
reprogramming the flight software to reinitialize certain subsystems at
frequent intervals. After this effect was observed in 1997, NASA decided to
fly HST’s new 486 computer processor on a space shuttle mission (the same
mission that carried John Glenn) to find out if it would work correctly in
orbit, since an upset in the main flight control computer would be a much
larger disruption to Hubble’s mission than an upset in a science instrument.
After the 486 processor passed its qualification flight it was installed on
HST by the third service mission in December 1999. At the time several people
commented on how far NASA was behind the technology curve, since the 486 chip
had largely been replaced by versions of the Pentium in the personal computer
market, without understanding that the Pentium had not yet undergone the
testing that would have been needed to qualify it for space flight.

Amsat has performed its own radiation testing of chips for which no data could
be found in the literature, aided in this effort by several of our members who
work in hospital radiology departments and who have access to lethal radiation
sources. A number of integrated circuits of interest to Amsat builders have
been tested in this way.

Sometimes commercial chips are flown in space on demonstration missions to
test their behavior in non-mission-critical applications before using them in
more critical applications. The new IHU-2 computer on AO-40 is an example of
this, it was flown in the hope that it would be a prototype for future
spacecraft computers on upcoming Amsat spacecraft. 

Radiation effects are of two types, one type is a “single event upset”
which can cause bits to flip or logic circuits to change state but no
permanent damage is done, and corrective action such as reloading the memory
cell or resetting the microprocessor can fix the problem when it is detected.
Another effect is due to the “total dose” which the chip has received over
its lifetime, in which damage to the semiconductor lattice eventually causes
the device parameters to change slowly over time. Transistor current gain can
decrease and junction leakage current can increase as more and more radiation
is received, until eventually the circuit ceases to function properly. The
Galileo spacecraft in orbit around Jupiter was showing signs of total dose
failure prior to its demise in the atmosphere of that planet.

There are steps that circuit designers can take to minimize radiation risk, by
de-rating parts to lower voltage and current levels, by designing circuits to
detect when a part “latches-up” and cut off power before it destroys
itself, and by techniques such as error detection and correction (EDAC)
circuits to detect and correct memory errors by use of redundant bits in the
memory array. The fact that AO-40 has been recording a greater number of
memory errors in the past few days simply shows that the EDAC is working as it
is supposed to. It is also common to attach metal shields to chips to try to
reduce radiation dosage and prolong the chip’s lifetime in space. 

It is true that Amsat’s design methods are of necessity somewhat different
than those of the commercial and government satellite builders, since our
resources are lacking compared to theirs, but Amsat’s satellite builders are
aware of the current state of the art and do take into account the environment
in which our satellites will live.

To dig deeper please check out:
http://www.govcomm.harris.com search on “radiation hardened”.

Dan Schultz N8FGV

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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