siliconchip.com.au Australia’s electronics magazine July 2019 19
ing schemes for memory, such as
the BCH (Bose–Chaudhuri–Hoc-
quenghem) cyclic error correction
scheme. BCH (250, 32, 45) can pro-
vide 99.9956% correctness even
with a 10% memory bit error rate
(1 byte in every 711 would still be
defective). Robust error correcting
codes have a high computational
overhead.- the use of redundancy such as mul-
tiple redundant computers and soft-
ware, as used on the Space Shuttle.
With three or more computers, they
can ‘vote’ if they do not all agree
(see below) - the use of multiple error correction
schemes - keeping multiple copies of critical
information - the use of a watchdog timer that will
reset a computer if the expected be-
haviour does not occur after a cer-
tain amount of time
Testing techniques
Electronic components can be test-
ed for radiation hardness by expos-
ing them to radiation from sources
such as particle accelerators, radio-
active elements like californium and
actual testing in space. The correct
application of statistical techniques
to determine true error rates is very
important.
Radiation and CubeSats
CubeSats are popular, low-cost sat-
ellites often built on a tight budget and
with commercial off-the-shelf (COTS)
components.
The question is often asked if radi-
ation hardening of CubeSats is nec-
essary.
The answer varies depending on
the CubeSat mission, but in general,
CubeSats have limited lifetimes in low
earth orbit, where radiation is a much
less serious threat than in other orbits.
The limited expected life in orbit
also limits the requirement for exten-
sive radiation hardening measures.
Radiation hardening in CubeSats is
usually achieved through software,
component redundancy and good
component choices.
A standard Android phone has
been used as the control device on a
CubeSat.
On the other hand, the Lunar Ice-
Cube CubeSat mission to the moon
uses a radiation-hardened computer- see photo on page 12.
For more information on CubeSats,
refer to the SILICON CHIP article on
that topic in the January 2018 issue
(siliconchip.com.au/Article/10930).
Commercial radiation hard-
ened devices, past and present
As mentioned above, early elec-
tronic devices were less susceptible
to radiation because of their large fea-
ture sizes.
One such example is the Fairchild
Micrologic Type G three -input NOR
gate from 1961, as used on the Apollo
guidance computer (see Fig.8).
The RCA 1802 from 1976 (Fig.9)
was one of the first microprocessors
available in a radiation hardened ver-
sion, fabricated using silicon on sap-phire. It used the Complementary
Symmetry Monolithic Array Com-
puter (COSMAC) 8-bit architecture.
The chip is still made today by In-
tersil, and sold as a high-reliability
device, although its exact radiation
resistance is unstated.
It was and is used in the Galileo
Probe, Hubble Space Telescope, Ma-
gellan spacecraft and various other
satellites.
The processor, in its bulk silicon
version, was also popular with hobby-
ists. Further information on this chip
is at the following links: siliconchip.
com.au/link/aaq1 (device history)
and siliconchip.com.au/link/aaq2
(regarding its use in amateur radio
satellites).
The Space Shuttle had a Data Pro-
cessing System which comprised four
IBM AP-101S General Purpose Com-
puters with identical hardware and
software, and a fifth computer with
identical hardware but different soft-
ware which had the same goals as the
software in the other four computers.
The computers would vote on any
result, and any system in disagree-
ment with the others would have its
result excluded.
While not explicitly stated, it is like-
ly that this voting system took into ac-
count the possibility of data process-
ing errors due to radiation events or
for other reasons and the redundancy
would ensure a correct result.
A description of the system can be
seen at: siliconchip.com.au/link/aaq3
Two current devices of interest that
are radiation-hardened for space ap-Fig.10: the radiation-hardened Vorago RH-OBC-1
onboard computer and avionics board for spacecraft,
specifically designed for CubeSats.
Fig.11: the Ramon GR712RC, a radiation-hardened chip for
space applications. It contains a dual-core LEON3FT SPARC
V8 processor and was being used by the SpaceIL “Beresheet”
lunar lander (see SILICON CHIP, November 2018; siliconchip.
com.au/Article/11296). It uses Ramon’s proprietary “RadSafe”
technology, with a dedicated design including circuitry to
monitor radiation, monitoring of chip junction temperatures,
error correction logic, hardened flip-flops, redundant circuit
elements and a watchdog timer to reset of the chip if it crashes.