Rad Chips

By Mike Deliman

deliman_lg.jpgIn a recent EEtimes Article (requires login) NASA has begun looking for a suitable processor for continued manned space exploration.  Current space robots (MRO, etc) run BAEsystem's Rad750, based on PowerPC 750, and descended from AFRL's Rad6000 design.  The Rad6000 was the first 32-bit computer hardened for space use.  In the world of computers, these aren't huge horse-power producing machines.  In fact, in today's world, they're both kind-of throwbacks to earlier times.  If these are old designs, why are they still in use?

And.. what's the big deal?  Why is there a "search" for appropriate computing power?  Can't we just grab one of today's chips, put it through whatever process it is, and launch it into space?  And why are these space processors so … well, underwhelming?  The Rad750 is based on PPC750 – which has been around for a long time, and at 133Mhz / 260 MIPS, this one looks like it's been around a long time too!

So… what's the deal?  Why is it that these state-of-the-art satellites are flying chips that look like they're so old?

Par of the answer is that these chips aren't just like the chips they were based on.  These chips are much more robust.  They've been redesigned to be able to function in environments very high in radiation.  But the process of making them resilient – able to withstand that much radiation – can't be allowed to make the chips fundamentally different from the chips they were based on.  The rad-hard versions still have to operate on the same data and instructions and have the same results as the chips they were based on.

It takes a long time to take a regular chip and modify it to be able to handle the effects of radiation.  It can take years to change the physical layout, test, and correct problems.  This is why a given rad-hard chip might take 5 to 10 years to appear on the market after the chip it's based upon has been released.   Once it's been rad-hardened, it has to be tested to show how much radiation it can tolerate, and that it's functionally equivalent to the chip it was based on.  This is a fundamental milestone in the life of such a chip – a critical milestone.  Over the past decade the processes of hardening CPUs have become slightly more streamlined, but it still takes years to make the design changes.

Hardness vs. Tolerance

There are a lot of chips out that claim varying degrees of radiation tolerance – the ability to operate under periods of increased radiation.  Many of these use newer CPU designs, and many are faster than their rad-hard cousins.  But are they good for the same environments?

Radiation tolerant designs might be good enough for terrestrial usage – in industrial and scientific high-energy applications, etc, where the chips will not be constantly exposed to  radiation, and occasionally subjected to intense exposure ("storms" in the solar wind, for example).  Colliders, generation plants, and reactors might not need "rad hard" parts.  Even satellites in Low Earth Orbit (LEO) might have enough shielding from the Earth's magnetic fields to only really need ruggedized or tolerant parts.

But when you're talking about anything beyond LEO, you need to design for high levels of radiation.  The Sun periodically throws magnetic tantrums, ejecting gigantic clouds of superheated matter.  These Coronal Mass Ejections (CME's) are the force that drives the Northern lights and Geo-magnetic storms on Earth, and solar radiation storms in space.   These storms are powerful enough to destroy computers.   Sometimes even satellites in low-earth orbit.To date there are only two thirty-two bit CPUs that are radiation hardened to handle more than a few kilo-rads of radiation – the Rad6000 and Rad750 from BAE are hardened to handle up to 1 Mega-rad.  These chips are in the Mars Exploration Rovers, Stardust, Mars Recon Orbiter, and the about-to-be-launched  STEREO solar observatory (launches Wed. 6:30pm).  STEREO is off into deep space to study the sun with a pair of solar observatories.  We'll be able to see the sun doing it's stuff in 3-D, and learn more about what creates events like Coronal Mass Ejections (CMEs).  In short, CMEs are like hurricanes in the solar wind – they are characterized by intense bursts of radiation (sub-atomic particles, etc). 

Tolerant designs can handle – live-through – life in Low Earth Orbit, but they can't handle those huge storms.  Even in LEO, they may be damaged by CMEs – which can actually cause the Earth's Magnetic Field to Collapse!  When this happens, the normal protection provided by the Earth's magnetic field is gone – even the earth gets showered with particles (causing the Northern Lights).

What's in the future?

We're already designing new rad-tolerant chips with new materials.  Eventually, as we understand more about high-energy events like CMEs, we'll be able to make specific design changes to make computers more resilient to the effects of Space Weather.  In the mean time, designs will have to take the traditional conservative approach – and space computers will remain "a little behind the curve".  Someday, though, we'll be able to design computers that are immune to most of the day-to-day high energy events in space.

References:

STEREO twin satellites launch today at 6:30pm

BAE news – see page 4 for designs involving Rad750

Space Daily  Space Radiation reports

Air Force Research Labs (see project# 10 – Rad750)

3-d animations of CME's causing Earth's magnetic field to collapse on DigitalRadiance.com .

Today's Space Weather on Spaceweather.com

NOAA POES auroral activity maps