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Computers in Space Examined
Wil Harris writes "There's an article about the computers used in space missions over at bit-tech this morning. It covers the processor types and speeds, why space stations are less powerful than the laptops that astronauts take up with them and why tape storage is still de rigeur. An interesting and concise couple o' pages."
also, older components tend to use larger signaling thresholds, which makes a big difference considering the mag-flux caused by radiation in space is much higher than on earth. id guess those tapes are also done with a high bias, as platters could be wiped with a decent flare, and fine-process cmos chips could be knocked out completely with a suprisingly small charge. even a small spike in power from a line surging or regulator going bad could take down some hard-disks, while all you have to do is rewind the tape and it's good.
you only need enough cpu power to handle some basic tasks and send the rest down to earth. considering most of the software is in c or assembler a 486 is an awful lot of power for most tasks.
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Because of the density of memory now-day, bit-flipping is becomming a real problem for home PCs and workstations running with an uptime of over a week. Bit-flipping happens all the time and even on your PC. It just may be happening in a region of the wafer that does not currenly have anything important addressed to it...hence not an issue. But someplace, somewhere, a slashdot reader is getting a bitflip causing data-rott once it's commited to the harddrive. By the way, these bit-flips are causes cosmic rays.
If you serious about data integrity and stability, you would be foolish not to use ECC. You may take a 5% performace hit, but it's best to get used to it. If you need that extra 5%, then upgrade your processor to make up for it.
Life is not for the lazy.
Unfortunately, radiation hardening a processor involves altering the fabrication process (some processes - e.g. SOI - are more resistant to bit flips than others), inserting guard rings, adding self-checking logic, and a bunch of other changes. Doing all of this stuff takes time (and money) so space-ready processors typically lag COTS processors by a generation or two. Example: the current "hot new" rad-hard processor is the RAD750, which is a rad-hard version of the venerable PowerPC 750.
Having said all of that, some small, risk-tolerant missions do use standard COTS processors (PowerPCs and StrongARMs are popular, as are industrial embedded processors like the Hitachi SuperH line). But you won't tend to find them in most NASA projects.
Some 9 years ago I worked on some chip design for Hughes and ESA.
Back then, we used 1.2um on 4" (or 6" in the new fab) wafers - and everything was built on a sapphire substrate instead of a silicon substrate to make them radiation hard (when they went through the van allen belt).
It was dull, as every single chip had about 12 inches of paperwork from QA. Every *instance* of every chip had its own paperwork, I mean. It was also dull because they wanted tried and tested tech, not any of this new fangled sub-micron stuff.
That was then. Can anyone let me know how much things have changed?
Better idea. Go read the Honor Harrington series. (Go to baen.com, click on Free Library, and select "On Basilisk Station".) While they have hyperdrives and gravity propulsion, space battles are heavily dependent on orbital vectors, base velocities, missile loadouts, and missile counter-defenses. The author (David Weber) does a really good job of showing how a space battle might play out. Oh, and it is quite exciting. :-)
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You beat me to it; good thing I decided to re-check before I hit submit (that's what happens when you take half an hour to compose your reply). Anyway, Weber is definitely an interesting source for the possible future history of space warfare. Pretty much everything he writes is based on historical precedent, but he does (in my estimation) a great job of bringing it into the future and into the three-dimensional (or maybe a few more) world of space.
The FPGAs on Spirit and Opportunity seem to be overlooked. NASA's new Reconfigurable Scalable Computer (RSC) Project for space applications is exploring using FPGAs (instead of CPUs) which offer increased performance and radiation tolerance at a fraction of the power consumption.
What's past is NOT ALWAYS prologue for the future!
See Rover FPGAs and RSC.
Future NASA space computers may not look like what most expect.
What's past is NOT ALWAYS prologue for the future!
I work for NASA on the manned programs and my experience is that hard drives are a headache on long term space missions.
u z_iss_010427.html
The laptops onboard Space Station are primarily IBM laptops (many of which will soon be running Linux - yeah!). While the drives are easy to replace, they fail fairly often (compared to other space hardware) and new ones need to be launched. The software on the drives also becomes corrupted frequently (maybe once every few weeks), requiring the crew to waste time recopying the software from CD. While these COTS laptops and hard drives were cheap up front (almost zero development cost, custom stuff would have been tens of millions of dollars) we are paying for it now because we waste a lot of operational time fixing them.
The Honeywell Command and Control computers (the primary flight computers onboard, which are triple redundant and manages core systems in the US segment) used to have a 300 megabyte hard drive to store flight software.
In 2001 during a shuttle mission, hard drive problems caused ALL THREE of those computers to crash simultaneously in a massive cascading failure. While it never got a lot of press, recovering from that took several days and an effort reminiscent of Apollo 13. You can read a contemporary article on it here: http://www.space.com/missionlaunches/launches/soy
When we got the things back and did a post-mortem, it turned out that the hard drive had a design flaw where the arm was dragging across the disk during power down and scratching it, which eventually led to failure.
They were replaced with solid state units shortly thereafter (which were already in the development pipeline). No moving parts, and much less problematic.
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