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Star Cores: Computers in space

Posted on 3 Sep 2008 at 14:26

Fast-forward to the present day, and the same painstaking approach to developing and testing computer components destined for space remains intact. The Phoenix's descent to the polar regions of Mars earlier this year may have marked another notable landmark for space exploration, but the computing technology inside it was far from cutting-edge. In fact, the 33MHz processor on the radiation-hardened RAD6000 single-board computer inside the shuttle can trace its legacy right back to the IBM PowerPC architecture that was previously to be found inside Apple Macs.

Yet, despite its modest credentials, the RAD6000 can be found in dozens of NASA programmes. Vic Scuderi, business area manager for BAE Systems' Space Electronics division - the company behind the RAD6000 - spoke to PC Pro about the challenges involved in building hardware and software for use in space. "We've been told it's become the workhorse of the industry," Scuderi said of the RAD6000. "It's been proven in space. One of our measurements isn't so much that we've got the latest and greatest, but that we're able to show by heritage this computer is able to do what it's supposed to do."

Harsh environments

The stresses that computing hardware has to withstand in such extreme conditions include temperatures that can vary between plus and minus 120C, massive shock and vibration during launch and landing, and - probably the most threatening - radiation. Ionisation of the semiconductor materials leads to a slow degradation of transistor performance, resulting in increased current leakage and a shifting of switching thresholds. Sufficient accumulation will eventually cause failure of the chips comprising the computer system.

Scuderi claims the RAD6000 was developed to withstand both single-event upset and total-dose radiation, and in serious doses. "A human being can't take more than 400 or 500 rads [the unit of measuring radiation]," he said. "When satellites are orbiting in extreme radiation belts, hardware has to be designed to cope with 500,000 to 1,000,000 rads."

This is achieved by combining radiation hard-by-design and hard-by-process techniques. The former ensures that custom-designed components can divert single-event upset radiation (similar to a lightning rod) to avoid damaging semiconductor materials, while the latter involves the silicon process used to create the chips from the ground up, so they can withstand radiation over time. This dual approach reduces the reliance on redundancy, where more than one of a particular system is present to reduce the effect of damage or failure caused by radiation.

This methodical, ruthlessly disciplined development process ensures failures are kept to a minimum. "When I purchase an integrated circuit," said Scuderi, "I need to know when it was made, what lot it was part of, what testing was done, its certificates of conformance, and so on. What could have been a $3 commercial product becomes a $10,000 part, so when we reach our final testing, if a part failure occurs, we can trace it all the way back to the origins and find the source of the failure."

Raphael Some, New Millennium Program Technologist at NASA's Jet Propulsion Laboratory, told us: "The philosophy is to test to levels beyond those expected. Usually a safety factor of two to three is applied to environmental stresses at the component level, and somewhat less at the subsystem and system levels."

Unsurpassed reliability might be the primary requirement for space hardware, but if it can't match its dependability with the most parsimonious use of energy, it's about as likely to fly as an astronaut with the measles. Power is scarce, but every bit of power consumed produces the equivalent amount of heat, and dissipating that heat from a spacecraft in a vacuum isn't easy. There are usually large black radiators for this purpose that perform essentially the opposite function to the solar panels they visually resemble.