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Message: <blockquote data-quote="imhotep" data-source="post: 30299412" data-attributes="member: 562115">It's not only the reliability. Don't forget that it's in outer space.  You got to protect the electronics from Cosmic Rays. Galactic Cosmic Rays are like bullets. Hence the electronics have to be well shielded. But no shileding is perfect.Voyager 1 and 2 each carry six onboard computers, originally organized as a distributed system consisting of three dual-redundant computers: the Computer Command System (CCS), the Attitude Articulation Control System (AACS), and the Flight Data System (FDS). Without these six embedded computers, which have operated continuously for nearly half a century, the two spacecraft would never have reached the Solar System’s outer planets, and all the scientific data collected by the instruments on board the spacecraft would never have made it back to Earth.The CCS – designed by the Jet Propulsion Laboratory (JPL) in Pasadena, California – controls all major spacecraft systems, monitors the spacecraft’s health, maintains temperatures inside of the spacecraft, manages the AACS and FDS computers, and controls the eleven onboard scientific instruments by sending them commands. The CCS employs an 18-bit instruction word with a 6-bit opcode and a 12-bit address, and it has an 18-bit data word.To control development costs, the CCS is nearly identical to the embedded computer developed for the Viking spacecraft that went to Mars, with the addition of interface ports for the FDS and AACS. The CCS is constructed entirely of TTL logic chips, because that’s how things were done in the early 1970s; It was the heyday of the 7400 series TTL family, which was dominated by Texas Instruments. The paired CCS computers use dual-redundant plated-wire read/write memory, which works like magnetic-core memory but uses wire plated with a magnetic coating instead of ferrite beads. The CCS is an interrupt-driven computer and runs bare-metal code. There is no operating system.The AACS has a very similar architecture to the CCS and therefore also traces its lineage to the earlier Viking spacecraft computer. This computer handles attitude control for the spacecraft and controls articulation of the scan platform, which was mounted on a boom to give the spacecraft’s imaging instruments a moving platform for a better field of view. The AACS controls the spacecraft’s boom servomotors and hydrazine thrusters and is responsible for keeping Voyager’s large dish antenna pointed at the Earth so that contact isn’t lost. Superficially, at least, the architecture of the CCS and AACS seem to have more in common with the DEC PDP-9.The FDS was custom designed for the Voyager spacecraft because JPL needed a faster computer to format, store, and transmit images (the data that we most identify with the Voyager missions) and to send the spacecraft’s science and engineering telemetry data back to Earth. Unlike the other two computer systems used on Voyager, the FDS is not built with TTL chips. It’s the first computer based on CMOS chips to be flown in space.Instead of plated-wire memory, the FDS employs volatile CMOS RAM for read/write memory. This choice was heretical in JPL spacecraft design back in the day. JPL preferred nonvolatile memory so that the spacecraft computer could survive a temporary power loss. However, the Voyager spacecraft are powered by plutonium-fueled, nuclear-thermoelectric generators, and the FDS had a direct connection to the generator’s output, so a power loss indicates much bigger problems on the spacecraft than a mere computer glitch.Part of the data formatting performed by the FDS includes forward error correction (FEC) using Golay coding. As the two Voyager spacecraft get more and more distant, their signals become weaker, the radio channel becomes noisier, and so the signal-to-noise ratio falls. Golay coding allows data sent to Earth to survive three bits of reception error per data word. However, Golay coding also doubles the number of bits sent, thus cutting effective channel bandwidth in half.JPL enhanced the FDS capabilities on Voyager 2 when the original Jupiter/Saturn mission was extended to the outer planets. The enhancements included image compression and a switch to Reed-Solomon FEC for image processing. Reed-Solomon codes incur significantly less overhead than the original Golay FEC code and are now widely used for data storage and communications applications. The Voyager FDS software was a pioneer in its use of this coding algorithm.Both FDS enhancements allow Voyager 2 to push more data through the increasingly diminished radio bandwidth as the spacecraft travels farther and farther away from Earth, but at a cost. The enhancements require full-time use of the second, redundant FDS computer for the new image-processing algorithms because one FDS computer is no longer sufficient to run all the FDS software. So, the price for the enhancements was a loss of FDS redundancy. It’s important to note that the enhancements were possible only because they could be uploaded to Voyager’s computers as software upgrades.PS: Nowadays, for eg the Ingenuity Helicopter on Mars uses FPGAs. Unlike on the Earth, Mars presents with high radiation levels. The newer FPGAs that are manufactured with a 28nm FDSOI (Fully-Depleted-Silicon-On-Insulator) process that’s inherently radiation-tolerant will be used for later probes.</blockquote>

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