Processori di bordo e SEU

C’é una conversazione interessante su Facebook a proposito dei processori usati nelle missioni Voyager e Galileo. Non so se posso copiare il thread, ma se avete FB date un’occhiata qui:

In sostanza si dice che il fenomeno dei SEU (single event upset) era sconosciuto prima delle Voyager, e che fu scoperta la vulnerabilità dei processori AMD 2901, usati da Galileo, grazie ai dati delle Voyager. Questo permise un parziale riprogetto dei processori di Galileo, che diversamente sarebbe stata un fallimento. Cito la parte interessante:

"Galileo’s processors and memories were chosen in 1977. Voyager had not yet reached Jupiter, so hardware decisions were based on 1973-1974 Pioneer data. Nothing was known about SEU vulnerability, so no space for error detection and correcting codes and no provision for special shielding was made. Some incorrect imaging commands sent by sequencers in the Pioneers were later tagged as SEUs. Voyager’s clocks were slowed by Jovian radiation so that the computers were forced out of synchronization occasionally. By 1980-1981, the nature of the SEU problem became apparent. Sulfur ions from Jupiter’s volcanic moon, Io, were being whipped up to high energy by the Jovian gravity. In 1982, Galileo Project Chief Engineer B. Gentry Lee was assigned the job of determining how bad the SEU problem could be and finding a solution. Lee arranged for cyclotron tests at the University of California’s Berkeley campus in which computer and other electronic parts were submitted to bombardment by high-speed particles. Results indicated that the 2901 chips used in the attitude control computers were highly SEU sensitive, with 20% - 50% of hits causing probable software failures. RCA 1802s used in the Command and Data Subsystem were actually much less sensitive, being of older, and thus less dense, technology.

Attitude control engineer Kopf commented, “It is not worth flying the mission if you cannot get rid of the SEU problem.” Failures were most likely at the most critical part of the probe mission when the orbiter is very near Jupiter. In order to avoid possible further delays in an already much postponed mission, Lee searched for solutions along two tracks. One solution would use a radiation-hardened processor built by Trecor called the RHEC-Radiation Hardened Emulating Computer-1750A. Even though it is an emulator capable of imitating the 2901, a new retargeted HAL compiler would be needed. The cost of this solution would be $20 million. Lee’s second solution was to contract with Sandia National Laboratories to custom make radiation-hardened 2901s. No software needed be changed, just new ICs were necessary, and they cost $5 million. Due to cost considerations and the inherent attraction of retaining the already created and largely validated software, the Sandia solution was chosen. As a footnote, it is interesting that if the Galileo had launched on time, a sufficient understanding of the SEU problem would not yet have been available, and a doomed spacecraft carrying an unknown time bomb would have been traveling toward an unfriendly Jupiter waiting to hurl ion thunderbolts at it."

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