<story><title>Nasa’s Mars Rover Opportunity Concludes a 15-Year Mission</title><url>https://www.nytimes.com/2019/02/13/science/mars-opportunity-rover-dead.html</url></story><parent_chain><item><author>tzs</author><text>Those &amp;quot;have N copies and compare them&amp;quot; systems then have the issue that whatever does the comparison is a single point of failure. You could use multiple comparison units and have a second level comparison check them, but then that is your single point of failure, and so on.&lt;p&gt;I&amp;#x27;ve been told, but never actually looked it up, that there is a theorem that proves you always have to have at least one single point of failure.&lt;p&gt;I don&amp;#x27;t know if the following is actually true, or just a rumor, but I&amp;#x27;ve heard that at least one aircraft whose mission called for very high reliability didn&amp;#x27;t have a comparison unit: the redundancy extended all the way to having the 3 independent flight control computers each control a separate actuator on each flight control surface. If one of the systems went bad and tried to move the surface incorrectly, the other two would physically overpower it.&lt;p&gt;That still has a single point of failure, but now that point is the control surface itself. If your control surface itself has failed it no longer matters if the 3 computers controlling it agree.</text></item><item><author>luizfzs</author><text>I saw a video once saying that SpaceX uses general purpose computers on the rockets instead of specific purpose hardware. If I&amp;#x27;m not mistaken, they amount to 6 and have a checking system to assure the output of them is the same.</text></item><item><author>wiremine</author><text>Was looking through Opportunity&amp;#x27;s Wikipedia page, and found this cool comparison of the embedded systems used in Mars rovers:&lt;p&gt;&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Comparison_of_embedded_computer_systems_on_board_the_Mars_rovers&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Comparison_of_embedded_compute...&lt;/a&gt;&lt;p&gt;That lead me to the RAD6000 page, which was new to me:&lt;p&gt;&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;IBM_RAD6000&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;IBM_RAD6000&lt;/a&gt;&lt;p&gt;&amp;quot;Reported to have a unit cost somewhere between US$200,000 and US$300,000, RAD6000 computers were released for sale in the general commercial market in 1996&amp;quot;&lt;p&gt;Anybody know why the per unit cost is so high? Low yields or is it that much more expensive?</text></item></parent_chain><comment><author>Strilanc</author><text>&amp;gt; &lt;i&gt;I&amp;#x27;ve been told, but never actually looked it up, that there is a theorem that proves you always have to have at least one single point of failure.&lt;/i&gt;&lt;p&gt;In what context? There&amp;#x27;s a theorem that arbitrarily-reliable computation can be done with noisy components, as long as the noise is below some threshold (e.g. picture less than 1 error per 10 operations). [1]&lt;p&gt;1: von Neumann, J. (1956). &amp;quot;Probabilistic Logics and Synthesis of Reliable Organisms from Unreliable Components&amp;quot;, in Automata Studies, eds. C. Shannon and J. McCarthy, Princeton University Press, pp. 43–98 &lt;a href=&quot;http:&amp;#x2F;&amp;#x2F;www.cyclify.com&amp;#x2F;wiki&amp;#x2F;images&amp;#x2F;a&amp;#x2F;af&amp;#x2F;Von_Neumann_Probabilistic_Logics_and_the_Synthesis_of_Reliable_Organisms_from_Unreliable_Components.pdf&quot; rel=&quot;nofollow&quot;&gt;http:&amp;#x2F;&amp;#x2F;www.cyclify.com&amp;#x2F;wiki&amp;#x2F;images&amp;#x2F;a&amp;#x2F;af&amp;#x2F;Von_Neumann_Probabil...&lt;/a&gt;</text></comment>
<story><title>Nasa’s Mars Rover Opportunity Concludes a 15-Year Mission</title><url>https://www.nytimes.com/2019/02/13/science/mars-opportunity-rover-dead.html</url></story><parent_chain><item><author>tzs</author><text>Those &amp;quot;have N copies and compare them&amp;quot; systems then have the issue that whatever does the comparison is a single point of failure. You could use multiple comparison units and have a second level comparison check them, but then that is your single point of failure, and so on.&lt;p&gt;I&amp;#x27;ve been told, but never actually looked it up, that there is a theorem that proves you always have to have at least one single point of failure.&lt;p&gt;I don&amp;#x27;t know if the following is actually true, or just a rumor, but I&amp;#x27;ve heard that at least one aircraft whose mission called for very high reliability didn&amp;#x27;t have a comparison unit: the redundancy extended all the way to having the 3 independent flight control computers each control a separate actuator on each flight control surface. If one of the systems went bad and tried to move the surface incorrectly, the other two would physically overpower it.&lt;p&gt;That still has a single point of failure, but now that point is the control surface itself. If your control surface itself has failed it no longer matters if the 3 computers controlling it agree.</text></item><item><author>luizfzs</author><text>I saw a video once saying that SpaceX uses general purpose computers on the rockets instead of specific purpose hardware. If I&amp;#x27;m not mistaken, they amount to 6 and have a checking system to assure the output of them is the same.</text></item><item><author>wiremine</author><text>Was looking through Opportunity&amp;#x27;s Wikipedia page, and found this cool comparison of the embedded systems used in Mars rovers:&lt;p&gt;&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Comparison_of_embedded_computer_systems_on_board_the_Mars_rovers&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Comparison_of_embedded_compute...&lt;/a&gt;&lt;p&gt;That lead me to the RAD6000 page, which was new to me:&lt;p&gt;&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;IBM_RAD6000&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;IBM_RAD6000&lt;/a&gt;&lt;p&gt;&amp;quot;Reported to have a unit cost somewhere between US$200,000 and US$300,000, RAD6000 computers were released for sale in the general commercial market in 1996&amp;quot;&lt;p&gt;Anybody know why the per unit cost is so high? Low yields or is it that much more expensive?</text></item></parent_chain><comment><author>mehrdadn</author><text>This is something I&amp;#x27;d always wondered about and never gotten a straight answer to. You&amp;#x27;re the first person I&amp;#x27;ve seen who explicitly confirmed my suspicion that every system has a single point of failure. Would love to find a reference for this...</text></comment>