from Hacker News

Roundoff Error and the Patriot Missile (1992)

by auslander on 9/22/19, 4:16 AM with 10 comments

  • by matchagaucho on 9/22/19, 7:49 PM

    The computer science behind TBM interception is a fascinating problem with very high stakes consequences, as occurred on February 25, 1991.

    I was an active duty Patriot Technician and Systems Mechanic (24T) during that time; and even though we were rebooting our systems regularly to diminish the impact of this roundoff error, there were other critical timing issues during the intercept stage.

    TBM interception involves the science and math behind a "bullet hitting a bullet". These are extremely high velocities converging on each other.

    To Raytheon's credit, they were iterating rapidly and released patches almost daily as new data was collected.

    Even when we managed to launch a Patriot to engage an oncoming Scud, success was dependent on the proximity fuzed warhead detonating at just the perfect predictive moment ahead of the projectile. This timing was perfected over a decade in White Sands NM using lower velocity drones. But Patriot's software was not optimized for TBM-scale velocity (it is now).

    A "perfect" hit typically resulted in a shower of hot metal and undetonated debris raining down on civilian populations.

    From a game theory perspective, this is basically a no-win situation. You're just trying to minimize collateral damage once a theater of operations escalates to using TBMs.

  • by sixstringtheory on 9/22/19, 10:06 PM

    I’ll never forget my Computational Methods professor discussing exactly this project, and how a lack of knowledge or care about things like the conditioning of your routines, or compounding rounding errors in floating point, could literally get people killed.

    Most of the students laughed at this idea.

  • by auslander on 9/22/19, 4:53 AM

    Also: "As a stopgap measure, the Israelis had recommended rebooting the system's computers regularly."

    https://en.wikipedia.org/wiki/MIM-104_Patriot#Failure_at_Dha...